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:

• 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. Most people get over these side effects without a change in the amount of phenytoin they're taking. But if the side effects persist, it is usually a sign that the level of phenytoin in the body is too high. A change in the dose or type of phenytoin may help. No one should stop taking phenytoin or change the amount they take without their doctor's guidance, however.

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 if a drug rash occurs with phenytoin.

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, but not always, 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.

Long-term use of phenytoin also has been found to cause weakening of the bones. Bone disease is even more likely if a combination of seizure medicines is used. People taking phenytoin should exercise, take vitamin D supplements, and eat foods rich in calcium. Some non-food sources of calcium, such as Tums and Os-Cal, reduce absorption of phenytoin. They can be used but should not be taken within about 2 hours after taking the phenytoin.

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.