I'm locking this thread and adding our company response below. As others have said, in the vast majority of instances you will not be asked for this documentation, but it's important to have it just in case.
Short version:
If you're bringing medications, it's important to have them properly documented in case of inspection at customs or airport security. Using the original containers may not be practical if the containers are very large, or if you have several. In that case, you might bring a letter from your doctor listing your medications, and/or a copy of your prescriptions. In case you need to replace or refill a prescription while you're traveling, the copy usually cannot be filled by a foreign pharmacy but will be useful to a local doctor who can write a similar prescription. Be sure to keep your documents and medication with you in your hand luggage, rather than checked luggage, in case your bag doesn’t show up with the plane when you do!
Detailed version:
When packing medication for a trip, here are a few basic rules to follow:
First, make sure you carry all your important prescription medication on the plane with you so that it's not confiscated or lost if your luggage is delayed. We encourage you to pack light, fitting all your things into one carry-on size bag, which, hopefully, your medication will fit into. If it doesn’t, pack it in a separate small bag and keep that bag with you. Keep in mind that many airlines allow only one carry-on; so, if you have two bags, you may have to check one (again, don’t check your important medication).
Second, try to carry all your medications in their original containers. If those containers are too large for travel, ask your pharmacy to label smaller bottles with "official" labels that name the medication. Additionally, have copies of the original prescriptions with you. Your doctor can help with this (but ask him or her to please use legible handwriting). Also, ask him or her to put the generic names for the medicines in the event they don’t have a specific brand where you are traveling. Pharmacies in foreign countries often do not honor prescriptions from outside their borders, but if you need replacement medicine, the prescriptions will give guidance and contact info to the local doctor. Of course, having the properly labeled containers and prescriptions also helps you get through customs/security in airports—you don’t want the customs agents to think you are carrying illegal drugs.
Third, carry extra medication. If you're going on a three-week trip, take enough for four weeks. You want to be prepared should your flight home be delayed due to weather or other unforeseen circumstances.
A good organization to check out is International Association for Medical Assistance to Travelers (http://www.iamat.org/index.cfm; 716-754-4883). They have an online database and publish a pocket-sized booklet that lists English-speaking doctors in most countries and major cities. They also give information on recommended vaccines or other medical information.
When it comes to vitamins, again, you will want to make sure you have the original containers as loose vitamins in a plastic bag could be any kind of illegal drug in the eyes of airport security.
Hopefully this helps!