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Phenytoin

Supportive Care

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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®).

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Possible side effects

  • Nausea and vomiting
  • Feeling drowsy

Symptoms of an allergic reaction include: rash, hives, itching, chills, fever, headache, muscle ache, shortness of breath, coughing, tightness in the throat, swelling of the face or neck.

Not all patients who take phenytoin will experience these side effects. Common side effects are bolded, but there may be others. Please report all suspected side effects to your doctor or pharmacist.

Find more information on side effects.

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Possible late effects

  • 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.

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Tips for patients and families

Be sure to discuss all questions and instructions with your doctor or pharmacist. 

  • 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.

Drug at Home:

  • 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.
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Always check the medication label. Many medicines look the same. Do not take any new medicine, including over-the counter medicines or supplements, without checking with your doctor or pharmacist. Talk to your doctor or pharmacist about what to do in the case of a missed dose, allergic reaction, or medical emergency. 

This information is not intended to be a complete guide and is not a substitute for talking with your provider. Medicine names, appearance, formulas, or availability may vary by clinic location. Always follow the instructions given by your care team.

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