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.
The dosing of phenytoin is patient specific. It may be given once, twice, 3, or 4 times daily. Doses are often adjusted to find the optimal dose based on measurement of blood levels. Taking phenytoin with food may reduce some of the side effects. Elderly patients, debilitated persons, and patients with certain kidney or liver diseases may need lower doses. The suspension should not be given at the same time as tube feedings since tube feedings bind to phenytoin and reduce its absorption. The recommended adult dose is 100 mg two to four times daily. Some patients may require 200 mg three times daily. Patients stabilized on 100 mg three times daily may receive 300 mg once daily of the extended release capsules.
US BOXED WARNINGS :
CARDIOVASCULAR RISK ASSOCIATED WITH RAPID INFUSION RATES:
-The rate of IV administration should not exceed 50 mg per minute in adults and 1 to 3 mg/kg/min (or 50 mg/min, whichever is slower) in pediatric patients because of the risk of severe hypotension and cardiac arrhythmias.
-Careful cardiac monitoring is needed during and after IV administration of this drug.
-Although the risk of cardiovascular toxicity increases with infusion rates above the recommended infusion rate, these events have also been reported at or below the recommended infusion rate. Reduction in rate of administration or discontinuation of dosing may be needed.
NARROW THERAPEUTIC INDEX:
-This drug should be considered a narrow therapeutic index (NTI) drug as small differences in dose or blood concentrations may lead to serious therapeutic failures or adverse drug reactions.
- Generic substitution should be done cautiously, if at all, as current bioequivalence standards are generally insufficient for NTI drugs.
-Additional and/or more frequent monitoring should be done to ensure receipt of an effective dose while avoiding unnecessary toxicities.
Consult WARNINGS section for additional precautions.