MT-2 (Melanotan II)

Developed at the University of Arizona in the 1980s as a sunless tanning peptide. It is the near-twin of bremelanotide (PT-141), differing only at one end group, and was never approved in any country.

Activates several melanocortin receptors (MC1R, MC3R, MC4R, MC5R). MC1R drives skin pigmentation; MC4R (and possibly MC3R) is linked to erections, sexual desire, appetite suppression and yawning. Hitting several receptors explains why tanning, sexual and gastrointestinal effects appear together.

Early University of Arizona studies in small groups of men reported increased pigmentation, spontaneous erections, nausea and yawning. A placebo-controlled crossover in 20 men with erectile dysfunction reported erections without stimulation and increased desire. Development for sexual dysfunction stopped in 2000 in favor of bremelanotide, and cosmetic tanning development was dropped. In mice it suppressed melanoma growth, which conflicts with human case reports; long-term skin-cancer risk and human pharmacokinetics are unknown.

Trials reported nausea, yawning, flushing, spontaneous erections, sleepiness, reduced appetite and darkening of skin and moles. Case reports describe eruptive and atypical moles, melanomas, priapism requiring surgery, muscle breakdown with kidney injury, renal infarction and brain swelling. Forensic testing of illicit product found powder only about 30% pure, and FDA notes immunogenicity risk.

Never approved in the US or any other country; FDA issued a warning letter in 2009, and regulators in the UK, Ireland, Norway and Denmark have warned against it. FDA lists it as 'nominated but withdrawn' from Category 2, and it does not qualify for 503A compounding; it is reportedly scheduled for FDA advisory committee review by early 2027. Not named on the WADA list, but as an unapproved substance it falls under S0 (prohibited at all times).