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Generic name: fosphenytoin [ fos-FEN-i-toyn ]
Brand names: Cerebyx, Sesquient
Dosage form: injectable solution (100 mgPE/2 mL; 500 mgPE/10 mL)
Drug class: Hydantoin anticonvulsants

Medically reviewed by on May 10, 2024. Written by Cerner Multum.

What is fosphenytoin?

Fosphenytoin is an anticonvulsant that is used to prevent or control seizures. Fosphenytoin is used short term or when other forms of phenytoin cannot be given.

Fosphenytoin may also be used for purposes not listed in this medication guide.

Fosphenytoin side effects

Stop using fosphenytoin and get emergency medical help if you have signs of an allergic reaction (hives, difficult breathing, swelling in your face or throat) or a severe skin reaction (fever, sore throat, burning eyes, skin pain, red or purple skin rash with blistering and peeling).

Seek medical treatment if you have a serious drug reaction that can affect many parts of your body. Symptoms may include: skin rash, fever, swollen glands, muscle aches, severe weakness, unusual bruising, or yellowing of your skin or eyes.

Fosphenytoin may cause serious side effects. Call your doctor at once if you have:

Common side effects of fosphenytoin may include:

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.


Use only as directed. Tell your doctor if you use other medicines or have other medical conditions or allergies.

Before taking this medicine

You should not use fosphenytoin if you are allergic to fosphenytoin or phenytoin, or if you have:

Tell your doctor if you have ever had:

Patients of Asian ancestry may have a higher risk of developing a rare but serious skin reaction to fosphenytoin. You may need a blood test to determine your risk of this skin reaction.

Do not start or stop seizure medication during pregnancy without your doctor's advice. Fosphenytoin may harm an unborn baby, but having a seizure during pregnancy could harm both mother and baby. Preventing seizures may outweigh these risks. Tell your doctor right away if you become pregnant.

There may be other seizure medications that are safer to use during pregnancy.

If you receive fosphenytoin during pregnancy, be sure to tell the doctor who delivers your baby. Both you and the baby may need medication to prevent bleeding during delivery and just after birth.

Fosphenytoin can make birth control pills less effective. Ask your doctor about other birth control options such as an injection, implant, skin patch, vaginal ring, condom, diaphragm, cervical cap, or contraceptive sponge.

Ask a doctor if it is safe to breastfeed while using this medicine.

How is fosphenytoin given?

Fosphenytoin is injected into a muscle or a vein by a healthcare provider.

You will be watched closely for 10 to 20 minutes to make sure this medicine does not cause harmful effects.

Your breathing, blood pressure, heart rhythm, oxygen levels, and other vital signs will be watched closely.

This medicine can affect the results of certain medical tests. Tell any doctor who treats you that you are using fosphenytoin.

Seizures may increase if you stop using fosphenytoin suddenly.. Ask your doctor before stopping the medicine.

What happens if I miss a dose?

Call your doctor for instructions if you miss a dose of fosphenytoin.

What happens if I overdose?

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222. An overdose can be fatal.

Overdose symptoms may include weakness, nausea, vomiting, feeling light-headed, fast or slow heart rate, tingling, muscle spasms, seizures, or slow breathing (breathing may stop).

What should I avoid while using fosphenytoin?

Avoid drinking alcohol.

What other drugs will affect fosphenytoin?

Sometimes it is not safe to use certain medications at the same time. Some drugs can affect your blood levels of other drugs you take, which may increase side effects or make the medications less effective.

Using fosphenytoin with other drugs that make you drowsy can worsen this effect. Ask your doctor before using opioid medication, a sleeping pill, a muscle relaxer, or medicine for anxiety or seizures.

Many drugs can affect fosphenytoin. This includes prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible interactions are listed here. Tell your doctor about all other medicines you use.

Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use this medication only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.