This is what the law says:
Code of Federal Regulations
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Section 1301.26 Exemptions from import or export requirements for personal medical use.
This page includes changes to the Code of Federal Regulations since April 1, 2004. View Key.
Any individual who has in his/her possession a controlled substance listed in schedules II, III, IV, or V, which he/she has lawfully obtained for his/her personal medical use, or for administration to an animal accompanying him/her, may enter or depart the United States with such substance notwithstanding sections 1002-1005 of the Act (21 U.S.C. 952-955), providing the following conditions are met:
(a) The controlled substance is in the original container in which it was dispensed to the individual; and
(b) The individual makes a declaration to an appropriate official of the Bureau of Customs and Border Protection U.S. Customs Service stating:
(1) That the controlled substance is possessed for his/her personal use, or for an animal accompanying him/her; and
(2) The trade or chemical name and the symbol designating the schedule of the controlled substance if it appears on the container label, or, if such name does not appear on the label, the name and address of the pharmacy or practitioner who dispensed the substance and the prescription number. , if any; and
(c) The importation of the controlled substance for personal medical use is authorized or permitted under other Federal laws and state law.(c) In addition to (and not in lieu of) the foregoing requirements of this section, a United States resident may import into the United States no more than 50 dosage units combined of all such controlled substances in the individual's possession that were obtained abroad for personal medical use. (For purposes of this section, a United States resident is a person whose residence (i.e., place of general abode-- meaning one's principal, actual dwelling place in fact, without regard to intent) is in the United States.) This 50 dosage unit limitation does not apply to controlled substances lawfully obtained in the United States pursuant to a prescription issued by a DEA registrant.
[62 FR 13952, Mar. 24, 1997]
http://www.deadiversion.usdoj.gov/21cfr/cfr/1301/1301_26.htm
and:
The above guidance does not specify that a U.S. citizen may import an unapproved drug only with a prescription from a U.S. licensed physician, or that a foreign citizen may import an unapproved new drug only with a foreign prescription. Rather, to ensure that the importation is for personal use only (and not for resale), and to ensure that the use of the unapproved new drug sought to be imported into the U.S. is supervised and does not represent an unreasonable risk, the guidance provides that the individual affirm in writing that the drug is for his or her personal use, and provide either the name and address of the U.S. licensed physician who will supervise its use
or some evidence that the treatment was begun in a foreign country and that the drugs are being imported to continue/conclude the already begun treatment. Thus, while not the only documentation, either a U.S. or foreign prescription, along with an affirmation of personal use, could be supplied as evidence that this factor exists.
http://www.fda.gov/ora/import/pipinfo.htm
From Customs own brochure:
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.
\Please note that only medications that can be legally prescribed in the United States may be imported for personal use. Be aware that possession of certain substances may also violate state laws. As a general rule, the FDA does not allow the importation of prescription drugs that were purchased outside the United States. Please see their Website for information about the enforcement policy for personal use quantities.
***Then see above from FDA own site...
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