It might not be medication related. It could be Acanthosis Nigricans, a skin condition associated with insulin resistance, overweight, PCOS, Type 2 diabetes. (You needn't suffer from all of those conditions in order to develop it. You can find a lot of information about it through Google.

Here are a few of the links I thought had decent information. (The third link has some rather graphic photos, so anyone who is sensitive to such things may want to skip over it.)

http://www.aocd.org/skin/dermatolog...is_nigrica.html

http://www.mayoclinic.com/health/ac...DSECTION=causes

http://dermatology.cdlib.org/DOJvol...hosis/katz.html

http://www.webmd.com/skin-problems-...ricans-overview