Note that you have to declare the purchase to customs or take a chance with inspections. Check out the legality of bringing in the drugs and consider being prepared to provide the documentation needed.
We ALWAYS declare what we have (don't want to risk Nexus revocation) and have never, ever had a problem (you can bring back prescription drugs for personal use). In the five years we have been doing this only one time did the CBP agent in the booth as to see the bag with the receipt, he took a cursory look and then waived us on.