Q: Does a finding about one transfer to the other?

No study among the research behind this page gave Melanotan-I and Melanotan-II to the same people, so every side-by-side a reader meets was assembled afterwards rather than measured in one experiment. That matters most where the two look closest, which is the tan. Both have one, produced by different methods on different bodies. The 1996 pilot concludes that these results demonstrate that MT-II has tanning activity in humans given only 5 low doses every other day by subcutaneous injection, which is a statement about three men. The 2000 study concludes that the tanning induced by MT-I in the face and forearm is associated with a significant increase in the eumelanin content of the human skin, which is a statement about seven volunteers. Ten people in total, on two different instruments, is not a basis for putting either molecule above the other. The 2026 systematic review that read 68 peer-reviewed studies does not separate them either.

Question 1

Melanotan-I and Melanotan-II: which molecule is which?

Accepted Answer

The two names belong to two different molecules, and the pair was worked on together from the start. A 1998 review of the field is filed under the title Discovery and development of novel melanogenic drugs. Melanotan-I and -II. That title is the one place in this record where both are handled as one programme. After that they part company. The 1996 pilot introduces Melanotan-II as a cyclic heptapeptide analog of alpha-melanocyte stimulating hormone (alpha-MSH), which in plain words is a ring of seven links modelled on a hormone the body already makes. The tanning study names the other one as [Nle4-D-Phe7]alpha-MSH, also called Melanotan-I (MT-I), which is that same hormone's own sequence with two positions altered. A reader does not need the chemistry: one name denotes a shortened ring, the other the parent hormone with two substitutions, and a result carrying one name does not transfer to the other on its own.

Question 2

What has each one been measured on in people?

Accepted Answer

Three human studies sit behind the two names, and they do not share a single endpoint between them. Melanotan-I owns one of them. Seven normal volunteers (six males and one female) with tanning skin types III or IV (Fitzpatrick scale) were given 10 daily subcutaneous injections of a superpotent synthetic analog of alpha-melanocyte stimulating hormone (alpha-MSH) over two weeks. Fitzpatrick types III and IV mean skin that tans easily, and the endpoint was pigment, read by a light meter and then by a laboratory test on forearm skin. Melanotan-II has the other two, built differently again. The 1996 pilot gave it to three male volunteers on alternating days against saline. In the 2000 study, Melanotan II was administered to 20 men with psychogenic and organic ED using a double-blind placebo-controlled crossover design, ED meaning erectile dysfunction, and stiffness was tracked for six hours by a bedside device. Three studies, three designs, and the one endpoint both molecules share is a tan measured once on each.

Question 3

Why do the two human records not line up?

Accepted Answer

Set the three studies side by side and the gaps are structural: the molecules went to different populations, for different purposes, on different instruments. Melanotan-I's tanning result is laboratory chemistry. One week after MT-I treatments ended, there was a mean (SD) 49% (+/- 17.6%) increase in forehead skin PTCA levels compared to baseline. PTCA is the laboratory stand-in for eumelanin, the brown-black pigment in a tan, and no equivalent assay was ever run on the other molecule. Melanotan-II's reading in the 1996 human pilot is a reflectance meter and a pair of eyes: two subjects had increased pigmentation in the face, upper body and buttock, as measured by quantitative reflectance and by visual perception 1 week after MT-II dosing ended. So one was assayed and the other observed, and the two readings are not the same kind of number. Putting a percentage from a biopsy beside a count of men who appeared darker is arithmetic rather than comparison.

Question 4

Which molecule does the animal work belong to?

Accepted Answer

Every animal experiment among the research behind this page used Melanotan-II. The first molecule appears in none of them. That gap is easy to miss, because the findings later travel under the bare word melanotan. Rats were used to map how an erection is produced. The conclusion was that central and peripheral melanocortin pathways are recruited by melanotan-II, depending on its route of delivery. In plain words, the site of the injection changes which nerve route answers. Mice were used to ask whether it reaches the brain. That work calls it well known for the anorexic effects it elicits in rodents, meaning it blunts appetite in rats and mice. Set against all of that, the Melanotan-I column among the research behind this page is empty. A sentence about what melanotan does in animals is a sentence about one of the two molecules rather than about both.

Question 5

What questions did the animal work actually ask?

