Most of the time, everyday medications don't cause much fuss when you leave the United States or when you return. But depending on which country you're traveling to in Europe, exactly what you're carrying – and what sort of a day the customs or security screener has been having – you may be held to a set of specific rules about carrying medications across borders.

Original Bottles and Prescriptions

The Transportation Security Administration, which screens your luggage as you leave the U.S., isn't particularly concerned about whether medications are in their original bottles with the prescription label. But they do warn that state laws on transporting medication vary, and when you re-enter the United States, U.S. Customs and Border Protection requires you to have your medications in the original container with the doctor's prescription printed on it. Keeping your meds in the original bottles will satisfy border control in most European countries too, so it's a great way to avoid the headaches, confiscated medicine and missed flights that can occur if you're held up during screening.

Alternatives to the Original Bottle

If for some reason you simply can't keep your medications in the original bottle, pack them in a pill case or a clear zip-close bag. Then carry a copy of your prescription, along with a note from your doctor explaining which medications you take and why they're medically necessary in such a quantity.

How Much and Where to Pack It

You're allowed to carry medications in any "reasonable quantity" as you enter and leave the United States. So bring enough for your trip, plus at least a few days' extra in case of delays – it's usually okay up to a maximum of 90 days. That medication can go in either your checked or carry-on luggage, but, assuming it's really vital for your well-being, you should always pack it in your carry-on so you'll have immediate access, even if your checked bag(s) go missing. The TSA has strict rules about how much liquid you can pack in your carry-on, but you're allowed to carry liquid medications in excess of that amount – as long as you let the TSA screeners know about the extra liquid before they start screening your bags, and as long as you're ready to prove its medical necessity.

Proving Medical Necessity

Your doctor is a valuable ally in the quest to keep you and your medications together, especially if you're packing extra needles, oxygen tanks, lots of liquid medication or anything else that might raise eyebrows at a security screening point. A note from your doctor explaining medical necessity is usually enough to get you through security with those items. You can also ask your doctor for a list of the active ingredient(s) in each medication, along with their generic equivalents. This information – along with a copy of the prescription – is invaluable if you find yourself stranded and need an emergency resupply. Major bonus points if you have both lists translated into the language of your destination country.

Learn the Rules of Your Destination

As a general rule, taking this conservative approach to packing your medicine – packing it in its original bottles and carrying a copy of the original prescription, along with a doctor's note of medical necessity for any unusual items – will get you through most screening checkpoints in Europe too. Keep those documents handy as you go through customs into Europe, because you may be asked to prove your medication's authenticity.

With all that said, country-specific rules about medication can vary, even within Europe. To be completely safe, check with the embassy in your destination country as you begin planning your trip. They'll be able to tell you what you can take across the border in your packed luggage, and exactly how to prove its medical necessity.