Dose-related side effects
The most common side effects are neurotoxic and dose-related. They include:

• sedation
• impaired memory
• slurred speech
• nystagmus
• decreased coordination
• confusion
• dizziness
• headache

If these problems do not diminish within several days, a reduction in the dose of phenytoin often will solve the problem. Problems with sedation also may be helped by splitting the dose or giving the largest dose at bedtime.

The most common side effects of long-term use affect appearance. About 20% to 40% of these patients notice a problem with gingival hyperplasia. This effect appears to be dose-related and is more common in children than in adults. Its occurrence and its associated problems can be minimized by good dental care, including vigorous brushing, daily flossing, and regular dental care. It usually resolves within a few months if phenytoin is discontinued.

Other appearance-related side effects of long-term use of phenytoin include

• hirsutism
• acne
• coarseness of facial features

Hirsutism is more of a problem for people with light complexions. It can be controlled with hair removal creams. Most acne can be treated effectively with facial hygiene, antibiotics, ointments, or lotions.

Dyskinesias, including chorea, dystonia, tremor, and asterixis, may be caused by phenytoin. Also, a small number of patients report sensory peripheral polyneuropathy.

Cerebellar atrophy may be another long-term effect of phenytoin, especially if high doses are used. Damage severe enough to produce significant problems is uncommon, however.

Idiosyncratic reactions
Rash is a common allergic reaction seen in about 5% to 7% of those who use phenytoin. This reaction usually occurs early in the course of therapy, within 5 to 17 days after the initial dose. Most patients will need to stop taking phenytoin and replace it with a different antiepileptic medication. The rash itself may require treatment with an antihistamine or steroid if it is severe.