Portugal has decriminalised all opiates but doesn't sell any OTC. Getting a script requires a certain type of doctor but they tend to be quite open-handed once it's clear you aren't down spiralling. Fentanyl patches kick it old school for easy drug extraction. Benzos not OTC but need a script which all doctors can give. No special status doctor required.
France has OTC codeine as does the UK, Ireland, Poland. Mixed with ibuprofen or paracetamol (acetaminophen) usually. Benzos not OTC anywhere in Europe or USA that I've found but easily scripted if you don't mind paying a private doc and smiling like a civilised human being.
Mexico: tramadol and codeine is OTC but many pharmacies have bottom-feeder doctors attached who'll script anything for a $5 consult including opiates and bentos.
Australia had no OTC opiates or benzos with doctors who were far too contentious imho. Japan has no OTC opiates or bentos. India last time I was there was similarly lax as Mexico. Brazil similar to Portugal.
You can transport virtually any personal supply across borders with an appropriate verifiable doctor's letter / script copy. Worst case if you're transporting personal supply without any backup, it's going to get trashed (and you'll be left with an apology but no meds). Common sense should prevail here. Keep the meds on your person and not packed away - hidden in plain sight - imho, if it's pharmaceuticals and plausibly own-use supply only.
Apologies if all this info is well known already. Just joined this site so figured I should try to contribute. Be seeing you..!