I know that Medical marijuana is schedule I in the US, and that it is illegal federally, but what about schedule II-IV drugs? Do other countries have rules about which substances are controlled? At the moment, I will be flying from the US to Canada and Mexico, but in the future, I may be going to Europe. I don't take narcotics at the moment, but I do take Clonazepam for psychiatric management.
You need to learn the status of the medication in your destination. This is harder than you might think. A decongestant that is available over the counter in Canada is banned in New Zealand and since it's not by prescription, I had to get a special letter from my doctor (NZ customs helped with the wording) explaining why I had it with me.
If the medication is not controlled in the destination, you can just bring it. If the landing card asks about medications, declare it even though it's not controlled. When they ask what you have, you can say "Tylenol" or whatever and they will be happy. If it's by prescription only, again declare it, bring it in the prescription bottle with the label showing your name and if you can, bring a receipt and any other paperwork to prove this has been prescribed to you. If it's banned in the country, see if there are doctor-letter exemptions and arrange for the letter in plenty of time.
I have crossed a LOT of borders with prescription meds and other than looking at the label, there has not been much delay or any problem. I don't know if they are trained to know what various pills actually look like, but nothing of mine has ever had more than a visual inspection. One reason for that may be that I don't bring thousands of pills, just the amount I might need for the length of the trip.
In the particular case of marijuana, it is still illegal in Canada. Do not bring any. Best case is that you will be refused admission. Worse cases include being arrested for smuggling. I have seen that so many times on Border Security. And not just for the smokable leafy form. Chocolates, tea, you name it, if they find it, they react as they would to cocaine or any other street drug. And they generally find it (the smell is easy to detect.)
In the U.K. Clonazepam is Class C, possession can get you 2 years in prison, an unlimited fine, or both. Guidance on how to travel with it legally while visiting the U.K. is here https://www.gov.uk/travelling-controlled-drugs In essence, you need to prove it’s prescribed to you and you may need to get a licence.