What are the consequences of accepting unknown substances from strange men?

I'm pretty sure this is just a mutagen like all the others you find. There is no indication in the game that this one should be different. I used the mutagen and encountered no noticeable effects later.

You should know that the mutagens are permanent, so never use the standard or lower versions of the mutagens, only the greater ones (if they exist for that kind of mutagen).

I've found out this decision could affect The Witcher 3 in a bit.