I just came to recheck this thread because I thought I hadn't read any reports of such and wondered if I was the only one. I've been applying Melanon to moles and spots for about a week now and noticed itchiness only on some of the places I've applied it. My best guess was that it's akin to the itch I get on wounds as they are healing and that the spots where it didn't itch are relatively damage free. The spots that itched the most have been itching less and perhaps that supports the idea.
It's DMSO that causes the itchiness but like others mentioned adding a bit of water or even more ethanol/vodka to dilute it should eliminate that issue. Allergic reactions tend to come with a rash so unless you had that it is probably just the DMSO.