You should have it in a labeled container, if you usually get say six months worth at a time, ask for a container labeled with the information for the smaller amount you are carrying.

From the know before you go brochure, available here:

http://www.cbp.gov/linkhandler/cgov/...eforeyougo.pdf

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.


While you probably won't need all the documentation, it's way better to have it and not need it then need it and not have it. How strict of course depends on what the medication is (not asking, just saying)