PT-141 (bremelanotide): what two phase 3 trials, an FDA label and a cancelled men’s programme actually show

PT-141 (bremelanotide): what two phase 3 trials, an FDA label and a cancelled men's programme actually show

The page that stood here from April 2026 was called "PT-141 peptide (bremelanotide): mechanism, dosage and research". It had a dosage section, a reconstitution calculator and a storage guide, and in three months Google showed it 2,500 times and sent six people to it. I took the calculator out of this version and I would have taken it out regardless of the traffic, because the FDA letters of 24 August 2026 cite exactly that kind of content, on exactly this compound, as evidence that a vendor is selling a drug. What is left is the record. Bremelanotide is unusual among the peptides I write about: it is approved, so there is a label, an FDA review, two phase 3 trials, an extension study, independent re-analyses and twenty years of SEC filings from the company that made it. Most of what the internet says about PT-141 is either copied from the men's programme that was cancelled in 2007 or is about melanotan II. I have tried to keep them apart.

Michael Phelps

Founder & Peptide Research Specialist, PrymaLab

Research reference · First published April 4, 2026 · Rewritten from primary sources October 10, 2026 · ~45 min read

What is PT-141, and how is it related to melanotan II?

PT-141 is Palatin Technologies' code for bremelanotide, a synthetic cyclic heptapeptide that activates melanocortin receptors. It is the carboxylic-acid form of melanotan II, the tanning peptide, and it became a drug because of a side effect: volunteers in melanotan II tanning studies at the University of Arizona reported spontaneous erections.

Mac Hadley, who ran those studies, wrote in 2005 that the sexual effects of melanotan II "were discovered accidentally while studying the effects of the peptide and related analogs on human skin pigmentation (tanning)".^[1] Wessells and colleagues then gave melanotan II to ten men with psychogenic erectile dysfunction in a double-blind crossover: eight developed erections, with penile tip rigidity above 80% for 38.0 minutes against 3.0 on placebo (p=0.0045), along with nausea, stretching and yawning.^[2] A second ten-man study in organic erectile dysfunction produced erections after 12 of 19 injections against 1 of 21 on placebo, and severe nausea after 4 of 19.^[3] In the review of both studies, increased desire was reported after 13 of 19 melanotan II doses against 4 of 21 placebo doses.^[4]

The molecule Palatin took forward differs by one change. King and colleagues described PT-141 in 2007 as "the carboxylate derivative of MT-II", meaning the C-terminal amide of melanotan II is replaced by a free carboxylic acid.^[5] Everything else in the ring is the same. That one functional group's difference matters legally far more than it matters pharmacologically: bremelanotide went through a full development programme, 43 completed studies and about 3,500 participants by the sponsor's count, two-year carcinogenicity studies and an FDA review, and melanotan II went through none of it.^[6][7] The two compounds are routinely confused online, and the confusion runs in both directions: melanotan II is marketed as a substitute for PT-141, and PT-141 is credited with melanotan II's tanning effects, which it does produce at the receptor level (hyperpigmentation is on the Vyleesi label) but was never developed for.

What is the structure of bremelanotide?

Bremelanotide is Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH: an acetylated norleucine followed by a six-residue ring closed by a lactam bond between the aspartate side chain and the lysine side chain, ending in a free carboxylic acid. Molecular formula C₅₀H₆₈N₁₄O₁₀, molecular weight 1,025.2, CAS 189691-06-3.

PubChem records the compound as CID 9941379 with InChIKey FFHBJDQSGDNCIV-MFVUMRCOSA-N and lists its identifiers: UNII 6Y24O4F92S for the free base, ChEMBL CHEMBL2070241, DrugBank DB11653, KEGG D06569.^[8] The Vyleesi label describes the drug substance as "a synthetic, cyclic heptapeptide with a free acid at the carboxyl terminus and an acetylated amino group at the amino terminus", supplied as the acetate salt (UNII PV2WI7495P), with one to two acetate equivalents per molecule, in a prefilled autoinjector.^[7][9] The FDA pharmacology review notes a convenient coincidence: at a molecular weight of 1,025, "1 ng/mL ~ 1 nM", so plasma concentrations in ng/mL read directly as nanomolar.^[10]

