Which one is approved?

Melanotan I — afamelanotide, marketed as Scenesse, approved for erythropoietic protoporphyria.

Why does searching 'melanotan' find the wrong one?

The bare name resolves to afamelanotide's record. Melanotan II needs the numeral, or CID 92432 directly.

MT-2 (Melanotan II)

Melanotan I and Melanotan II Are Not the Same Compound

JMWritten & reviewed by Jack Muncaster · Founder, UK PeptidesLast reviewed 2026-08-234 cited sources

They are different molecules with different structures and different regulatory status. Melanotan I is afamelanotide — linear, thirteen residues, 1,646.8 Da, marketed as Scenesse for erythropoietic protoporphyria. Melanotan II is cyclic, seven residues, 1024.2 Da, and holds no authorisation anywhere.

Key facts

Melanotan I: Afamelanotide, CID 16197727
MT-I mass: 1,646.8 Da, C78H111N21O19
MT-I structure: Linear, 13 residues
MT-I status: Approved — Scenesse, for EPP
Melanotan II: CID 92432
MT-II mass: 1024.2 Da, C50H69N15O9
MT-II structure: Cyclic, 7 residues
MT-II status: Approved nowhere

The numbering is misleading

Melanotan I and II sound like successive versions of one thing. They are not. They differ by 622 Da, by six residues, and by whether the peptide is a ring or a chain. Nothing about the naming conveys that these are structurally distinct compounds developed along different lines.

What Melanotan I actually is

Afamelanotide, also known as NDP-MSH — a linear thirteen-residue analogue that keeps the parent hormone's length while substituting residues for stability. It is marketed as Scenesse and is approved for erythropoietic protoporphyria, a condition causing severe phototoxicity. Wensink and colleagues reviewed its use in 2021; Kim and colleagues reviewed it in the American Journal of Clinical Dermatology in 2016.

Research material referenced

MT-2 10mg — third-party HPLC tested

View — £19.99

What Melanotan II is

A cyclic heptapeptide — the receptor-binding core compressed into a ring closed by a lactam bridge. Substantially smaller, structurally quite different, and never taken through regulatory assessment anywhere.

The database trap

Searching PubChem for melanotan returns CID 16197727 — afamelanotide, the approved one. Melanotan II requires searching the full name with the numeral. This is the most consequential identifier collision found on this site, because it points from an unlicensed compound toward a licensed one and so invites exactly the wrong inference.

Why the difference in status is not arbitrary

Afamelanotide was developed for a specific serious condition, assessed by regulators, and approved with an indication, a label and a monitoring context. Melanotan II went through none of that. The gap between them is a full regulatory process, and it does not narrow because the names look sequential.

The inference to avoid

That afamelanotide's approval says anything about Melanotan II. It does not — different molecule, different structure, different evidence, different assessment. This is the same error pattern as reading mecasermin's approval as covering IGF-1 LR3, and it is even easier to fall into here because the names are numbered.

Quick reference

	Melanotan I	Melanotan II
Also called	Afamelanotide, NDP-MSH	MT-2
Structure	Linear, 13 residues	Cyclic, 7 residues
Mass	1,646.8 Da	1024.2 Da
PubChem CID	16197727	92432
Status	Approved (Scenesse)	None

Extended research context

The MT-2 (Melanotan II) deep dive

Deep dive: the identifier collision that points the wrong way

This site has now found several plausible-looking identifiers that resolve to the wrong molecule. A PubChem record for tirzepatide cited as retatrutide. Thymosin beta-4's CAS quoted as TB-500's. An unrelated organic acid returned for KPV. This one is the most consequential of the set, and the reason is direction. Searching PubChem for melanotan returns CID 16197727 - afamelanotide, at 1,646.8 Da for C78H111N21O19. That is Melanotan I, a linear thirteen-residue peptide, and it is an APPROVED MEDICINE, marketed as Scenesse for erythropoietic protoporphyria. Melanotan II is CID 92432: cyclic, seven residues, 1024.2 Da, 622 Da lighter, and assessed by no regulator anywhere. So the wrong record does not merely mislead about mass. It resolves from an unlicensed compound toward a licensed one, and the sequential naming - I and II - makes them look like versions of a single product rather than the structurally distinct compounds they are. Anyone verifying a certificate by searching the bare name lands on an approved drug's record and may never notice which compound it names.

