Melanotan I vs Melanotan II: Receptor Selectivity, Clinical History, and Research Comparison (2026)

# Melanotan I vs Melanotan II: Receptor Selectivity, Clinical History, and Research Comparison (2026)

> For research use only. Melanotan I (afamelanotide) and Melanotan II are sold on this platform as laboratory reagents for in vitro and preclinical work. The only licensed form of either compound is the afamelanotide implant approved for erythropoietic protoporphyria; research-grade vials are not that product. Melanotan II is not licensed as a medicine anywhere, and regulators in the UK, EU, and Australia have acted against its sale for tanning. Nothing on this page describes how to administer either compound.

Melanotan I (MT-1) and Melanotan II (MT-2) came out of the same University of Arizona programme in the 1980s, and both are engineered versions of alpha-melanocyte-stimulating hormone (alpha-MSH). One became a licensed orphan drug. The other became a grey-market tanning injectable with a case-report literature of its own. The difference between them is a single design decision: MT-1 keeps the full 13-residue linear chain, while MT-2 is cut to seven residues and closed into a ring. That ring is what gives MT-2 its activity at MC3R and MC4R, and with it the erections, nausea, and appetite effects that MT-1 lacks.

Quick comparison

The melanocortin receptor family

Alpha-MSH is a 13-amino-acid fragment of pro-opiomelanocortin (POMC). It activates four of the five melanocortin receptors; MC2R responds only to ACTH.

Native alpha-MSH is degraded within minutes in plasma. Both Melanotans carry the same two stabilising substitutions: norleucine at position 4 in place of the oxidation-prone methionine, and D-phenylalanine at position 7, which raises receptor affinity and resists proteolysis. Hadley and Dorr, who led the Arizona programme, describe the design goal as "superpotent, prolonged acting, enzymatically resistant" analogues that could be used to map what each receptor does (PMID 16412534).

Melanotan I (afamelanotide)

Structure and selectivity

MT-1 is [Nle4, D-Phe7]-alpha-MSH: the full-length peptide with the two substitutions and nothing else changed. Because it retains the native backbone, it keeps alpha-MSH's preference for MC1R. Its pigmentation effect is MC1R-driven eumelanin synthesis, and central MC4R-type effects are not part of its clinical profile.

From tanning candidate to orphan drug

Arizona tested MT-1 clinically for skin tanning in the 1990s (PMID 16412534). Clinuvel later developed it as a 16 mg subcutaneous implant for erythropoietic protoporphyria (EPP), a genetic photodermatosis in which sunlight causes severe pain. Two randomised, double-blind, placebo-controlled trials published in the New England Journal of Medicine in 2015 (74 EU patients over 180 days, 94 US patients over 270 days) found longer pain-free sun exposure with afamelanotide than placebo in both trials, fewer phototoxic reactions in the EU trial (77 versus 146), improved quality of life, and mostly mild adverse events (PMID 26132941). The EMA approved the implant in 2014 and the FDA in October 2019. Other dermatological uses, including vitiligo and solar urticaria, remain investigational.

Stability

A 2026 forced-degradation study of afamelanotide identified fourteen degradation products under acid, base, oxidative, UV, and 60 °C stress, formed by truncation, methylation, deacetylation, and oxidation (PMID 41547183). The norleucine substitution removes the methionine oxidation site, but the peptide still degrades on every stress axis tested. For a laboratory this means the storage conditions on a research vial are not a formality; light exposure and warm storage both produce measurable products.

Melanotan II

Structure

MT-2 keeps only the core message sequence of alpha-MSH (residues 4 to 10) and closes it with a lactam bridge between the side chains of Asp5 and Lys10. Dorr and colleagues give the structure as Ac-Nle4-Asp5-His6-D-Phe7-Arg8-Trp9-Lys10-NH2, cyclised, and describe it as "superpotent" in melanotropic assays (PMID 8637402). The ring fixes the peptide in a conformation that binds MC3R, MC4R, and MC5R well, which is why MT-2 is used as a pan-melanocortin agonist in receptor pharmacology.

