Sometimes one substance, such as a medication, changes the way the body absorbs, digests, or excretes another one. This is true not only for prescription medicines, but also for over-the-counter medicines, dietary supplements, herbal products, a few kinds of food (such as grapefruit juice), and even cigarettes!

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

Seizure medicines (including carbamazepine) often affect each other. This is why people who start to take a second seizure medicine in combination (or stop taking a combination) may need a change in the dose of the first medicine.

How does carbamazepine affect other medicines?
Carbamazepine makes birth control pills less effective, so the chances of becoming pregnant are greater. Women who use pills for birth control should talk to the doctor who prescribed them right away if they start taking carbamazepine. The same is true for other forms of hormonal birth control, like Depo-Provera or implants. Barrier forms of birth control, such as condoms, IUDs, and diaphragms, are not affected by carbamazepine.

Carbamazepine also affects the way the body handles many other seizure medicines. Some of these are:

• valproate (Depakote)
• phenytoin (Dilantin or Phenytek)
• clonazepam (Klonopin)
• topiramate (Topamax)

How do other medicines affect carbamazepine?
Some medicines can cause carbamazepine to build up in the blood. Having too much carbamazepine in the blood makes people feel dizzy, unsteady, or sleepy.

Some of the medications and other substances that may raise carbamazepine levels include:

• Depakote and other valproate seizure medicines
• some medicines for high blood pressure, including diltiazem (Cardizem) and verapamil
• fluoxetine (Prozac)
• certain antibiotics, including erythromycin
• cimetidine (Tagamet, also available without a prescription)
• Darvon and Darvocet
• certain anti-fungus medicines, including Diflucan (fluconazole), Nizoral (ketoconazole), and Flagyl (metronidazole)
• grapefruit juice

On the other hand, some medicines reduce the amount of carbamazepine in the blood. More seizures may occur unless a higher dosage of carbamazepine is taken.

Medicines that may lower carbamazepine levels include:

• phenytoin (Dilantin or Phenytek)
• oxcarbazepine (Trileptal)
• felbamate (Felbatol)
• phenobarbital
• primidone (Mysoline)