The D-phenylalanine at position 4 and the lactam ring are what make this family "superpotent" relative to the natural hormone alpha-MSH: the ring locks the pharmacophore His-D-Phe-Arg-Trp into its receptor-binding conformation and the D-amino acid resists proteolysis.^[5] Melanotan II has the identical ring and ends in -NH₂; afamelanotide (melanotan 1) is a linear 13-residue alpha-MSH analogue and is a different molecule altogether.^[11] A 2026 analytical paper mapped the degradation products of bremelanotide acetate by high-resolution mass spectrometry, including epimerisation at the chiral centres, which is the kind of impurity profile an identity test on research material has to resolve.^[12]

How does PT-141 work, and why does the label say the mechanism is unknown?

Bremelanotide activates several melanocortin receptors, in the label's order of potency MC1R, MC4R, MC3R, MC5R and then MC2R, and the effect on desire is attributed to MC4R in the hypothalamus. The label states plainly that "the mechanism by which VYLEESI improves HSDD in women is unknown". The receptor pharmacology is measured; the link from receptor to behaviour is a hypothesis with mixed animal support.

The FDA review gives the numbers. In competitive binding against radiolabelled NDP-alpha-MSH, bremelanotide bound MC1R and MC4R most tightly, then MC3R, then MC5R, and, by the sponsor's statement (FDA received no source data), negligible binding at MC2R, the adrenal ACTH receptor (Kd above 1,000 nM). In MC4R-transfected cells it raised cAMP with an EC50 of 1.1 nM, reaching 68% of the maximal response to alpha-MSH, and its binding Ki at MC4R is about 14 nM.^[10] Free plasma concentration after a labelled dose is roughly 63 nM, above the Ki for both MC1R and MC4R, which is why the label says binding at those two receptors is "most relevant" at therapeutic levels.^[10][7] The MC1R binding is what produces pigmentation. The MC3R binding may be what produces nausea, in the FDA reviewer's reading. The MC4R binding is credited with the desire effect, the blood-pressure rise and the slowed gastric emptying that underlies the drug interactions below.^[10]

The behavioural hypothesis, laid out in an industry-authored review, is that MC4R agonism on presynaptic neurons in the medial preoptic area of the hypothalamus increases dopamine release, and that this shifts the balance toward excitation in a circuit that is inhibited in HSDD.^[13] Rat data support part of it: systemic PT-141 produced erections in rats and primates and activated hypothalamic neurons in the same region that projects to the penis.^[14][15] A 31-woman crossover fMRI study at Imperial College found that a single dose increased self-reported desire for up to 24 hours and changed activity in the cerebellum, supplementary motor area and amygdala-insula connectivity in response to erotic stimuli.^[16] Against that, a 2025 study in female Syrian hamsters found bremelanotide did not alter MC3R or MC4R expression in the mesolimbic dopamine system and "failed to enhance sexual reward" in a conditioned-place-preference test, which led the authors to conclude it does not act on the ventral tegmental area to nucleus accumbens circuit.^[17] In mice, FDA noted, the peptide crosses the blood-brain barrier "with a significant delay", with brain radioactivity peaking at about 10% of plasma, so central MC4 receptors "might be only partially activated".^[10] The honest summary is the label's: the receptor is known, the mechanism is not.

One more piece of mechanism belongs here because it explains the safety profile. MC4R agonists raise blood pressure: FDA's review summarised that subcutaneous infusion of an MC4R agonist raises blood pressure in humans and that people with loss-of-function MC4R mutations have less hypertension.^[10] MC4R agonism also reduces food intake, which is why Palatin later tested bremelanotide in obesity: in the first of two phase 1 trials in obese women it cut intake by about 400 kcal a day and weight by 1.3 kg against placebo over 16 days, and a 2024 to 2025 phase 2 of low-dose bremelanotide with tirzepatide reported 4.4% weight loss against 1.6% over eight weeks, after which Palatin said it would not fund further development on its own and was looking for a partner.^[18][19]

How is PT-141 different from sildenafil and tadalafil?

