What are the rules about bringing prescription drugs to Turkey? Do you need a letter from your doctor?
Not for all medications. For most, just having the label on the bottle or a copy of the prescription is sufficient. But if you take Pain meds, meds for anxiety or depression, ADHD, or sleeping pills (among others) then do some searching on the web about what might be illegal or highly restricted in Turkey. For those, you at least need a letter from your Doctor, at worst you may not be able to travel with them.
Also, take only what you need, plus a bit of a buffer, so if going for a couple weeks, don't take a 90 day supply. Chances are, no one will ask or check, but be prepared.
This is what the Turkish Consulate in LA says about that topic:
https://losangeles-cg.mfa.gov.tr/Mission/ShowInfoNote/410626
If you have a prescription for a controlled substance, you may have bigger challenges. You should contact the Turkish Consulate before you leave to ensure what you want to bring is allowed in Turkey.