What the human studies found

The 1996 pilot phase I study in three healthy men is the clearest description of MT-2's effect profile. Over two weeks of alternating saline and MT-2 injections, the authors recorded mild nausea at most dose levels, somnolence and fatigue in one subject at the highest level, a stretching-and-yawning complex that correlated with the onset of spontaneous penile erections lasting one to five hours, and increased pigmentation of the face, upper body, and buttocks in two of three subjects one week after dosing ended (PMID 8637402).

Wessells and colleagues then ran a double-blind, placebo-controlled crossover in ten men with psychogenic erectile dysfunction. Eight of ten developed clinically apparent erections after MT-2; mean duration of tip rigidity above 80% was 38 minutes with MT-2 versus 3 minutes with placebo (p = 0.0045). Nausea, stretching and yawning, and decreased appetite were more frequent than on placebo but did not require treatment (PMID 9679884). This work led to bremelanotide (PT-141), an MT-2 derivative that Palatin took through phase III and that the FDA approved in 2019 as Vyleesi for hypoactive sexual desire disorder in premenopausal women.

The grey market and its case reports

MT-2 never completed development as a medicine, and by the late 2000s it was being sold online as a tanning injectable. Breindahl and colleagues bought MT-2 vials from three online shops and analysed them by LC-UV-MS/MS. Vials from two shops contained unknown impurities of 4.1 to 5.9%, and the MT-2 content ranged from 4.32 to 8.84 mg although every shop claimed 10 mg (PMID 24771717). A Chicago poison-centre report described a man who injected internet-purchased MT-2 (confirmed by mass spectrometry against a reference standard) and developed sympathomimetic symptoms, rhabdomyolysis with creatine kinase peaking at 17,773 IU/L, and acute kidney injury, requiring three days in intensive care (PMID 23121206). Both papers are about consumer injection of an unlicensed product; they are cited here because they are the only independent analyses of what a commercial MT-2 vial contains.

Head-to-head

Receptor pharmacology

MT-1's usefulness is that it isolates MC1R. MT-2's usefulness is that it does not. In a melanocyte or melanoma cell line, MT-1 reports on MC1R signalling with little else in play. In a hypothalamic slice, a feeding study, or a spinal-cord preparation, MT-2 is the standard non-selective agonist, usually paired with SHU9119 or another MC3R/MC4R antagonist to assign the effect to a receptor.

Central effects

Every human study of MT-2 recorded nausea, yawning and stretching, and erections; these are MC4R effects and they appear at the same doses as pigmentation (PMID 8637402, PMID 9679884). The afamelanotide EPP trials, by contrast, reported mostly mild adverse events and no serious events attributed to the drug (PMID 26132941). For any experiment where a central melanocortin effect would confound the readout, MT-1 is the cleaner tool.

Regulatory status

Afamelanotide is a licensed prescription medicine in the EU and US as the EPP implant. The research-grade MT-1 sold on this platform is not that product and carries no licence. MT-2 has no marketing authorisation anywhere; the UK MHRA has issued warnings about its sale as an unlicensed medicine, and Australia schedules it as prescription-only. Our UK and Australia country guides cover the local position for research purchasers.

Which to choose

Supplier data: what the platform shows (September 2026)

The figures below are computed from the peptides.so listings table on 20 September 2026, restricted to product rows whose name pins a 10 mg vial and to list prices of at least $5. They are list prices in USD before shipping.

The two compounds cost about the same per vial at the median, so the old assumption that MT-2's shorter sequence makes it cheaper does not hold in the current market; MT-2's upper quartile is higher because more of its listings are multi-vial packs stored at pack price under the same product row. MT-1 has more suppliers (51 versus 36) and more of its listings are in stock, which is the reverse of the "MT-2 is easier to find" picture from a few years ago. Both product pages (Melanotan I 10 mg, Melanotan II 10 mg) list every current offer; use the compare tool to sort by price.

Per-milligram figures are only meaningful when the vial size is pinned; a $45 10 mg vial is $4.50/mg, and the four MT-1 and seven MT-2 listings above $100 are packs rather than expensive vials.

