Indications
Phenytoin is an anti-seizure medicine used to control several types of epilepsy, including tonic-clonic (grand mal) seizures and focal (partial) seizures. It is also used to prevent or treat seizures during or after brain surgery or head injury, and as an injection in hospital for prolonged seizures (status epilepticus).
How It Works
Nerve cells in the brain send signals using tiny channels that let sodium in. In epilepsy, some nerve cells fire too often and the activity spreads. Phenytoin slows the recovery of these sodium channels, which stops nerve cells from firing rapidly again and again and reduces the spread of seizure activity.
Dosage and Administration
The dose is set and adjusted by a doctor for each person, usually guided by blood levels of the medicine. Doctors often start adults on about 300 mg a day, taken once or in divided doses, then change it in small steps. Take it at the same time each day, with water, with or without food but in the same way each day. Shake liquid forms well and use a proper measuring device. Do not switch between brands or forms without advice, as levels can change. Injections are given only by a doctor or nurse. Never stop suddenly, as this can trigger severe seizures.
Interactions
Phenytoin interacts with many medicines. It can lower the effect of Warfarin (or sometimes raise it), hormonal contraceptives, Corticosteroids, Carbamazepine, many HIV and cancer medicines and some antibiotics. Levels of Phenytoin can be raised by Isoniazid, Fluconazole, Amiodarone, Omeprazole, Valproate, Chloramphenicol and alcohol taken heavily at once, or lowered by Rifampicin, regular heavy drinking, antacids and tube feeds. Folic acid in high doses can lower levels. Always tell your doctor and pharmacist about every medicine and supplement you take.
Contraindications
Do not use Phenytoin if you are allergic to it or to similar medicines (hydantoins), or if you have had a serious skin reaction or liver problem with it before. The injection is not used in people with a very slow heart rate or certain heart blocks. It must not be combined with Delavirdine.
Side Effects
Common side effects include:
- Dizziness, unsteadiness, shaky eye movements, blurred or double vision
- Drowsiness, slurred speech, confusion
- Swollen, overgrown gums (good tooth care helps)
- Extra body or facial hair, coarse facial features, acne
- Nausea, vomiting, constipation
Long-term use can weaken bones (low vitamin D) and lower folate levels. Rarely, it can affect the blood count, liver or lymph glands.
Get urgent medical help if you develop a spreading rash, blisters, peeling skin or mouth sores (people of Asian origin with a certain gene are at higher risk), fever with swollen glands, yellow skin or eyes, unusual bruising or bleeding, severe unsteadiness or confusion, thoughts of self-harm, or seizures that become more frequent.
Pregnancy and Lactation
Phenytoin can increase the risk of birth defects and may affect the baby’s growth and development. Do not stop it on your own if you become pregnant, as uncontrolled seizures are also dangerous; see your doctor promptly. Doctors usually advise folic acid before and during pregnancy, and vitamin K near delivery. Small amounts pass into breast milk; many women can breastfeed, but watch the baby for drowsiness or poor feeding and discuss with your doctor.
Precautions and Warnings
Regular blood tests may be needed to check levels, liver function and blood count. Use with care in liver disease, low albumin, kidney disease, diabetes, porphyria and in older people. It can cause drowsiness, so do not drive or use machines until you know how it affects you. Avoid alcohol. Keep good dental hygiene. Some people on anti-seizure medicines have mood changes or thoughts of self-harm; tell your doctor about any such change.
Overdose
Overdose can cause severe unsteadiness, jerky eye movements, slurred speech, vomiting, confusion, slow breathing and coma. Seek emergency medical help immediately.
Therapeutic Class
Anticonvulsant (hydantoin antiepileptic).
Storage
Store below 30°C in a dry place, away from light and out of the reach of children. Do not freeze liquid forms.
Prepared by the Oshud Alo team from standard drug references. For information only; always follow your doctor's advice.