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What is phenytoin?

Phenytoin sodium (also called Dilantin®) is used to prevent or treat seizures. It is also used to treat migraine headaches and pain in the nerves of the face and other areas of the body. It is available in these forms taken by mouth:

  • A 25 mg/ml orange liquid suspension
  • A 30-mg pink and white capsule
  • A 100-mg orange and white capsule
  • A 50-mg yellow chewable tablet

The injection form of phenytoin is a clear liquid given by vein (IV) or by muscle (IM). It is called Fosphenytoin (also called Cerebyx®).

How to give phenytoin

  • If you are taking the liquid suspension, shake the bottle well before you take each dose. The medicine will settle at the bottom of the bottle between doses.
  • Do not change the form of phenytoin (suspension, tablet, or capsule) you are taking without telling your doctor. If the doctor changes the type of pheytoin you are taking, your blood level needs to be checked to make sure it is stable.
  • For patients who are also taking tube feedings:
    • If you are receiving feedings by NG-tube, G-tube, or button, do not take your phenytoin dose while you are receiving your feedings. This will greatly decrease the phenytoin in your blood and increase the risk for seizures. You should stop the feedings 2 hours before and 2 hours after taking your phenytoin dose. Your doctor, pharmacist, or nurse will help you schedule your feedings around your phenytoin doses.
    • If you are taking the liquid suspension through the tube, you must mix the phenytoin dose with 3 times more water. After you take the medicine by tube, flush the tube with 20 ml water to make sure the medicine is not stuck in the tube.
    • Mix _______ ml of phenytoin with _______ ml of water, and give the phenytoin through the tube. Then flush the tube with 20 ml water.

Possible side effects of phenytoin


  • Nausea and vomiting
  • Feeling drowsy

Later (usually more than a day after treatment starts)

  • Red, irritated, bleeding, and swollen gums
  • Blurred or double vision
  • Decreased coordination
  • Feeling nervous
  • Muscle twitching
  • Slurred speech
  • Headache
  • Increased hair growth
  • Skin rash
  • Easy bruising, unusual bleeding
  • Increased blood sugar levels
  • Low blood counts
  • Trouble sleeping
  • Allergic reaction with these symptoms: fever, rash, liver function changes, and muscle aches

These are the most common side effects, but there may be others. Please report all side effects to the doctor or nurse.

In case of a severe side effect or reaction, call the doctor, nurse, or pharmacist at 595-3300. If you are outside the Memphis area, dial toll-free 1-866-2STJUDE (1-866-278-5833), and press 0 once the call is connected.

Special instructions for phenytoin

  • Do not stop taking this medicine until your doctor tells you to do so. If you stop taking it without being told, your chances of having a seizure may increase.
  • Store phenytoin at room temperature.
  • This medicine may cause you to feel dizzy and drowsy. Do not operate heavy equipment or drive a motor vehicle until you see how this medicine will affect you.
  • The doctor will slowly increase your dose of phenytoin when you first begin taking it.
  • If you develop a rash after starting phenytoin, it may mean that you have an allergy to this medicine. Call your doctor, nurse, or pharmacist right away. 
  • It is very important that you take phenytoin exactly as your doctor has told you. This will keep the blood levels constant during the day and night. If you miss a dose, follow these guidelines:
    • If the next dose is not due for at least 4 hours, take the missed dose right away.
    • If the next dose is due in less than 4 hours, do not take the missed dose. Take the next dose at the scheduled time. Never take a double dose unless your doctor or pharmacist tells you to do so.
  • It is very important that you brush your teeth, massage your gums, and use dental floss while taking phenytoin. See your dentist regularly.
  • Phenytoin may affect the way other medicines work. These medicines include warfarin, birth control pills, and theophylline.
  • Several medicines can cause your blood levels of phenytoin to increase or decrease. These medicines include: calcium-containing antacids (such as Tums®), carbamazepine, diazepam, fluconazole, ketoconazole, itraconazole, phenobarbital, promethazine, rifampin, sucralfate, and valproic acid. Alcohol (found in many over-the-counter cough/cold medicines) can increase the drowsy feeling caused by phenytoin.
  • Always tell your doctor if you are taking these medicines or if you start taking any new medicine while taking phenytoin.
  • The staff will draw blood samples regularly to check for changes in liver function.
  • This medicine can increase the risk of falls.

Blood levels of phenytoin

  • Blood levels of phenytoin will be checked regularly to reduce side effects and to improve treatment. It is best to draw blood for these checks just before the next dose is due. Therefore, do not take your next scheduled dose until after the blood level has been drawn. Your dose of phenytoin may change based on the blood level.
  • If you have blurred vision, decreased coordination, vomiting, or an increased drowsy feeling, please contact your doctor right away. These side effects can mean the level of phenytoin in your blood is too high.


This document is not intended to take the place of the care and attention of your personal physician or other professional medical services. Our aim is to promote active participation in your care and treatment by providing information and education. Questions about individual health concerns or specific treatment options should be discussed with your physician.

St. Jude complies with health care-related federal civil rights laws and does not discriminate on the basis of race, color, national origin, age, disability, or sex.

ATTENTION: If you speak another language, assistance services, free of charge, are available to you. Call 1-866-278-5833 (TTY: 1-901-595-1040).

ATENCIÓN: si habla español, tiene a su disposición servicios gratuitos de asistencia lingüística. Llame al 1-866-278-5833 (TTY: 1-901-595-1040).

تنبيه: إذا كنت تتحدث باللغة العربية فيمكنك الاستعانة بخدمات المساعدة اللغوية المتوفرة لك مجانا. .يرجى الاتصال بالرقم. 5833-278-866-1  (الهاتف النصي: 1040-595-901-1).