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http://customs.gov/xp/cgov/travel/vacation/kbyg/prohibited_restricted.xml#Medication

Medication

Rule of thumb: When you go abroad, take the medicines you will need, no more, no less.
Narcotics and certain other drugs with a high potential for abuse—Rohypnol, GHB, and Fen-Phen, to name a few — may not be brought into the United States, and there are severe penalties for trying to do so. If you need medicines that contain potentially addictive drugs or narcotics (e.g., some cough medicines, tranquilizers, sleeping pills, antidepressants, or stimulants), do the following:

Declare all drugs, medicinals, and similar products to the appropriate CBP official.
Carry such substances in their original containers.
Carry only the quantity of such substances that a person with that condition (e.g., chronic pain) would normally carry for his/her personal use.
Carry a prescription or written statement from your physician that the substances are being used under a doctor's supervision and that they are necessary for your physical well being while traveling.

U.S. residents entering the United States at international land borders, who are carrying a validly obtained controlled substance (other than narcotics such as marijuana, cocaine, heroin, or LSD), are subject to certain additional requirements. If a U.S. resident wants to bring in a controlled substance (other than narcotics such as marijuana, cocaine, heroin, or LSD) but does not have a prescription for the substance issued by a U.S.-licensed practitioner (e.g., physician, dentist, etc.) who is registered with, and authorized by, the Drug Enforcement Administration (DEA) to prescribe the medication, the individual may not import more than 50 dosage units of the medication into the United States. If the U.S. resident has a prescription for the controlled substance issued by a DEA registrant, more than 50 dosage units may be imported by that person, provided all other legal requirements are met.