Under the Federal Food, Drug and Cosmetic Act, travelers are strongly discouraged from bringing any prescription medications from Mexico into the United States. There are strict restrictions imposed for those who may need to import medications for their own personal use.
Mexico and the United States have varying definitions of what is considered a controlled or legal substance, but drugs such as LSD, Rohypnol and fen-phen can never be legally brought into the U.S.
Mexican law states that pharmacies in Mexico cannot fill prescriptions from other countries. Thus, in order to buy controlled medication in Mexico, you must have a prescription provided by a Mexican doctor.
If you are caught buying medication in Mexico without a proper prescription, you can be charged with possession of a controlled substance and be sentenced to up to 15 years in prison.
U.S. Customs and Border Protection (CBP) requires that any medication brought from Mexico be declared and accompanied by a prescription from a U.S. doctor. This is in addition to a Mexican prescription for the medicine. Any medication declared without a valid U.S. prescription will be confiscated.
Be sure to keep the medication you bought in Mexico in its original container and never have more than a single person would use in three months.