While the practice of reimporting drugs from Canada, Mexico, or other countries is still technically illegal (with the possible exceptions noted below), it is increasingly becoming a custom more honored in the breach than in the observance. The U.S. House of Representatives has passed three versions of bills that would allow consumers to import legal drugs for personal use. A similar measure, known as the Dorgan-Snowe Drug Importation bill, is currently before the Senate.
In the meantime, the mission of the FDA, as always, is to promote and protect the health of Americans. The mission of the U.S. Customs service is to enforce Federal laws and regulations as they pertain to imported substances such as drugs. And here's where the law gets kind of squishy.