Seen a lot of posts recently about long term users stating that Melanotan is becoming less effective for them. Perhaps there is a large scale supplier issue, but it appears that the MC1R receptor can become de-sensitized to Alpha MSH/Melanotan. MC1R is the main receptor that Melanotan binds to that causes tanning and according to a Clinuvel patent this receptor can become de-sensitized. So taking a break should make everything go back to normal, similar to any drug tolerance such as alcohol.
"administering to the subject an alpha-MSH analogue in an effective amount and time to induce melanogenesis by the melanocytes in epidermal tissue of the subject without inducing homologous desensitization of the melanocortin-1 receptors of the subject"
http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=9345911.PN.&OS=PN/9345911RS=PN/9345911