Phenytoin: A second-line option for seizure management
The bottom line
Phenytoin is a second-line option for treatment and prevention of seizures in children and adults. Phenytoin may cause sleepiness, confusion, slurred speech, unsteady gait, headache, incoordination, abnormal body movements, involuntary eye movements (nystagmus), unwanted hair growth, and overgrowth of gums. Severe allergic reactions are possible. It interacts with many drugs.
Prevention Tips
Store phenytoin up, away, and out of reach of children.
Take as prescribed.
Swallow whole; do not cut, crush or open extended-release capsules.
Chew chewable tablets.
Do not stop phenytoin suddenly.
Be aware of potential for interactions with other drugs, vitamins and herbals.
This Really Happened
A 46-year-old man on phenytoin for seizures overdosed on 60 extended-release 100 mg phenytoin pills in a suicide attempt. He presented with altered mental status, injury to his knee, and a cut on his forehead requiring sutures from a fall. He was lethargic, with slurred speech and ataxia (unsteady gait). His initial phenytoin blood level was elevated and continued to rise over the next several days. He became agitated and fell out of bed twice. His mental status worsened, as he was unable to follow commands, his speech was slurred, and his response to questions delayed. Dialysis to remove phenytoin more rapidly was performed on day 8 and phenytoin blood level decreased. Mental status improved on day 12. His mental status returned to normal on hospital day 14 at which point phenytoin blood level was within the therapeutic range. He was switched to another anticonvulsant and was discharged.
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What is phenytoin prescribed for?
Phenytoin is an anticonvulsant prescribed for treatment and prevention of seizures in both children and adults. The types of seizures that phenytoin is used for are generalized tonic-clonic seizures and complex partial seizures. It is no longer considered a first line treatment for seizures. Treatment of cardiac arrhythmias and trigeminal neuralgia are off-label uses of this medication.
How does phenytoin work?
Sodium channels are part of the nervous system that transmits electrical signals. When a person is experiencing a seizure, the brain cells generate and transmit signals at a very high frequency. Phenytoin blocks sodium channels in the brain and heart. It decreases abnormal electrical activity which makes it more difficult to excite the neuron such that it will be less likely to fire repetitively. Phenytoin blocks the spread of the seizure to nearby brain areas.
What are the side effects of phenytoin?
Phenytoin may cause sleepiness, confusion, slurred speech, unsteady gait, headache, incoordination, abnormal body movements, and involuntary eye movements (nystagmus). Other side effects include changes in taste, constipation, unwanted hair growth, and overgrowth of gums. Abnormal laboratory findings include hyponatremia (low sodium in blood) and, rarely, thrombocytopenia (low platelet count).
More serious side effects include severe reactions including rash, hives, peeling skin, mouth ulcers, sores around eyes, fever, sore throat, swollen glands, muscle pain, bruising, and/or swelling to the face, eyes, lips, tongue and throat as well as kidney or liver damage. Phenytoin can cause drug-induced liver injury which can be mild or develop into severe liver failure. Long-term treatment with phenytoin can cause cerebral atrophy (shrinkage of brain tissue).
Is phenytoin a controlled substance?
No. Phenytoin is not a controlled substance. It is available by prescription.
What should I avoid while taking phenytoin?
Be careful driving or operating machinery until you know if phenytoin makes you sleepy. It is best to avoid alcohol since it can affect phenytoin blood concentrations. There are many drug interactions with phenytoin including birth control pills, antivirals, antacids, blood thinners, antibiotics, antidepressants, other seizure medications, and corticosteroids.
Check with your doctor or pharmacist about potential interactions with prescription and nonprescription drugs, vitamins, and herbals, as well as whether it is okay to change phenytoin brands or formulations. Do not abruptly stop taking phenytoin since this can result in seizures.
Can phenytoin be used during pregnancy?
Phenytoin should be avoided in pregnancy if at all possible. It can cause birth defects including heart defects, cleft palate, and neural tube defects such as spina bifida. It may also result in delayed motor development in children exposed to it in utero.
What should I do if someone accidentally takes phenytoin or takes too much phenytoin?
Accidentally taking one pill is unlikely to result in significant toxicity. If you forget to take a dose, take it as soon as you realize that you missed it. Do not double up on your dose to try to ‘catch up.’ If you take an overdose of phenytoin, depending on the dose, you may experience nausea, vomiting, drowsiness, incoordination, dizziness, slurred speech, abnormal eye movements (nystagmus), coma, and shortness of breath. Rarely, seizures can occur when blood phenytoin levels are high.
Seek medical attention if you are experiencing serious side effects or have taken an overdose. If someone swallows phenytoin or if you have a question about using phenytoin safely, use the webPOISONCONTROL® online tool to get help, or call your poison center at 1-800-222-1222. Whether online or by phone, expert guidance is always free, confidential, and available 24 hours a day.