Sildenafil and tadalafil are PDE5 inhibitors: they act on the blood vessels of the penis and require sexual stimulation to produce an erection. Bremelanotide acts on melanocortin receptors in the brain and, in the early men's studies, produced erections without stimulation along with increased desire. The two classes were tested together once, in 19 men, in 2005. Bremelanotide is approved for women with low desire; it is not approved for erectile dysfunction in anyone.

The 2005 study randomised 19 men with PDE5-responsive erectile dysfunction to a crossover of low-dose sildenafil plus low-dose intranasal PT-141, sildenafil alone and placebo, and found the combination's erectile response "significantly greater" than sildenafil alone, with no new adverse events.^[20] That is the whole of the head-to-head literature. It is a 19-man RigiScan study, it used the intranasal formulation that was later abandoned, and it was never followed up. The Vyleesi label adds a limitation of use that answers the comparison from the regulator's side: the drug "is not indicated to enhance sexual performance".^[7] The question people are usually asking when they type "PT-141 vs Viagra" is whether PT-141 is a better erectile drug for men, and the record's answer is that it was never shown to be one and its development for that purpose was stopped by its own sponsor.

What did the RECONNECT phase 3 trials show for PT-141 in women?

Two identical 24-week trials randomised 1,267 premenopausal women with acquired, generalised HSDD to bremelanotide or placebo. On the co-primary endpoints, desire score rose by 0.5 to 0.6 points against 0.2 on placebo and distress fell by 0.7 against 0.4. The number of satisfying sexual events did not change. About 40% of treated women left before 24 weeks.

Kingsberg and colleagues published the two trials together in 2019. Study 301 ran from 7 January 2015 to 26 July 2016 and Study 302 from 28 January 2015 to 4 August 2016; 1,247 women were in the safety population and 1,202 in the modified intention-to-treat population; 85.6% were white, 96.6% were at US sites, mean age was 39.^[21] A majority (74% in Study 301, 67% in Study 302) also reported decreased arousal.^[7]

The scales matter for reading the table. The FSFI desire domain is two questions about frequency and intensity of desire over 28 days, scored 1.2 to 6.0; a change of 0.5 is a fraction of one step on either question. The distress item asks how often, over 30 days, the woman was bothered by low desire, scored 0 to 4; a change of 0.7 against 0.4 is a difference of 0.3 of a step.^[7] The minimal clinically important differences the sponsor set in advance were +0.6 and −1.0, derived from the phase 2b responder analysis, and the label's own responder thresholds were +1.2 and −1.0.^[22][23][7] The placebo-adjusted group differences did not reach the sponsor's own thresholds; the responder analyses, which count individual women who did, are where the sponsor's case for a clinically meaningful effect rests. Pre-specified subgroups (age, weight, contraceptive use, HSDD duration) mostly showed the same direction, with the exception that the desire gain reached significance only in women not using hormonal contraception.^[24]

The extension study is the other half of the efficacy story. Of 856 eligible women who completed the core phase, 684 entered a 52-week open-label extension and 272 finished it. Among those who stayed, desire and distress scores kept moving in the right direction (changes of 1.25 to 1.30 and −1.4 to −1.7 from baseline for women who had been on drug in the core phase), but the authors noted that fewer bremelanotide-treated than placebo-treated women completed the core phase and chose to continue, so "nonresponders are likely underrepresented", which "may result in an overestimation" of the effect.^[22] The FDA approval letter of 21 June 2019 went to AMAG Pharmaceuticals with three post-marketing requirements, a pregnancy registry, a pregnancy cohort study and a lactation study; the lactation study (ten women, 2025) is complete and its results were not in the label at the time of writing.^[25][26][27]

How big is the benefit, and who disputes it?

Statistically real, clinically small, and contested. The sponsor's responder analyses show about 58% of treated women versus 36% on placebo reporting a meaningful improvement. An independent re-analysis of the FDA data found that 72.7% of protocol-listed outcomes went unreported, that the odds of discontinuing for adverse events were twelve times higher on drug (number needed to harm, six), and that by completion and extension-enrolment behaviour participants "preferred placebo".

