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Phenytoin (FEN i toyn) has been approved by the FDA to treat tonic-clonic (grand mal) psychomotor (temporal lobe) seizures, as well as for the prevention and treatment of seizures occurring during or following neurosurgery.

Your epilepsy treatment should always be discussed with your healthcare provider before use. Based on their judgment and knowledge, a drug may be prescribed for other epilepsy types not included in the indications. For more information, please see the prescribing information.

How can you take phenytoin? (Available formulations)

Phenytoin is available in a few different formulations to take by mouth. You can take phenytoin as a chewable tablet, an extended-release capsule, and an oral suspension.

Who should not take phenytoin?

If you are allergic to phenytoin, fosphenytoin, ethotoin, or mephenytoin, as well as any of the inactive ingredients, then you should not take it. You should not take phenytoin if you have a history of liver problems caused by fosphenytoin or phenytoin

Other considerations may influence whether you should take phenytoin. Tell your healthcare provider if you:

  • have or have had depression, mood problems, or suicidal thoughts or behavior.
  • have an allergic reaction to similar medicines such as carboxamides, barbiturates, succinimides, and oxazolidinediones.
  • have kidney disease.
  • have liver disease.
  • have porphyria (a genetic enzyme disorder that causes symptoms affecting the skin or nervous system).
  • have diabetes or high blood sugar.
  • have a vitamin D deficiency or any other condition that causes thinning of the bones.
  • drink alcohol heavily.
  • are pregnant or plan to become pregnant.
  • are breastfeeding or plan to breastfeed.

What is important to know about taking phenytoin?

Do not stop taking phenytoin suddenly unless directed to do so by your healthcare provider.

As with all antiseizure medications, phenytoin should be withdrawn gradually to minimize the risk of causing or worsening seizures or status epilepticus. You should not stop using phenytoin suddenly unless your healthcare provider tells you to stop the medicine because of a serious side effect.

Tell your healthcare provider about all the medicines you take, including prescription and over-the-counter medicines, vitamins, and herbal supplements.

Taking phenytoin with certain other medicines may cause side effects or affect how well they work. Do not start or stop other medicines without talking to your healthcare provider. As there are a large number of medications that may interact with phenytoin, this is particularly important for this antiseizure drug. Phenytoin may also impact other medicines, so be sure to tell healthcare providers (including dentists) that you are taking it.

Phenytoin should not be taken with delavirdine.

Do not drink alcohol while you take phenytoin without first talking to your healthcare provider.

Drinking alcohol while taking phenytoin may change your blood levels of phenytoin which can cause serious problems.

Do not drive or operate machinery until you have gained sufficient experience on phenytoin to gauge whether it adversely affects your abilities.

Do not drive, operate heavy machinery, or do other dangerous activities until you know how phenytoin affects you. Phenytoin can slow your thinking and motor skills.

Phenytoin can cause overgrowth of your gums. Brushing and flossing your teeth and seeing a dentist regularly while taking phenytoin can help prevent this from happening.

Women or those who are/plan to become pregnant
Use in pregnancy

Phenytoin may harm your unborn baby if taken while pregnant.

Tell your healthcare provider if you are pregnant. Phenytoin increases the risk for serious birth defects but having a seizure during pregnancy could harm both the pregnant individual and baby. The benefit of preventing seizures may outweigh any risks to the baby. Your healthcare provider may need to adjust your dose if phenytoin is taken during pregnancy.

If you have received phenytoin during pregnancy, be sure to tell the healthcare provider who delivers your baby about your phenytoin use. Both you and the baby may need to receive medications to prevent excessive bleeding during delivery and just after birth.

If you become pregnant while taking phenytoin, talk to your healthcare provider about registering with the North American Antiepileptic Drug (NAAED) Pregnancy Registry. The purpose of this registry is to collect information about the safety of antiseizure medicine during pregnancy. You can enroll in this registry by calling 1-888-233-2334.

Use during breastfeeding

Phenytoin can pass into breast milk, but its effects on the nursing baby are not known. Tell your healthcare provider if you are breastfeeding. Your healthcare provider will consider the developmental and health benefits of breastfeeding along with your need for phenytoin and the potential effect on the infant from phenytoin or from your epilepsy.

Effect on birth control and fertility

Phenytoin may decrease the effectiveness of hormonal contraceptives, including birth control pills, injections, implants, skin patches, and vaginal rings. To prevent pregnancy after using phenytoin, use a barrier form of birth control: condom, diaphragm, cervical cap, or contraceptive sponge.

What are the side effects of phenytoin?

Phenytoin is approved by the FDA because it is safe and effective for the majority of people who take it. However, there are risks associated with all medicines. Some side effects caused by phenytoin can be very serious, and even life-threatening. It is important to be informed about these serious reactions and to be aware of their symptoms.

Common side effects

The most common side effects that were reported in studies of phenytoin are irregular movements of the eye (nystagmus), dizziness, drowsiness (somnolence), slurred speech, decreased coordination, mental confusion, and problems with movement and balance (ataxia).

Rare, but serious side effects
Suicidal thoughts or behaviors

Studies have found that people who take antiseizure medications including phenytoin may have suicidal thoughts or behaviors, which occur in approximately 1 in 500 patients. If you experience any thoughts or impulses to hurt yourself, you should contact your healthcare provider immediately.

Serious reactions, including serious blood or liver problems

Rare but life-threatening reactions involving the immune system or multi-organ hypersensitivity, which can cause serious blood or liver problems have been reported with phenytoin use. You may or may not have a rash with these types of reactions. Call your healthcare provider right away if you experience fever, frequent infections, severe muscle pain, swelling of your face, eyes, lips, or tongue, swollen lymph glands, unusual bruising or bleeding, weakness, fatigue, yellowing of your skin, or the white part of your eyes, trouble walking or seeing, seizures happening more often, or pain/tenderness in the area toward the top of your stomach (enlarged liver/spleen).

Risk of serious rash

Phenytoin may cause life-threatening rashes including Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN). While not all rashes are serious, there is no way to predict which ones will become life-threatening. Tell your healthcare provider right away if you have a skin rash, blistering or peeling of your skin, hives, or painful sores in your mouth or around your eyes.

Patients of Asian ancestry may have a higher risk of developing this serious skin reaction to phenytoin. Your healthcare provider may recommend a blood test before you start the medication to determine your risk.

Blood cell levels

Some patients taking fosphenytoin have reported that the numbers of blood cells are lower than they should be, which may weaken the immune system. Your blood may need to be tested often to monitor levels of blood cells. Call your healthcare provider right away if you have signs of infection such as:

  • fever
  • swollen glands, sore throat
  • easy bruising, unusual bleeding (nose, gums)
  • feeling very weak or tired
  • red or purple spots on your body
Cardiac effects

Phenytoin can cause problems with your heart, including a slow heartbeat. Let your healthcare provider know right away if you have any of these symptoms: dizziness, tiredness, feeling like your heart is beating slowly or skipping beats, or chest pain.