The act of simple possession of a controlled substance without a valid prescription violates 21 USC, Section 844 (up to 1 year in prison and $1,000 fine).
"It shall be unlawful for any person knowingly or intentionally to possess a controlled substance unless such substance was obtained directly, or pursuant to a valid prescription or order, from a practitioner, while acting in the course of his professional practice..." [url]http://www.deadiversion.usdoj.gov/21cfr/21usc/844.htm[/url]
According to the Consulate General, at least with respect to Mexico, you are required to have obtained the drugs from a licensed physician in Mexico, and hold a valid prescription from that physician upon re-entry.
So, not only are you required to have a prescription from your US physician, but also from the Mexican physician who prescribed the drugs when you re-enter the US. If you do not have both, you are committing a crime, and subject to those penalties, as well as penalties associated with violating 21 USC, Section 844.
These same laws are also what makes obtaining prescriptions from overseas pharmacies illegal. You will probably never get caught because they have bigger fish to fry. But, if you patronize the overseas pharmacies without a valid US prescription, you are in violation of not only 21 USC, but also in violation of laws that can be enforced by FDA concerning procurement of medications from pharmacies whose drugs are manufactured in foreign manufacturing facilities without FDA approval.
Most of the drugs that "mongers" obtain in these foreign countries come from manufacturing plants that are not FDA approved, and we couldn't produce a prescription for the meds if our lives depended on it.
US Customs is charged with enforcing these regulations upon our re-entry into the United States. If you violate these regulations, you can be charged with a Felony if they really want to push it.