Glen Spielmans re-analysed the phase 3 data from the FDA review documents in 2021. His findings: "72.72% of protocol-listed outcomes were not reported by Kingsberg et al."; adverse-event-induced discontinuation odds ratio 11.98 (95% CI 3.74 to 38.37), number needed to harm 6; and, combining completion of the core phase with enrolment in the extension, an odds ratio of 0.30 favouring placebo, number needed to harm 4. His conclusion was that "the drug is generally not useful".^[28] The trial authors replied in the same journal.^[29] Spielmans and Ellefson returned in 2024 with the eight of eleven registry-specified efficacy outcomes that had not been published, on which "effect sizes ranged from nil to small", and argued that the validity evidence for the two co-primary instruments in HSDD was "questionable, at best".^[30] The Drug and Therapeutics Bulletin, reviewing both approved HSDD drugs, put it in one sentence: "flibanserin led to an average of only one additional enjoyable sexual experience every two months, bremelanotide to none."^[31]

The counter-case is not nothing. The co-primary endpoints were met in both trials (p<0.001 for every comparison except distress in Study 302, p=0.005), the responder rates separated by about 22 percentage points, and a 2026 systematic review of 36 studies concluded that bremelanotide improved total FSFI, its desire and arousal subscales, and distress.^[21][32] Two independent pharmacology reviews land in the middle: "the overall clinical benefit appears to be modest".^[33][34] Placebo responses in female sexual dysfunction trials are large, which cuts both ways: it makes a drug effect hard to show and it means a 0.2-point placebo gain is normal.^[35]

The market has given its own verdict. AMAG paid $60 million upfront in January 2017 for North American rights, launched in September 2019, reported 1,300 prescribers and 3,000 prescriptions in the first four weeks, and in July 2020 terminated the licence, paying Palatin $12.0 million at closing and $4.3 million later to be rid of it.^[36][37][38] Palatin sold the drug itself for three years, reaching gross sales of $12.5 million and net sales of $4.9 million in the year to June 2023, then sold it to Cosette in December 2023 for $12 million upfront plus milestones.^[38][39] In June 2025 Cosette settled the outstanding sales-milestone and inventory obligations for a lump sum of $630,000 and took over the supply commitments; Palatin kept a 20% share of a $3 million Korean first-sale milestone it says it does not expect to receive.^[19] A drug that worked the way the marketing copy says would not have changed hands twice at falling prices.

What happened to PT-141 for men?

The men's programme ran from about 2000 to 2007, reached phase 2B with 1,020 patients in two intranasal trials, and stopped when FDA raised "serious concerns about the acceptable benefit/risk ratio", citing blood pressure. Those trial results were never published. The only large peer-reviewed trial in men is a 342-patient Iranian study that now carries an expression of concern. The erectile dysfunction IND was withdrawn in 2017, and no trial for a sexual indication in men is registered today.

The published PT-141 data in men are three small studies and one large one. Diamond and colleagues gave intranasal PT-141 to healthy men and to sildenafil-responsive patients: median time to peak concentration 0.50 hours, half-life 1.85 to 2.09 hours, an erectile response that was significant against placebo at the higher doses tested, first erections at about 30 minutes, flushing and nausea the commonest adverse events, and "a maximum-tolerated dose was not identified".^[40] Rosen and colleagues gave it subcutaneously: significant erectile response at the higher doses in healthy men and at both doses tested, in a crossover, in patients who had not responded adequately to sildenafil.^[41] The 19-man sildenafil combination study is described above.^[20] Then Safarinejad and Hosseini in Iran randomised 342 married men aged 28 to 59 who had not responded to sildenafil to intranasal bremelanotide or placebo and reported "positive clinical results" in 51 (33.5%) against 13 (8.5%), p=0.03, with more drug-related adverse effects.^[42] On 10 January 2023 the Journal of Urology issued an expression of concern about that paper.^[43] The same author's 2008 paper on intranasal bremelanotide in women with arousal disorder was retracted after reviewers "raised multiple concerning questions about the methodology, results, and statistical interpretation" and the author chose not to respond.^[44] Every "PT-141 works for men" claim that cites a controlled trial is citing one of these four papers or the two ten-man melanotan II studies, and the largest of them is under a cloud.