What a certificate of analysis should show

Breindahl's finding that labelled 10 mg vials held 4.3 to 8.8 mg is the reason to ask for a lot-specific certificate before ordering either compound. For MT-1 the certificate should report an HPLC purity and a mass-spectrometry identity consistent with ~1,647 Da; for MT-2, ~1,024 Da, with the cyclic structure confirmed (a linear precursor with the same residues would have a different mass). HPLC alone reports purity, not identity. See how to read a peptide COA and HPLC versus mass spectrometry for purity testing.

Frequently asked questions

What is the structural difference between Melanotan I and Melanotan II?

MT-1 is the full 13-residue alpha-MSH chain with norleucine at position 4 and D-phenylalanine at position 7. MT-2 keeps only residues 4 to 10 and closes them into a ring with a lactam bridge between Asp5 and Lys10. The ring is what broadens MT-2's receptor activity to MC3R and MC4R.

Why did MT-1 become a medicine and MT-2 did not?

MT-1's MC1R selectivity meant its clinical programme in EPP did not have to manage erections, nausea, and appetite suppression. MT-2's phase I and erectile-dysfunction studies documented those effects at every dose tested; the MC4R activity was later redirected into bremelanotide, which is the licensed descendant of MT-2.

Is Melanotan II banned?

It has no marketing authorisation anywhere. In the UK it is treated as an unlicensed medicine and the MHRA has warned against its sale; in Australia it is a prescription-only scheduled substance. Research-grade supply for laboratory use is a separate question governed by each jurisdiction's research-chemical rules; check the relevant country guide.

Which has more suppliers?

On peptides.so in September 2026, 51 suppliers list a 10 mg MT-1 vial and 36 list a 10 mg MT-2 vial. Median list prices are within a dollar of each other.

Can the two be used in the same experiment?

Yes; that is a standard design for separating MC1R-dependent from MC3R/MC4R-dependent effects. MT-1 serves as the MC1R-biased agonist, MT-2 as the pan-agonist, and an MC3R/MC4R antagonist such as SHU9119 assigns the difference.

How should either compound be stored?

Both are supplied lyophilised. The afamelanotide forced-degradation study found products forming under light, heat, oxidative, acidic, and basic stress, so lyophilised material should be kept cold, dark, and dry, and reconstituted solutions should be used promptly rather than held.

Related reading

• •Melanotan II research profile
• •PT-141 (bremelanotide) research profile
• •How to read a peptide certificate of analysis
• •Research peptides in the UK 2026
• •Research peptides in Australia 2026

References

1. Hadley ME, Dorr RT. Melanocortin peptide therapeutics: historical milestones, clinical studies and commercialization. Peptides. 2006;27(4):921-930. PMID 16412534

2. Dorr RT, Lines R, Levine N, et al. Evaluation of melanotan-II, a superpotent cyclic melanotropic peptide in a pilot phase-I clinical study. Life Sci. 1996;58(20):1777-1784. PMID 8637402

3. Wessells H, Fuciarelli K, Hansen J, et al. Synthetic melanotropic peptide initiates erections in men with psychogenic erectile dysfunction: double-blind, placebo controlled crossover study. J Urol. 1998;160(2):389-393. PMID 9679884

4. Langendonk JG, Balwani M, Anderson KE, et al. Afamelanotide for Erythropoietic Protoporphyria. N Engl J Med. 2015;373(1):48-59. PMID 26132941

5. Breindahl T, Evans-Brown M, Hindersson P, et al. Identification and characterization by LC-UV-MS/MS of melanotan II skin-tanning products sold illegally on the Internet. Drug Test Anal. 2015;7(2):164-172. PMID 24771717

6. Nelson ME, Bryant SM, Aks SE. Melanotan II injection resulting in systemic toxicity and rhabdomyolysis. Clin Toxicol (Phila). 2012;50(10):1169-1173. PMID 23121206

7. Chawathe A, Sharma N. Investigation of the stability profile of therapeutic α-MSH analogue: Insights from liquid chromatography-high resolution mass spectrometry analysis of afamelanotide. J Pharm Biomed Anal. 2026;272:117362. PMID 41547183

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For research use only. Melanotan I and Melanotan II are laboratory reagents for in vitro and preclinical investigation. Neither research-grade compound is approved for cosmetic tanning, self-administration, or any human or veterinary use. The clinical studies summarised above are cited as published literature, not as guidance.