I first used Melanotan II about 2.5 years ago. Nothing in excess, just 0.5mg per week in two divided doses. I know a lot of people that have used 5 - 10 times that dose. After a couple weeks I noticed I could not get an erection at all, and I had absolutely zero libido, or desire to even have sex. I was 21 at the time and went from having to masturbate at least once a day to never needing too at all.
I did have a sustained period of abstinence from use, for about 6 months. My libido never returned. I have been back using it at a very low dose of about 0.1-0.2 mg per week for around the last 12 months, with some breaks here and there.
Apparently, it has some control over dopamine binding in certain areas of the brain linked to reward. Maybe I downregulated dopamine receptors in these pathways?
http://www.ncbi.nlm....pubmed/11846623
Additionally seeing as it has known aphrodisiac qualities as a result of its Melanocortin-4 receptor agonism, maybe I down regulated this receptor as well
http://www.ncbi.nlm....pubmed/12172010
I thought it was a no brainer since my problems with libido coincided with my Melanotan use, however at the low doses I used, I would have thought sustained abstinence should have seen my libido return?