Very grey to put it mildly... the US laws, I believe, with respect to OP affiliates have been affected a fair bit by recent precedent setting cases.
I have no legal expertise, but we had client sites that were in various OP RX affiliate programs, including the infamous MyRXforLess.com site run out of Florida / Mexico by Bill Naron.
OP like casinos can be lucrative, but under even closer legal scrutiny since it directly involves human health.
Yes, probably best to consult an attorney or do some basic research for recent cases against online/offshore pharmacies. Note that it is much harder to directly market prescription meds to US clients, since most of the mainstream PPC players (such as Google AdWords, Microsoft Adsense, Yahoo Overture, etc) no longer accept non-prescription / non-accredited pharmaceutical ads.
Just my non-expert $0.02
Rob