The trials that would have settled it were Palatin's own. Its 2010 annual report describes two phase 2B double-blind trials of intranasal bremelanotide "conducted in 726 non-diabetic and 294 diabetic patients", which the 2007 report says had a three-month at-home treatment period, and claims that "over 30% of ED patients were restored to a normal level of function".^[45][46] Those results never appeared in a journal. What appeared instead was the 2007 annual report's account of FDA's response: "serious concerns about the acceptable benefit/risk ratio to support progression into Phase 3 as a first-line therapy in the general population", questioning "overall efficacy results and the clinical benefit of bremelanotide in both general and diabetic populations, citing blood pressure increases as its greatest safety concern".^[46] King Pharmaceuticals, Palatin's partner since August 2004, gave notice in September 2007 and the collaboration ended on 5 December 2007. Palatin's bremelanotide spending fell from $18.3 million to $3.2 million the following year, and its share price went from $2.09 to $0.17.^[47] The company blamed the intranasal route, attributing the blood-pressure and nausea problems "at least partially, to high doses resulting from variability in drug uptake with nasal administration", and ran two phase 1 subcutaneous blood-pressure studies in men, one a two-week repeat-dose study and one a crossover in 49 healthy men aged 45 to 65. It said in 2010 that it intended to start an in-clinic phase 2 of subcutaneous bremelanotide in men who did not respond to PDE5 inhibitors "as early as the first quarter of calendar 2011".^[45] No such trial appears on ClinicalTrials.gov or in any later filing. By 2013 the active programme was "for treatment of FSD" only.^[48] FDA's review of the women's application records that "IND 61706 [male ED, nasal spray] was withdrawn in 2017" and that "some pharmacology studies related to the male indication of erectile dysfunction" were left outside that review.^[10] The label states that Vyleesi "is not indicated for the treatment of HSDD in postmenopausal women or in men".^[7] A ClinicalTrials.gov query on 9 October 2026 returned ten bremelanotide studies, none for a sexual indication in men; the only two open to men are a 16-patient kidney-disease study and a 108-patient obesity study.^[49] The most recent men's publication I could find is a 2026 commentary asking whether bremelanotide should be considered for desire disorders in men, which is a question, not a trial.^[50]

What are the side effects of PT-141?

Nausea in 40% of treated women against 1.3% on placebo, flushing in 20%, headache in 11%, vomiting in 5%. Eighteen percent discontinued for adverse reactions against 2%. Blood pressure rises by up to 6/3 mmHg, peaking 2 to 4 hours after a dose, with heart rate down by up to 5 beats per minute. Focal hyperpigmentation of the face, gums or breasts occurred in 1% at labelled frequency and in 38% of women dosed daily for eight days.

Two label items are not in the table and matter more than their frequency suggests. The contraindication: Vyleesi is contraindicated in "uncontrolled hypertension or known cardiovascular disease", and "not recommended in patients at high risk for cardiovascular disease".^[7] The blood-pressure rise is modest and brief in the trial population, which was young and screened for normal or controlled blood pressure, and the phase 2b ambulatory monitoring study found placebo-adjusted systolic rises of about 3 mmHg, with peaks typically lasting under 15 minutes.^[52] It was large enough, in the older male population of the 2007 programme and with the variable nasal formulation, to end that programme. The interaction: bremelanotide slows gastric emptying, and in FDA's clinical pharmacology studies it cut the peak concentration of oral naltrexone by 64% and its exposure by 40%, and indomethacin's by 35% and 19%, while leaving sertraline, venlafaxine, pseudoephedrine and phentermine unaffected. The label tells patients to avoid it with oral naltrexone products for alcohol or opioid dependence, because a missed naltrexone level in that setting is a treatment failure with consequences.^[10][7]

The rest of the safety dossier is what an approved drug has and a research compound does not. Two-year carcinogenicity studies in rats (intranasal) and mice (subcutaneous, at up to 111 times human Cmax) found no increase in tumours; genotoxicity tests were negative; fertility was unaffected in mice at hundreds of times human exposure.^[7] FDA found no apparent increase in atypical nevi within the limits of the trials and noted that reversibility of hyperpigmentation "was documented in one-half of cases during the period of follow-up".^[10] Seven pregnancies occurred among more than 1,057 treated women, with no major congenital anomalies, but post-implantation loss rose three- to eight-fold in dogs at the lowest dose tested, about 16 times human exposure, so the label advises effective contraception.^[7] A human abuse-potential study against phentermine found responses no different from placebo, and the drug is not scheduled.^[10][53]

What are the pharmacokinetics of bremelanotide?

