Phenytoin is an oral and injectable anti-seizure medication first synthesized in 1908. Phenytoin was originally approved by the FDA in 1939.

Use as directed by your doctor

Many varied adverse effects can occur during phenytoin therapy including dizziness, drowsiness, difficulty focusing (vision), unsteady gate, tiredness, abnormal involuntary movements, nausea, vomiting, constipation, abdominal pain, and loss of appetite. Children and young adults can develop overgrowth of the gums during long-term therapy which requires regular treatment by a dentist. Good oral hygiene and gum massage may reduce the risk. Rashes can occur in between 1 in 20 and 1 in 10 persons; some may be severe. Additionally, darkening coloration of the skin may develop (more commonly in women). Phenytoin can produce unusual growth of hair in some patients. This reaction most commonly affects the arms and legs but can also affect the trunk and face; it may be irreversible.

: There are many potential drug interactions with phenytoin. Phenytoin can increase the metabolism (elimination) of many drugs, reducing their concentrations in the body. Drugs that may be affected include: digoxin, carbamazepine, clonazepam, corticosteroids (e.g. prednisone), cyclosporine, disopyramide, doxycycline, estrogens, felodipine, levodopa, lidocaine, methadone, mexiletine, oral contraceptives, paroxetine, quinidine, tacrolimus, theophylline, phenobarbital, and warfarin. Phenytoin can interact with these drugs not only when it is added to therapy but also when it is discontinued. In the latter case, the concentration of the other drugs may increase.

If you suspect an overdose call the emergency department or your doctor immediately.

Do not double a dose to make up for a missed one

Phenytoin is used to prevent and control seizures (also called an anticonvulsant or antiepileptic drug). It works by reducing the spread of seizure activity in the brain.

Tablets, capsules, and suspension should be kept at room temperature, 15-30°C (59-86°F).

Before taking phenytoin, tell your doctor or pharmacist if you are allergic to it; or to other anti-seizure medications (e.g., ethotoin, phenobarbital, ethosuximide, trimethadione); or if you have any other allergies. Before using this medication, tell your doctor or pharmacist your medical history, especially of: alcohol use, certain blood conditions (porphyria), diabetes, liver disease, lupus, folate or vitamin B-12 deficiency (megaloblastic anemia). This drug may make you dizzy or drowsy; use caution engaging in activities requiring alertness such as driving or using machinery. Limit alcoholic beverages because they may increase the risk of side effects. Alcohol may also affect your blood levels of this drug. Tell your doctor you are using phenytoin prior to surgery or any procedure that makes you unable to take it by mouth. If you have diabetes, this drug may increase your blood sugar levels. Check your blood (or urine) glucose level frequently, as directed by your doctor. Promptly report any abnormal results as directed. Your medicine, exercise plan, or diet may need to be adjusted. Vitamin D supplements may be necessary to prevent weakening of the bones (osteomalacia). Discuss this with your doctor. This medication is not recommended for use during pregnancy due to the possible risk of birth defects and fetal harm. Consult your doctor for more details. Phenytoin passes into breast milk. Therefore, breast-feeding while using this medication is not recommended. Consult your doctor before breast-feeding.