BMA Concise Guide to Medicine & Drugs


Brand name [phenytoin] Epanutin; [fosphenytoin] Pro-Epanutin

Used in the following combined preparations None


Drug group Anticonvulsant drug

Overdose danger rating High

Dependence rating Low

Prescription needed Yes

Available as generic Yes


Phenytoin is used to treat epilepsy. It decreases the likelihood of convulsions by reducing abnormal electrical activity in the brain. Fosphenytoin is a type of phenytoin given by injection for severe seizures. Phenytoin has also been used to treat other disorders, such as migraine, trigeminal neuralgia, and certain abnormal heart rhythms. Some adverse effects of phenytoin are more pronounced in children, so it is prescribed for them only when other drugs are unsuitable. The dose may be adjusted according to blood levels of the drug. Patients are recommended to remain on the same brand of phenytoin.


Your drug prescription is tailored for you. Do not alter dosage without checking with your doctor.

How taken/used Tablets, chewable tablets, capsules, liquid, injection.

Frequency and timing of doses 1–2 x daily with food or plenty of water.

Dosage range Adults 200–500mg daily (usually as a single dose). Children According to age and weight. Note: a small increase in the dose can cause a disproportionately high drug level in the blood.

Onset of effect Full anticonvulsant effect may not be felt for 7–10 days.

Duration of action 24 hours.

Diet advice Folic acid and vitamin D deficiency may occasionally occur. Make sure you eat a balanced diet containing fresh, green vegetables and dairy products.

Storage Keep in original container at room temperature out of the reach of children.

Missed dose Take as soon as you remember.

Stopping the drug Do not stop the drug without consulting your doctor; seizures may recur.


Seek immediate medical advice in all cases. Take emergency action if unsteadiness, severe weakness, confusion, or loss of consciousness occur.


Many of phenytoin’s adverse effects appear only after prolonged use. If they become severe, your doctor may prescribe a different anticonvulsant. Dizziness, headache, nausea, vomiting, and insomnia are common. More rarely, increased body hair or overgrowth of the gums may occur. Discuss with your doctor if any of these become severe. If you experience confusion or unsteadiness, or develop a rash, fever, sore throat, or mouth ulcers, contact your doctor at once. Long-term use of the drug may cause other adverse effects and disrupt control of diabetes (see Prolonged use).


General note Many drugs may interact with phenytoin, causing either an increase or a reduction in the phenytoin blood level. The dosage of phenytoin may need to be adjusted. Consult your doctor or pharmacist.

Oral contraceptives Phenytoin may reduce their effectiveness.

Antidepressants, antipsychotics, mefloquine, chloroquine, and St John’s wort These preparations may reduce the effect of phenytoin.

Warfarin The anticoagulant effect of this drug may be altered. An adjustment in its dosage may be necessary.


Be sure to tell your doctor if:

· You have long-term liver or kidney problems.

· You have diabetes.

· You have porphyria.

· You are taking other medicines.

Pregnancy The drug may be associated with malformation and a tendency to bleeding in the newborn baby. Folic acid supplements should be taken by the mother. Discuss with your doctor.

Breast-feeding The drug passes into the breast milk, but at normal doses adverse effects on the baby are unlikely. Discuss with your doctor.

Infants and children Reduced dose necessary. Increased likelihood of overgrowth of the gums and excessive growth of body hair.

Over 60 Reduced dose may be necessary.

Driving and hazardous work Your underlying condition, as well the effects of phenytoin, may make such activities inadvisable. Discuss with your doctor.

Alcohol Avoid. Alcohol increases the sedative effects of this drug.


There is a slight risk that blood abnormalities may occur. Prolonged use may also lead to adverse effects on skin, gums, and bones. It may also disrupt control of diabetes.

Monitoring Periodic blood tests are performed to monitor levels of the drug in the body and composition of the blood cells and blood chemistry.