In the label's pharmacokinetic data, bremelanotide given by the subcutaneous route was about 100% bioavailable, reached peak plasma concentration at a median of 1.0 hour, and had a terminal half-life of about 2.7 hours (range 1.9 to 4.0). It is cleared by hydrolysis of its amide bonds, with 64.8% of radioactivity recovered in urine and 22.8% in faeces.

The label gives mean Cmax of 72.8 ng/mL and AUC of 276 h·ng/mL at the approved dose, exposure that increases less than proportionally with dose, with peak concentration plateauing at higher doses; 21% protein binding; a volume of distribution of 25 L; clearance of 6.5 L/h.^[7] Exposure rises 1.2-, 1.5- and 2-fold in mild, moderate and severe renal impairment and 1.2- and 1.7-fold in mild and moderate hepatic impairment, and the label advises caution in severe renal or hepatic impairment because nausea and vomiting may be more frequent and severe.^[7] Intranasal administration was faster (median Tmax 0.5 h) and produced a 2.5-fold higher peak in the men's studies than the approved product, with the variability that ended the men's programme.^[40][7] A 2020 paper developed an ultra-sensitive LC-MS/MS assay specifically to measure plasma bremelanotide after oral administration, which tells you that oral absorption is low enough to need one.^[54] The short half-life is also the pharmacological reason the label can state that blood pressure returns to baseline within about 12 hours and that 16 days of daily use in a study produced no cumulative effect.^[7]

What does the approved label specify, and why does none of it apply to research material?

The Vyleesi label sets out conditions of use for one product, a prefilled autoinjector, and this article does not repeat them. Those conditions are facts about one pharmaceutical product with a measured content and a sterile, characterised formulation. They say nothing about any other material, and this article contains no protocol.

The label is also candid about the limits of its own instruction: "the duration of efficacy after each dose is unknown and the optimal window for VYLEESI administration has not been fully characterized".^[7] FDA's review explains why the label limits frequent use: very limited safety information with frequent use, a rising risk of hyperpigmentation, and more time each month spent with raised blood pressure.^[10] The phase 3 product came out of a 12-week phase 2b trial, which compared three dose levels with placebo and took the highest into phase 3; that trial's primary endpoint was satisfying sexual events, which the pooled higher doses did improve (+0.7 against +0.2 a month, p=0.018) and which phase 3 then failed to replicate as a secondary endpoint.^[55][21] The phase 3 endpoints then changed again: FDA's review records that, at the sponsor's request and with the agency's agreement in September 2016, after the two trials' last patient visits in July and August 2016 and before the database was locked, the desire and distress scores became the co-primary endpoints and satisfying sexual events became the key secondary endpoint. That is the "shift in primary outcomes" the Drug and Therapeutics Bulletin objected to.^[10][31]

The earlier version of this page had a reconstitution calculator. It is gone, and the reason is on the FDA's website. The 24 August 2026 warning letters cite the sale of bacteriostatic water (Tex Peptides, Peptide Partners, NuScience, Peak Performance) and a "peptide guide" and "peptide calculator" (Royal Peptides) as evidence that products labelled for research are intended for human use.^[56][57][58] A page that describes the label of Vyleesi is describing an approved drug. A page that tells a reader how to turn a research vial into a dose is doing something else.

Why was the nasal spray abandoned?

Because nasal absorption was variable. Palatin's own account attributes the blood-pressure increases and the nausea and vomiting in the men's programme "at least partially, to high doses resulting from variability in drug uptake with nasal administration", and the approved product is a subcutaneous autoinjector.

