Most people who take phenytoin don't have too much trouble with side effects. That's one of the reasons it’s used so much. The most common complaints (usually not too severe) are:

• sleepiness
• jerking movements of the eyes
• decreased coordination
• shaking of the hands
• slowed thinking and movement
• memory problems
• slurred speech
• poor concentration

If these problems do not go away within several days, or are really bothersome, call the doctor. Sometimes the doctor can help with these side effects by changing the prescription:

• reducing the overall amount of phenytoin
• changing the amount taken at certain times, such as taking a greater proportion of the phenytoin at bedtime to reduce daytime sleepiness
• prescribing smaller doses, to be taken more often
• changing the prescription from regular phenytoin to Phenytek, a form of phenytoin that is released into the body more slowly and evenly

No one should stop taking phenytoin or change the amount they take or when they take it without their doctor's guidance.

People who have just started taking phenytoin (or who have just started taking a larger amount) should be careful during activities that might be dangerous, until they know whether they are having any side effects.

Allergic reactions
Approximately 1 in 10 people who take phenytoin have a red rash within the first few weeks of taking it. If this happens, tell the doctor or nurse right away, to be sure that it's not the beginning of a serious problem. It's rare for the rash to be serious, but don't ignore it. It's often necessary to switch to a different seizure medicine.

Long-term side effects
Some other side effects only appear after a person has been using phenytoin for a number of months or years. They are usually related to taking high doses. The most common ones affect appearance:

• overgrowth of the gums
• excessive hair on the face or body
• acne
• coarseness of facial features

Overgrowth of the gums (gingival hyperplasia) is more common in children than in adults. It can be reduced by vigorous brushing, daily flossing, and regular visits to the dentist, who may recommend additional treatments. If phenytoin is stopped, the gum problems won’t get worse, and in some cases will go away within a few months.

Some people who take phenytoin for several years may have damage to the nerves in their legs. Faulty reflexes can be seen in a minority of this group.

Damage to the part of the brain called the cerebellum may affect others who take phenytoin for a long time, especially if they take high doses. Damage severe enough to produce significant problems is uncommon, however.