(Trigger Warning: Not a question about actual history so much as historical technology)
I'm running a tabletop set about 4-500 years in the future, after human civilization destroyed itself through split factions warring for control and genetic modification changing the path of human evolution. Being 300 years in the future, most modern medicine will have been used, destroyed, or gone bad.
The tech level of the world is around the late 1800's where civilization has started back up and some groups can work steel and such while still salvaging old tech and refitting it.
I much prefer realism and hate telling a player things like "You have a first aid kit. Go nuts." I want to know what kind of things you would find in a first aid kit or doctor's bag in a setting like that. What kinds of plants could they find and use as drugs for various things? I know they would have alcohol and such. They also have limited capacity for modern chemistry, as some wealthy individuals might have access to precision gear. So, /r/askhistorians , what kinds of medicine, remedies, and drugs could be encountered and how rare or common would they be?
I'd be happy to provide any further info on the world itself if required, since I wouldn't be asking professionals/enthusiasts if simply googling an answer would be enough to satisfy my requirements.
Also, don't hesitate to tell me I'm stupid and uninformed, because I am.
You might find my reply to an earlier question relevant for your purposes.
Something I'd like to add: aspirin! It was first produced as we might recognize in the later 1800s. In fact, it was likely a significant cause in the decline of medical drug addiction in the early twentieth century. Scrupulous doctors and rational patients gravitated toward the non-addictive analgesic once it hit the market.