But the mechanism isn't what most assume
Choosing one over another usually depends on the objective under investigation and individual tolerance, rather than a universal best option

Common reasons Not getting any UV exposureMT-2 produces melanin but you need some UV to 'activate' it and see the tan Degraded peptide from improper storagecheck if solution is clear and properly refrigerated Dose too lowsome people need the full 500mcg to see results Unrealistic expectationsMT-2 won't make you darker than your genetic potential allows Not enough timefull results take 2-4 weeks of consistent loading Poor quality peptide from unreliable sourcealways verify with third-party testing (COA) Key Research [1]"Evaluation of melanotan-II, a superpotent cyclic melanotropic peptide, in a pilot phase-I clinical study" Dorr RT, Lines R, Levine N, Brooks C, Xiang L, Hruby VJ, Hadley ME, 1996 Finding: Pilot phase-I human study: subcutaneous melanotan-II induced measurable skin tanning in healthy male volunteers after only 5 low doses (every other day), with pigmentation increasing in the face, upper body and buttocks

Sultan Qaboos Univ Med J 15:e440e441 Savoia F, Ricci L, Patrizi A, Gaddoni G (2015) Congenital melanotic macules of the tongue
Timing: Inject at a consistent time each day usually before bed, fasted and rotate injection sites systematically