Deep dive: four stability features in seven residues

PubChem's IUPAC name for CID 92432 is unusually complete and reads as a full specification: N-acetyl-L-norleucyl-L-alpha-aspartyl-L-histidyl-D-phenylalanyl-L-arginyl-L-tryptophyl-L-lysinamide, cyclic (2-7)-peptide. Unpack it and there are four independent protease-resistance features in a molecule of seven residues. A lactam bridge joins the aspartate side chain at position 2 to the lysine side chain at position 7, closing the ring - which both locks conformation for receptor engagement and eliminates the free termini exopeptidases require. Position 4 is D-phenylalanine, the mirror image of the natural form, and proteases are stereospecific enough that they largely cannot process it. The N-terminus is acetylated and the C-terminus amidated, capping both ends. And position 1 is norleucine, which is not among the twenty proteinogenic amino acids at all - so even setting aside the cyclisation and the D residue, this peptide could never have been a gene product. It is a fully synthetic construction rather than a modified natural sequence, and it is among the most chemically robust compounds in this catalogue as a direct result.

Deep dive: why the safety literature is included rather than omitted

There is a published dermatological literature here, and the editorial choice was to state it. Reid and colleagues documented atypical melanocytic naevi following melanotan injection in the Irish Medical Journal in 2013. Eijmael and colleagues published a risk review in the Nederlands Tijdschrift voor Geneeskunde in 2022 - a national medical journal considering the topic worth addressing directly. Case reports have a real limitation: they establish an observation without a denominator, so they cannot establish causation or incidence, which is the same constraint that applies to pharmacovigilance disproportionality signals. But that limitation cuts both ways. A case report is not proof of harm and it is not dismissible either. What makes the absence of better data significant here is that no regulator has assessed this compound - so there is no label, no contraindications, no adverse event reporting requirement and no monitoring framework. The information that would normally exist for an injectable product simply does not, and what is available instead is what individual clinicians chose to publish after seeing something. Presenting the compound without that context would give an incomplete picture of what is actually known.

Research applications

▸Melanocortin receptor pharmacology and selectivity research
▸Cyclic peptide and lactam bridge design studies
▸D-amino acid substitution and protease resistance research
▸Structure-activity work on constrained peptide analogues
▸Comparative work on alpha-MSH derivatives
▸Analytical method development for cyclic peptides

Handling checklist

✓Verify against CID 92432, 1024.2 Da, C50H69N15O9, CAS 121062-08-6
✓Never verify by searching 'melanotan' alone - that returns afamelanotide at 1,646.8 Da
✓Require stereochemistry on the certificate - D-Phe4 is not detectable by mass
✓Protect from light; tryptophan at position 6 is photochemically reactive
✓Store lyophilised, cold, dry and dark
✓Expect no disulfide or oxidation satellites - no cysteine, no methionine
✓Quantification at 280 nm is available thanks to the tryptophan

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Searching PubChem for 'melanotan' to verify identity

Fix: That returns CID 16197727, afamelanotide, at 1,646.8 Da - an approved medicine and a different molecule. Use CID 92432 or search Melanotan II with the numeral.

✗ Treating Melanotan I and II as versions of one compound

Fix: They differ by 622 Da, by six residues, and by whether the peptide is cyclic or linear. Only Melanotan I has been assessed by regulators.

✗ Reading afamelanotide's approval as covering MT-2

Fix: Different molecule, different structure, different evidence, different assessment. Nothing transfers.

✗ Accepting a sequence written without stereochemistry

Fix: L- and D-phenylalanine have identical mass. A peptide with the L form at position 4 is a different compound that mass spectrometry cannot distinguish.

✗ Assuming a non-selective agonist affects only its intended receptor

Fix: Five melanocortin receptors share binding surfaces across different tissues. Engagement elsewhere is the same pharmacology in another place, not a side reaction.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Frequently asked questions

Are they versions of the same compound?: No. They differ by 622 Da, by six residues, and by whether the peptide is cyclic or linear.
Which one is approved?: Melanotan I — afamelanotide, marketed as Scenesse, approved for erythropoietic protoporphyria.
Why does searching 'melanotan' find the wrong one?: The bare name resolves to afamelanotide's record. Melanotan II needs the numeral, or CID 92432 directly.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

Written and reviewed by

Jack Muncaster · Founder, UK Peptides

Jack founded UK Peptides in Manchester after repeatedly receiving research compounds with missing or recycled paperwork. He is responsible for supplier selection, batch release decisions and the content published in this research library. Every article here is sourced to primary literature and every product page to a signed third-party certificate.

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