Sometimes one kind of medicine changes the way another kind of medicine works in the body. This is true not only for prescription medicines, but also for medicines you just pick up off the shelf at the store. It's also true for herbal products, vitamins, a few kinds of food, and even cigarettes!

Any time a doctor suggests a new prescription, be sure to talk about what other medicines you are already using. If two kinds of medicine affect each other, the doctor may want to prescribe something else or change the amount to be taken.

Taking phenobarbital will lower the level in the blood of several other seizure medicines:

• Tegretol, Carbatrol (carbamazepine)
• Lamictal
• Zonegran
• Gabitril
• Depakote, Depakene (valproic acid)

Medicines often used for other disorders are also affected. It is especially important to check with your doctor before taking any of these:

• Antidepressants
• Allergy medicines
• Coumadin (warfarin)
• Theophylline
• Tagamet (cimetidine)
• Oral contraceptives (birth control pills)
• Narcotic pain relievers such as Percocet
• Sedatives such as Nembutal and Seconal
• Steroids such as Medrol and Deltasone
• Tranquilizers such as Xanax and Valium

Reviewed February 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board.