Well, some cities have laws against going through someone else's garbage. Most places, though, it's legal, and it's a common investigatory technique for law enforcement if it's legal for anyone to do. If there's a law against it (or if the law does not exempt them), law enforcement would need a search warrant to go through someone's garbage.

Even if there's no "garbage law," dumpster diving in a pharmacy's trash could be considered illegal if it's seen as an attempt to illegally obtain controlled substances.

But, yes, pharmacies throw away expired drugs and bottles of liquids that are not fully empty.

Trampy