Personal prescription medicines are allowed almost everywhere, within limits designed to prove personal use. The most common ceiling is up to ninety days' supply for ordinary prescription drugs, with some countries drawing the line at thirty, and shorter caps for anything controlled. Quantities beyond your realistic personal need start looking commercial and invite questions.
The rules are usually stricter than travelers expect in two respects. First, some substances that are routine at home are controlled or banned elsewhere; codeine painkillers, ADHD stimulants, strong sedatives, and certain cold remedies are the classic traps, and a few countries require advance import permits for them. Second, the medicine should travel in original packaging with the pharmacy label matching your passport name, ideally with the prescription or a doctor's letter.
Before flying, check the destination's rules through its embassy or health ministry website, especially for anything psychoactive or injectable. If your treatment needs more than the allowed supply, ask about the import certificate procedure rather than packing extra and hoping.