Having visited various dark market sites (Silk Road, Silk Road 2.0, Black Market Reloaded, Sheep Marketplace, Agora.etc), I must say that prescription drugs account/once accounted for a significant part of revenue for many of these dark markets. Most popular drugs are viagra, cialis.etc, but there are a large number of vendors selling drugs such as Daraprim.
The only medication I could find that I have a legitimate prescription for was lorazepam, and it was definitely a lot cheaper. Of course, I didn't buy it, because I have free coverage. I think we need to dump Big Pharma for good, but the obvious obstacle to that is the prohibition of marijuana and psychedelic drugs.