Every early efficacy study but one used the nasal route: Diamond's 2004 men's study (Rosen's 2004 subcutaneous study was the exception), the 2005 sildenafil combination, the 2006 study in 18 women with arousal disorder, the intranasal phase 2 in women that ran from February 2006 to May 2007, and the two phase 2B trials in 1,020 men.^{[40][20][59][60][45]} In the intranasal studies the peak plasma concentration was 2.5 times that of the approved product.^[7] After FDA's 2007 response, Palatin moved the women's programme to the subcutaneous route, ran the subcutaneous phase 2b dose-finding trial, and never returned to the nose.^[45][55] The intranasal women's results from 2006 to 2007 were never published; the company's own summary says the premenopausal results were "confounded by a significant placebo effect".^[48] So a nasal spray of bremelanotide is a formulation its developer tested in more than a thousand people and walked away from on pharmacokinetic grounds. That is the whole of what the primary record says about it.

Is PT-141 legal? FDA, DEA, WADA and the August 2026 warning letters

Bremelanotide is an FDA-approved prescription drug in the United States, with no approval found anywhere else; it is not a DEA-controlled substance and is not on the WADA 2026 Prohibited List. Research-grade bremelanotide sold with claims about libido, arousal or HSDD is, in FDA's words in five warning letters dated 24 August 2026, an unapproved new drug, whatever the label says.

The August 2026 letters are the operative document for anyone in this market. FDA reviewed vendor websites in July 2026 and wrote to Royal Peptides that "'Tirzepatide,' 'Semaglutide,' 'Retatrutide,' 'SS-31 (Elamipretide),' 'PT-141,' 'Tesamorelin,' and 'BIMORELIN' are unapproved new drugs", quoting the vendor's own PT-141 page, which described the compound as having "been tested in clinical trials as a treatment for hypoactive sexual desire disorder", as evidence of intended use.^[57] To Tex Peptides it named "'PT-141 (Bremelanotide),' and 'Bacteriostatic Water 0.9% Benzyl Alcohol (10 ml)'" among the unapproved drugs, quoted the claim that PT-141 "is specifically researched for its ability to increase arousal and desire", and treated the bacteriostatic water as a drug because it supplied the means to prepare an injection.^[56] The other three letters of the same day, to Peptide Partners, NuScience Peptides and Peak Performance Peptides, name the compound too, as "PT-141 (Bremelanotide)", "PT-141 Peptide (Bremelanotide)" and "PT-141".^[62][63][64] A law-firm review of the wave covers four of the five and lists PT-141 among the products named in each; across the letters FDA relied on claims, a "peptide guide" and "peptide calculator", and bacteriostatic water, and rejected research-use-only disclaimers.^[58] The existence of an approved product does not shelter a third party's vial; if anything it sharpens the point, because the vial is being sold for the thing the approved product is approved for.

Outside the United States the drug is, for practical purposes, unapproved. Palatin's filings from 2007 to 2026 mention no EMA submission; the European path ended when the Richter licence was terminated by mutual agreement in September 2015, before phase 3 finished.^[65] Health Canada's product database returns nothing for bremelanotide or Vyleesi.^[66] The Korean bridging trial (193 women, completed May 2023) has produced no commercial sale; Palatin wrote in September 2026 that it "do[es] not expect to receive this milestone payment in the foreseeable future, if at all".^[70][19] As of Palatin's last disclosure in August 2023, China's only sales had been under Hainan's free-trade-port exemption from national registration.^[69]

How does bremelanotide compare with melanotan II?

Same ring, different ending, different world. Bremelanotide has a label, a negative carcinogenicity programme and a measured 1% hyperpigmentation rate at labelled frequency. Melanotan II has no approval anywhere, a place on FDA's compounding safety-risk list and the WADA S0 category, and a case-report literature of melanoma, encephalopathy, systemic toxicity and priapism.

