There’s no medical evidence on that question, but Melanotan II has been used by very many people for many years now.
Melanotan II works by stimulating the alpha-melanocyte receptor, which promotes formation of melanin in response to sun exposure. When a substantial amount of MT-II has been taken within recent “memory” of the skin cells, an individual tans as if he were a genetically darker type.
Some find MT-II unsuitable for them because of development of moles, or because of uneven skin darkening. But for most it works very well with no problems, including when used long term.
A possible additional side effect is greater tendency to developing an erection, typically beginning several hours after taking the drug. Depending on the individual, dosing may need to be at least 1 mg for this to occur. I’d recommend against exceeding 2 mg at a time. For those specifically seeking this effect, bremalanotide (PT-141) would be another option. It’s reported to share MT-II’s pro-erectile effect, but with no effect on tanning.
Melanotan II has an initial loading phase, and – if desired – a maintenance phase after that. Or, its effect can be allowed to slowly wear off over a period of months, and loading can be repeated.
Usually, 10-30 mg total use is required during the loading phase, with paler individuals requiring more than relatively darker individuals. It’s best to start with very low dosage, such as only 0.25 mg per day, and then work up if desired. Many can tolerate 2 mg/day, but it’s possible that risk increases with such use, and it usually should not be necessary to load up that fast. Even 0.5 mg/day will allow loading with 30 mg in only two months, or 1.0 mg/day will allow that much loading in just 30 days.
To maintain the same tanning ability, ongoing per-year usage after this needs to be about 2-3 times the loading dosage, as an estimate. So for example if you needed 30 mg of loading to get the result you want, to maintain it you might need about 60-90 mg total Melanotan II over the course of the year. You’d already have injected 30 mg of that, so there would be 30-60 mg (typically three to six vials) remaining to inject over the next 11 months.
You could be very precise and figure this as 2.72 – 5.45 mg/month or 0.09 – 0.18 mg/day. But there’s no need to do that. Instead, it would be fine to figure it as a vial per 2-4 months. Once starting on a vial, it could be injected at an amount such as 0.5 mg/day until the vial is finished.
This is easier than having to inject it at all the time, and gives equivalent results.
The frequency of starting maintenance vials can be varied according to personal judgment of effect. There’s no problem with underestimating, or even with going a year or more without maintenance, because it’s always easy to catch back up.
Of course, sun is still required to get a tan. MT-II facilitates tanning, but does not directly cause it.