Accepted Answer

Not tanning questions, which is the oddest part of the comparison. The animal work on Melanotan-II is mostly brain work, a long way from the reason anybody searches the name. Glycosuria means sugar spilling into the urine, and it is one endpoint that came up. In that study, melanotan II in mutant mice reversed alterations in glucose tolerance and glycosuria. In a mouse model of binge drinking, both NAL and MTII blunt binge-like EtOH drinking, where NAL is naltrexone, a medicine used in alcohol dependence, and EtOH is alcohol. Two more went further afield. A 2023 study read memory and anxiety in zebrafish (Danio rerio) given Melanotan-II on a rich diet. A 2005 rat study looked at appetite and reported that melanotan-II was not correlated with any parameter linked to DIO, which is diet-induced obesity. Line those up and the mismatch is the finding: the human endpoints are skin colour and erection, the animal endpoints sugar, alcohol, memory and appetite.

Question 6

What separates them at the receptor?

Accepted Answer

One descriptive word in the record separates them, and it belongs to Melanotan-II alone. Its own investigators call it a non-selective melanocortin receptor agonist, meaning it switches on more than one receptor in that family rather than only the one that drives pigment. That is where its non-tanning effects begin. A 2007 mouse study traces the appetite effect to the centrally located melanocortin receptors, MC3R and MC4R, both of which sit inside the brain rather than in skin. Even that is not the full account, because in a 2018 mouse study a drop in body heat was preserved in mice lacking any one of melanocortin receptors 1, 3, 4, or 5. Melanotan-I is written up in these same records by strength rather than by reach: it is called a superpotent melanotropin, which says how hard it drives pigment and nothing whatsoever about what else it reaches. The comparison a reader wants is between two selectivity profiles, and only one has ever been set down rather than inferred.

Question 7

Why does so much published work just say melanotan?

Accepted Answer

Because a great deal of the published work does exactly that. The word turns up with no number in review after review, and the finding inside it then cannot be pinned to either molecule. A 2019 review of sudden crops of moles is the clearest. It sorted 179 patients by whatever seemed to have set the moles off, and one of its three buckets reads immunosuppressive agents, chemotherapy or melanotan (41%). Melanotan sits there with two unrelated categories and no numeral after its name. The pattern repeats. A 2022 review of online misinformation writes about Melanotan (an unlicensed and untested form of α-melanocyte-stimulating hormone), again with no number given. A 2025 history calls it the entirely novel invention of MelanoTan injections, as a single object. A 2026 systematic review fuses the pair outright, writing unregulated melanotan I and II use. Two molecules, one subject, one verdict. What those reviews report as harm belongs on the melanotan safety page.

Question 8

What happened to Melanotan-I after the tanning study?

Accepted Answer

This is where the two molecules genuinely stop being equivalent, and where the record is thinner than the version most readers have met. A 2015 German review puts both sides inside one sentence. It records afamelanotide as a prescription medicine for a rare inherited illness in which sunlight causes pain, that illness being erythropoietic protoporphyria. In the same breath the review says that structurally related α-MSH derivatives are available via the internet. What that sentence never does is say which relative is which. It groups them as structurally related rather than naming afamelanotide as Melanotan-I, so the step from one name to the other is one these records stop short of taking. A 2014 tanning review is just as loose, writing about α-melanocyte-stimulating hormone analogues as one undivided family. The 2015 review does pin one finding on the first molecule by name, reporting that Melanotan I leads to the activation of dysplastic nevi, dysplastic nevi being moles whose cells look irregular under a microscope. What is known about that sits on the melanotan safety page.

Question 9

Is PT-141 the same as Melanotan-II?

Accepted Answer

They are two different compounds. The question keeps coming up because one effect measured for Melanotan-II is the effect PT-141 is known for. No source among the research behind this page names PT-141 or bremelanotide. So nothing on this page can set the two molecules beside each other. What these records hold is what Melanotan-II itself did in twenty men. In that trial, in the absence of sexual stimulation, Melanotan II led to penile erection in 17 of 20 men, and increased sexual desire was reported after 13/19 (68%) doses of Melanotan II vs 4/21 (19%) of placebo (P<0.01). The rat work underneath that is specific about route. Central and peripheral melanocortin pathways are recruited by melanotan-II, depending on its route of delivery, so where the needle goes changes which pathway answers. The honest answer is two separate records rather than one. Reading a 2000 Melanotan-II result as a PT-141 result is the same mistake as reading a Melanotan-I tan as a Melanotan-II one.

Question 10

Does a finding about one transfer to the other?