The structural relationship is exact: melanotan II is Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂ and bremelanotide is the same sequence ending in -OH.^[11][5] Both bind MC1R, so both darken skin; both bind MC4R, so both affect desire, appetite and blood pressure. The pharmacology does not explain the gap in their safety records. The gap is that one was studied and the other was not. FDA's compounding page summarises the melanotan II literature as "serious adverse events including melanoma, posterior reversible encephalopathy syndrome, sympathomimetic toxidrome and priapism".^[61] The individual reports: a 39-year-old man who had injected melanotan II bought online and presented with a blood pressure of 151/85, a heart rate of 146, dilated pupils, sweating, tremor and rhabdomyolysis;^[73] eruptive atypical nevi in a man with prior melanoma who had injected a synthetic alpha-MSH peptide (the paper does not say which), which lightened when he stopped, prompting the authors to warn that synthetic alpha-MSH peptides "can drive proliferation of neoplastic melanocytic cells in predisposed patients";^[74] nevi that changed under dermoscopy during use in ways that made melanoma hard to exclude;^[75] an oral mucosal melanoma in a 22-year-old woman using a melanotan II nasal spray;^[67] and five melanomas in situ in a 2026 patient with tanning-bed, melanotan and anabolic hormone exposure.^[76] None of these establishes causation, and a mouse study found topical melanotan II suppressed melanoma growth, so the carcinogenicity question for melanotan II is open rather than answered.^[77] For bremelanotide it was asked formally, in two species over two years, and answered negatively.^[7] Both compounds turn up in the same black-market seizures: an Italian forensic laboratory characterised melanotan II and bremelanotide together in 2021 as sexual-function and tanning agents "not legally marketed".^[78] The melanotan comparison covers the tanning side.

Where PrymaLab fits

PrymaLab supplies research-grade bremelanotide through the PT-141 research listing and offers identity and purity testing for laboratories verifying the material they hold. It is a laboratory reagent. It is not Vyleesi, it was not made under the controls that produced the label figures above, and this page exists to put the primary record next to the marketing claims so that the two can be told apart. The sexual health peptide archive collects the related references.

One analytical note follows from the chemistry. Bremelanotide and melanotan II differ by 1 Da in mass (1,025.2 against 1,024.2) and in a single functional group, which puts them within reach of each other on a low-resolution instrument and makes the free acid a plausible hydrolysis product of the amide. The 2026 degradation study also identified epimers, which share the exact mass of the parent.^[12] An identity test that distinguishes the two compounds, and the parent from its epimers, needs high-resolution mass spectrometry with chromatographic separation, not a nominal-mass match. That is the standard the PT-141 test is built to.

What this article does not settle

• Whether bremelanotide works in men. The 1,020-patient phase 2B data were never published; the one large published trial has an expression of concern; no trial for a sexual indication in men is registered.^[45][43][49]
• Whether the women's effect is clinically meaningful. The sponsor's responder analyses and the independent re-analyses read the same data differently, and the pooled effect sizes of the 2026 systematic review are behind a paywall.^[21][28][32]
• The mechanism. The label says unknown; the hamster data contradict the dopamine hypothesis; the human fMRI study is one crossover of 31 women.^[7][17][16]
• Whether PT-141 was first identified as a metabolite of melanotan II. The primary sources call it the carboxylate derivative; I found no primary document showing it was isolated from humans given melanotan II.^[5]
• Melanotan II and melanoma. Case reports only, with a contrary mouse study; no controlled data either way.^[74][77]
• Approval outside the US. No EMA record found is not the same as proof no application was made; the Korean regulatory outcome was not found.^[65][19]
• Breast-milk transfer. The 2025 lactation study is complete; its results were not in the label at the time of writing.^[27]

PT-141, bremelanotide, Vyleesi, melanotan: what the names mean

PT-141 is Palatin Technologies' development code and the name the research market uses. Bremelanotide is the international nonproprietary name; bremelanotide acetate is the salt in the approved product. Vyleesi is the US trade name; Rekynda was the trade name AMAG announced in 2017 before launch.^[36] Melanotan II (MT-II, melanotan 2) is the C-terminal amide parent compound, never approved. Melanotan 1 is afamelanotide, a linear alpha-MSH analogue approved as Scenesse for erythropoietic protoporphyria, which is a different molecule and a different story. Alpha-MSH is the natural 13-residue hormone all of these imitate. MC4R is the melanocortin 4 receptor credited with the desire effect; MC1R is the pigmentation receptor. HSDD is hypoactive sexual desire disorder, the approved indication; FSFI-D and FSDS-DAO item 13 are the two questionnaire scores that were the co-primary endpoints; SSEs are satisfying sexual events, the secondary endpoint that did not move. RECONNECT is the name for the two phase 3 trials, Studies 301 and 302. NDA 210557 is the approval; IND 61706 was the men's erectile dysfunction application, withdrawn in 2017.

Frequently asked questions

References