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Fosphenytoin sodium (fos FEN i toyn) has been approved by the FDA to treat tonic-clonic status epilepticus and as prevention and/or treatment of seizures occurring during neurosurgery. Fosphenytoin can also be substituted, for short-term use, for oral phenytoin. Fosphenytoin should be used only when oral phenytoin administration is not possible.

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 fosphenytoin? (Available formulations)

Fosphenytoin is available as a solution for intravenous or intramuscular injection and is administered by a healthcare provider.

Who should not take fosphenytoin?

If you are allergic to fosphenytoin, ethotoin, mephenytoin, or phenytoin, as well as any of the inactive ingredients, then you should not take it.

You should not take fosphenytoin if you have a history of liver problems caused by fosphenytoin or phenytoin. If you have a slow heartbeat, heart block (a condition in which electrical signals are not passed normally from the upper chambers of the heart to the lower chambers), or the heart rhythm disorder Adams-Stokes syndrome, then you should not take fosphenytoin.

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

  • have heart disease.
  • have kidney disease.
  • have liver disease.
  • have low blood pressure.
  • have porphyria (a genetic enzyme disorder that causes symptoms affecting the skin or nervous system.)
  • have diabetes.
  • drink alcohol heavily.

What is important to know about taking fosphenytoin?

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

Taking fosphenytoin 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 many medications may interact with fosphenytoin, this is particularly important for this antiseizure drug.

Fosphenytoin should not be taken with delavirdine.

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

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

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

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

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

Boxed warning

Fosphenytoin 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 fosphenytoin 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.

Cardiovascular risk

You may experience serious or life-threatening low blood pressure or irregular heart rhythms while you are receiving fosphenytoin or afterward. Tell your healthcare provider if you have or have ever had irregular heart rhythms or heart block, as they may not want you to receive fosphenytoin. Also, tell your healthcare provider if you have or have ever had heart failure or low blood pressure. If you experience any of the following symptoms, tell your healthcare provider immediately: dizziness, tiredness, irregular heartbeat, or chest pain.

You will receive fosphenytoin in a medical facility, and a healthcare provider will monitor you carefully while you are receiving the medication and for about 10 to 20 minutes afterward.

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

Fosphenytoin may harm your unborn baby if taken while pregnant.

Tell your healthcare provider if you are pregnant. Fosphenytoin increases the risk for serious birth defects. If you have received fosphenytoin during pregnancy, both you and the baby may need to receive medications to prevent excessive bleeding during delivery and just after birth. Having a seizure during pregnancy could harm both the pregnant individual and the baby. The benefit of preventing seizures may outweigh any risks to the baby. Your healthcare provider may need to adjust your dose if fosphenytoin is taken during pregnancy.

If you become pregnant while taking fosphenytoin, 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, the active form of fosphenytoin, 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 fosphenytoin and the potential effect on the infant from fosphenytoin or from your epilepsy.

Effect on birth control and fertility

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

What are the side effects of fosphenytoin?

Common side effects

For adult patients, the most common side effects of fosphenytoin are itching, irregular movements of the eye (nystagmus), dizziness, drowsiness (somnolence), and problems with movement and balance (ataxia). For pediatric patients, the most common side effects are vomiting, nystagmus, and ataxia.

Rare, but serious side effects
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 fosphenytoin 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

Fosphenytoin 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 fosphenytoin. Your healthcare provider may recommend a blood test before you start the medication to determine your risk.

Blood cell levels

For some patients, fosphenytoin may lower the numbers of blood cells, 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