The best amount is the amount that completely controls seizures without causing troublesome side effects. It depends on many factors, which are different for every individual. Follow the doctor's directions. Call if you have any questions.

To avoid unwanted side effects, the doctor will prescribe a low dose to start and increase it gradually until seizures are controlled. Most adults do well with 300 to 500 milligrams per day, to be taken in one or two doses. Seniors often need less. Cigarette smokers often need more. Children (until they reach their teens) require a higher dose relative to their weight. They usually need to take phenytoin two or three times a day.

No one should stop taking phenytoin or change the amount they take without talking to the doctor first. Stopping any seizure medicine all at once can cause a problem that may be life-threatening.

Don’t use more phenytoin than the doctor prescribes. Talk to the doctor about what to do if one or two extra capsules are taken by accident. For a larger overdose, call a poison control center or emergency room right away.