What are you using it for?

As a First Aid instructor, I don't have the authority that trusbx or beachdoc have, but my personal belief is that a FAK and a PSK should be two separate items. You may need both in a survival situation, but they serve two separate purposes.

I carry Tylenol (regular or extra-strength) in my own FAK, but it's for my own personal use - Saint John Ambulance protocols prohibit me from giving any medication to patients.

If you are taking it for your own personal use, then my advice would be to take medication that you will personally use. If you have a prescription, take your prescription pills or potions.

Never share prescription medication with anyone else. (The exception is if you are in contact with a doctor, can describe the patient's signs and symptoms accurately enough for him/her to make a diagnosis, and get verbal authorisation to administer the meds.)

For giving to others, I would use great caution. Your fellow passenger may be in great pain, but if you give them codeine and it causes their death, you may be criminally liable.

If it's an OTC drug (aspirin, acetaminophen, sugar cubes) and they can take it themselves, then I personally don't see anything wrong with leaving the bottle where they can reach it. If it's anything stronger, I'd check with a doctor first.

I, personally, cannot envision a situation where I would have to give prescription medication to a fellow traveller without being able to obtain a doctor's authorisation, either because it's his own prescription, or by radio/telephone.