Any time a doctor suggests a new prescription, be sure to talk about what other medicines, supplements, herbs, and vitamins are already being taken. Sometimes one kind of medicine changes the way another kind of medicine works in the body. If two kinds of medicine affect each other, the doctor may prescribe something else or change the amount to be taken.

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.

How does valproic acid affect other medicines?
Valproic acid may increase the blood level of certain other seizure medicines, including

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

The levels of these medications in the blood may increase markedly—in some cases, more than double. If a person taking phenobarbital is also given valproic acid, a rapid rise in phenobarbital levels can lead to extreme tiredness, slurred speech, and other signs of intoxication.

Valproic acid has similar but less significant effects on:

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

Adding valproic acid may slightly increase side effects from each of these.

If combination therapy using valproic acid and another seizure medicine is begun, the dosage of the first medicine may need to be adjusted. Similarly, if a seizure medicine previously used in combination therapy is to be stopped, the dosage of the remaining medicine may need 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.

Several other seizure medicines cause valproic acid to leave the body much more quickly, so that higher doses may be needed to achieve the desired effect. These medicines include:

• Tegretol or Carbatrol (carbamazepine)
• Dilantin or Phenytek (phenytoin)
• phenobarbital
• Mysoline (primidone)

If Lamictal (lamotrigine) is given to someone already taking valproic acid—especially a child—the chances of a potentially life-threatening rash will be increased. Adding the Lamictal slowly reduces this danger.

If a combination of valproic acid and Klonopin (clonazepam) is given to a person who has been having absence seizures, prolonged absence seizures may occur.