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. For instance, aspirin (ASA) may increase the side effects from taking valproic acid.

If someone who is taking valproic acid—especially a child—also starts to take the seizure medicine called Lamictal, the chances of a very dangerous rash will be increased. Adding the Lamictal slowly reduces this danger.

Taking both valproic acid and another seizure medicine, Klonopin, sometimes causes absence seizures to be much longer.

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.

How does valproic acid affect other medicines?
Valproic acid affects the way the body handles many other seizure medicines. Some of these are:

• Felbatol (felbamate)
• Lamictal (lamotrigine)
• Mysoline (primidone)
• phenobarbital

If a person taking one of these medicines starts taking valproic acid, the first medicine will stay in the body longer. If the person keeps taking the same amount as before, soon there will be too much of it. This may cause side effects like extreme tiredness and slurred speech. The person might seem to be drunk.

A similar thing could happen with several other seizure medicines, but the effects for people taking these would probably not be too noticeable:

• Tegretol or Carbatrol (carbamazepine)
• Zarontin (ethosuximide)
• Ativan (lorazepam)
• Dilantin or Phenytek (phenytoin)

All of these effects mean that if a person taking another seizure medicine starts taking valproic acid too, the amount of the first medicine may need to be changed. Or if a person has been taking valproic acid along with another seizure medicine, stopping either one of them means that the amount of the other one probably will have to be changed.

How do other medicines affect valproic acid?
Some other medicines do affect the way valproic acid works in the body. Make sure that the doctor is aware of all the medicines being used.