Accepted Answer

No study among the research behind this page gave Melanotan-I and Melanotan-II to the same people, so every side-by-side a reader meets was assembled afterwards rather than measured in one experiment. That matters most where the two look closest, which is the tan. Both have one, produced by different methods on different bodies. The 1996 pilot concludes that these results demonstrate that MT-II has tanning activity in humans given only 5 low doses every other day by subcutaneous injection, which is a statement about three men. The 2000 study concludes that the tanning induced by MT-I in the face and forearm is associated with a significant increase in the eumelanin content of the human skin, which is a statement about seven volunteers. Ten people in total, on two different instruments, is not a basis for putting either molecule above the other. The 2026 systematic review that read 68 peer-reviewed studies does not separate them either.

Question 11

How do melanotan injections compare with other sunless tanning agents?

Accepted Answer

This is the comparison with the most work behind it, and melanotan is its thinnest entry. A 2026 systematic review set melanotan beside DHA, forskolin and carotenoids, DHA being dihydroxyacetone, the browning agent in a bottle of fake tan. The review reports that DHA induces pigmentation via the Maillard reaction, which is the browning chemistry of toast running in the dead outer layer of skin. Nor is it a clean option: concerns persist about DHA-related cytotoxicity, genotoxicity, and systemic absorption, meaning harm to cells, harm to genes, and uptake into the body. A 2026 survey looked at 37 self-tanning items on sale, and all products contained dihydroxyacetone (DHA). Melanotan appears in that survey only as a warning, in a line noting that troxerutin, melanotan, and melanoidins raise safety and regulatory concerns. The difference is one of kind, not degree. A cream colours dead skin. An injection asks living cells to make pigment, and only the second carries a 2025 history saying that tanning injections, moreover, introduced a new host of health risks.

Question 12

Do the two names appear in the same regulatory records?

Accepted Answer

They do, in exactly one file. The entries sit as separate lines rather than as a comparison. The FDA's recall records hold compounded injections under both names, all withdrawn for the same reason. One entry reads Melanotan I 200mcg/mL (2mg/ml), a) 1.5mL-vial, b) 5 mL-vial, Refrigerate, Tailor Made Compounding, and the reason given is Lack of Assurance of Sterility. Two further entries carry the second molecule: Melanotan II, 10mg injection, pulled for Lack of sterility assurance, and a second vial withdrawn over deviations from Current Good Manufacturing Practices (CGMP) that call into question the sterility of products intended to be sterile. Read carefully, those lines compare pharmacies rather than molecules. Each is a judgement about how a compounder filled a vial. The trial register leans one way. Its single live entry is Melanotan II (MT-II) as an Adjunct to NB-UVB Phototherapy for Repigmentation in Stable Nonsegmental Vitiligo, a Phase 2 study listed as recruiting. NB-UVB is narrowband ultraviolet B light, and among the research behind this page nothing comparable is registered under the first name.

Question 13

What would a head-to-head have to do?

Accepted Answer

A genuine comparison of the two is a specific experiment, and describing it shows how far this field is from having run one. It would administer both molecules to the same group of volunteers, in the same weeks, and read a single agreed endpoint on each. Some parts already exist. Both human tanning results were read a week after dosing stopped, so the window is shared: the first was taken one week after MT-I treatments ended, and the second by visual perception 1 week after MT-II dosing ended. The forearm biopsy assay exists too, applied to only one of the two. Everything else is missing. The two studies recruited different volunteers, in different years, and neither was designed with the other in mind. Until somebody runs both arms together, the sentence a reader is hunting for has to be inferred from two unmatched studies rather than read off a measured result.

Question 14

Where does each molecule's harm record sit?

Accepted Answer

Mostly under the second molecule, and the melanotan safety page carries the detail rather than this one. The imbalance is worth naming, because it shapes every comparison a reader finds elsewhere. The published harm record names Melanotan-II far more often than the first molecule. The 2026 systematic review that covers both writes them as one phrase, unregulated melanotan I and II use, instead of as two columns with two totals. The single harm these records pin on the first molecule by name is the mole finding set out above. The supply literature divides the same way. A 2018 analysis of falsified medicines lists image-enhancing polypeptides (e.g., human growth hormone, melanotan II) among the things found in seized material, naming the second molecule and not the first. That is a fact about which name appears in casework rather than a measure of which molecule carries more risk.

Study	Who	Design	What was measured
Melanotan-I tanning study	Seven volunteers (six men, one woman), skin types III or IV	10 daily injections under the skin over two weeks	Pigment, by light meter and a lab test on forearm skin
Melanotan-II, 1996 pilot	Three male volunteers	Alternating days against saline	Tan
Melanotan-II, 2000 study	20 men with erectile dysfunction	Double-blind, placebo-controlled crossover	Erection, tracked for six hours by a bedside device

Melanotan comparisons and stacks