Phenytoin

Key facts

• Phenytoin (sold as Dilantin) is an anticonvulsant used to control generalised tonic-clonic and focal seizures, including seizures after brain surgery or a severe head injury.
• You take it daily as tablets, capsules, chewable tablets or liquid; doses are adjusted slowly because clearance varies greatly between people.
• Phenytoin has a narrow therapeutic index: the gap between a helpful blood level and a toxic one is small, so regular blood tests are needed to keep levels safe.
• Seek urgent care for unsteadiness, slurred speech, confusion, or a severe skin reaction.

What phenytoin treats

Phenytoin treats generalised tonic-clonic seizures and focal seizures in epilepsy. It is also used short-term to prevent seizures after brain surgery, severe head injury, or in some intensive-care settings. It is not used for absence seizures, which it can make worse.

How phenytoin works

Phenytoin blocks sodium channels in nerve cells, slowing the repeated firing that spreads a seizure through the brain. Because the body clears phenytoin unevenly as the dose rises, small increases can produce disproportionately large rises in blood level.

Before you take it

• Tell your prescriber about liver disease, low blood pressure, heart rhythm problems, or a history of blood disorders before starting.
• Phenytoin interacts with many medicines that affect liver enzymes, including some antibiotics, antifungals, other anticonvulsants, and warfarin; doses may need adjusting either way.
• Regular blood tests to measure phenytoin levels are required, especially when starting treatment, changing dose, or adding a new interacting medicine.
• Careful dental care matters: phenytoin commonly causes gum overgrowth, so regular brushing, flossing and dental checkups reduce it.

Side effects

Common effects include drowsiness, unsteadiness, gum swelling and overgrowth, and mild tremor.

Stop and seek urgent medical care for any of these:

• Slurred speech, severe unsteadiness, blurred or double vision, or confusion.
• A widespread or blistering rash, fever, or mouth sores.
• Unusual bruising or bleeding.
• Yellowing of the skin or eyes.

Safety essentials

• Phenytoin's narrow therapeutic index means levels only a little too high can cause toxicity such as unsteadiness, slurred speech and confusion, while levels only a little too low can let seizures return. Never change your dose or brand without medical advice.
• Regular blood-level monitoring is essential, particularly after any dose change, illness, or new interacting medicine.
• Gum overgrowth is common and can be reduced with careful daily oral hygiene and regular dental review.
• Do not stop phenytoin abruptly. Stopping suddenly can trigger seizures, including a prolonged and dangerous seizure state.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.