It seems to me that this could be equally profitable compared to street drugs if you take the inflated price for common prescription medicine and health care in the US into account. If you source prescription medicine from let’s say Canada or Mexico to undercut the official street prices in the US your risk of getting cought or run into trouble with law enforcement or rival gangs seems much lower. Also neighbors would not call the police on a guy that sources cheap insulin for the old guy living next door, or what am I missing?
The profit margins are low because it’s easy and cheap. There are a lot of places already doing this and they have real (mostly international) businesses dealing with medication from real pharmaceutical companies. Ask a trans person or a serious bodybuilder for recommendations.