In S01E21 – Symbiosis, we are introduced to a planet which specializes completely in the manufacture of a single drug, all of which is exported to a neighbouring planet. The relevant part is about 6 minutes into this video:
The lesson continues in the following video. There is then a discussion of liberal tolerance. Dr Crusher finds the relationship distasteful and exploitative. Captain Picard reminds her that it’s not the Federation’s place to impose human values on other societies.
Unfortunately, the rest of the episode is a Saved by the Bell-quality sermon about the dangers of addictive drugs, ending with Picard cynically going against the spirit, if not the letter, of the Prime Directive in order to rid the planet of the scourge of narcotics.