We use an enhanced drivers lic. It's true that you dont need anything to GO to Mexico, it's getting back into the USA that's the trick. I have only crossed at Algodones and we just walked in, no questions asked, no guards, no police, nothing. You go through a brief questioning on your way back into the USA. Long lines can form late in the day.
Op already knows the routine but others may not. You can NOT possess any controlled substances WITHOUT a prescription. Someone may sell to you but you cant posses them so be fully aware of what you are doing.
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.
PROOF