Melanotan I (afamelanotide) is an MC1R-selective agonist focused on skin pigmentation research; Melanotan II activates MC1R, MC3R, MC4R, and MC5R — its broader receptor profile underlies its sexual function and appetite research effects absent in Melanotan I.
Research reference only — all information on this page summarises peer-reviewed scientific literature and does not constitute medical advice. View full compound profiles: Melanotan I · Melanotan II
Mechanism Comparison
Both peptides are synthetic analogues of α-MSH, but with different receptor selectivity profiles. Melanotan I (afamelanotide) is engineered for MC1R selectivity, driving melanocyte stimulation and eumelanin synthesis for photoprotection research. Melanotan II is a cyclic lactam with activity at MC1R, MC3R, MC4R, and MC5R — MC4R agonism mediates the penile erection and appetite-suppressing effects not seen with Melanotan I. This receptor breadth also explains Melanotan II's more complex side-effect profile in research populations.
Side-by-Side Attributes
| Attribute | Melanotan I | Melanotan II |
|---|---|---|
| Receptor selectivity | MC1R-selective | MC1R + MC3R + MC4R + MC5R (broad) |
| Structure | Linear α-MSH analogue ([Nle4,D-Phe7]-α-MSH) | Cyclic lactam analogue |
| Skin pigmentation (MC1R) | Primary effect — significant eumelanin induction | Present (MC1R activity) |
| Sexual function effect (MC4R) | Absent (no MC4R activity at standard doses) | Present — penile erection in rodent and human research |
| Appetite suppression (MC4R) | Absent | Present (MC4R-mediated) |
| FDA status | Approved — Scenesse® (afamelanotide) for EPP | No FDA approval; investigational |
| Half-life | ~1–2 hours (in implant form: 60-day release) | ~30–60 minutes |
| Nausea incidence in research | Low (MC1R selective) | Higher (multi-receptor, including MC4R nausea pathway) |
Key Research Points
- 1Melanotan I (afamelanotide) is FDA-approved as Scenesse® for erythropoietic protoporphyria (EPP) — the only FDA-approved melanocortin agonist. Its development path focused specifically on MC1R-mediated photoprotection.
- 2MC4R agonism is the receptor-level explanation for Melanotan II's sexual function research effects; the MC4R pathway mediates penile erection through downstream NO signalling in corpus cavernosum tissue — an effect not produced by MC1R-selective Melanotan I.
- 3Melanotan II's multi-receptor profile produces a broader side-effect profile in research populations, including nausea (MC4R-mediated) and spontaneous erections in male subjects, which distinguishes its pharmacology clearly from the more focused Melanotan I.
- 4Researchers studying MC1R biology (photoprotection, melanoma-risk signalling, skin pigmentation mechanisms) should use Melanotan I / afamelanotide for specificity; researchers studying MC4R-mediated sexual function or appetite should use Melanotan II or the MC4R-selective agent PT-141.
- 5PT-141 (bremelanotide) is a direct Melanotan II metabolite that was developed specifically for sexual function research and received FDA approval for hypoactive sexual desire disorder in premenopausal women.
Frequently Asked Questions
What is the difference between Melanotan I and Melanotan II?
Melanotan I (afamelanotide) is an MC1R-selective synthetic α-MSH analogue that primarily drives melanocyte stimulation and eumelanin production for photoprotection research; it has minimal MC4R activity. Melanotan II is a cyclic analogue with broad melanocortin receptor activity — MC1R, MC3R, MC4R, and MC5R. MC4R agonism produces sexual function effects (penile erection) and appetite suppression not present with Melanotan I. Melanotan I is FDA-approved (Scenesse® for EPP); Melanotan II is not FDA-approved.
Why does Melanotan II cause erections but Melanotan I does not?
The erection response to Melanotan II is mediated by MC4R (melanocortin 4 receptor) agonism. MC4R is expressed in the hypothalamus and paraventricular nucleus, and its activation drives downstream NO (nitric oxide) signalling in penile tissue via a CNS-mediated pathway. Melanotan I is engineered for MC1R selectivity and has minimal MC4R activity at standard doses — which is why it does not produce the erectile and appetite-suppressing effects observed with Melanotan II.
Is Melanotan I or Melanotan II FDA-approved?
Melanotan I (afamelanotide) received FDA approval in 2019 as Scenesse® (implant formulation) for prevention of phototoxic reactions in adults with erythropoietic protoporphyria (EPP) — it is the only FDA-approved melanocortin receptor agonist. Melanotan II has not received FDA approval and remains an investigational compound. PT-141 (bremelanotide), a metabolite of Melanotan II, received separate FDA approval as Vyleesi® for hypoactive sexual desire disorder.
Deep Dive
For extended mechanism analysis, trial data, and regulatory context, see the full research article:
Melanotan 1 vs Melanotan 2: Melanocortin Research Comparison →Full compound profile
Melanotan I
Full compound profile
Melanotan II