I was wondering would herb lore (making drugs from gathered plant extracts) come under the heal or the knowledge nature skill?
Depends on the properties of the drug/herb. Finding the herbs themselves would require a Knowledge (Nature) check that has a DC depending upon the herb type and its rarity.
Now, planning to make something out of said herbs into drugs would technically require a Craft (Alchemy) check to create said drugs. Treat harmful drugs like poisons, and treat helpful drugs like healing potions or bandages or ointments or what have you.
You'd have to come up with some way to translate time spent gathering herbs into raw materials for the Craft (Alchemy) skill. I'd consider using the mechanics for Profession, so you make money by spending time doing Profession (Herbalist) checks wandering around in the woods gathering materials, and then use those raw materials in a Craft (Alchemy) check to generate a final product with a mechanical benefit.
That's probably the most rules-accurate way to do it; I'd probably consider just using Knowledge (Nature) or Survival instead of Profession (Herbalist), but the Profession skill is the skill used to directly produce money by spending time - Craft skills are supposed to produce tangible objects.