You are allowed to buy a quantity of prescription drugs which would be considered for personal use. If you buy a drug be aware of what it is and why you are taking it. It's not their job necessarily but I have heard that border patrol will quiz people at times because people will buy drugs for others back in the USA and that is not legal. I have to comment on this. yes it is illegal to cross back into the states with drug that need a prescription. we were there on year and asked the purple drug store about it and they asked how much we wanted. we told him and he took a label for a legal drug ,put in onthe bottles and put the illegal drug in there and sealed it up. he did this right in front of us. and yes we made through customs. talk about sweating blood that day.