Generic name: phenytoin (injection) [ FEN-i-toyn ]
Brand names: Dilantin, Phenytoin Sodium, Prompt
Drug classes: Group I antiarrhythmics, Hydantoin anticonvulsants

Medically reviewed by Drugs.com on Oct 5, 2023. Written by Cerner Multum.

What is phenytoin?

Phenytoin injection is an anticonvulsant medication that is used to treat a prolonged seizure (status epilepticus). Phenytoin injection is also used to prevent seizures during a surgery.

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

Phenytoin side effects

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 in your eyes, skin pain, red or purple skin rash that spreads and causes 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.

A phenytoin injection may cause life-threatening heart problems. Tell your caregiver if you feel weak, light-headed, or short of breath during or after an injection.

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

• a light-headed feeling, like you might pass out;

• confusion, unusual thoughts or behavior;

• fever, chills, sore throat, swollen glands;

• red or swollen gums, mouth sores;

• easy bruising, unusual bleeding;

• pain, swelling, bruising, or other irritation where the injection was given;

• purple discoloration of your skin around the IV needle, or spreading away from where the medicine was injected (may occur several days after an injection);

• increased thirst, increased urination; or

• liver problems--loss of appetite, upper stomach pain, dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes).

Common side effects of Dilantin may include:

• drowsiness, confusion;

• slurred speech;

• abnormal eye movement; or

• problems with balance or muscle movement.

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.

Related/similar drugs

gabapentin, clonazepam, lamotrigine, lorazepam, pregabalin, topiramate, diazepam

Warnings

A phenytoin injection may cause life-threatening heart problems. This medicine is usually given by injection only if you are unable to take the medicine by mouth.

If possible before you receive a phenytoin injection, tell your caregivers if you have ever had serious heart problems, or slow heartbeats that have caused you to faint.

Your heart rate, breathing, blood pressure, and other vital signs will be watched closely. Tell your caregiver if you feel weak, light-headed, or short of breath during or after an injection.

Before taking this medicine

If possible before you receive a phenytoin injection, tell your caregivers if you have ever had:

• a heart condition called 2nd or 3rd degree "AV block";

• slow heartbeats that have caused you to faint;

• liver problems caused by phenytoin; or

• if you currently take delavirdine (Rescriptor).

Tell your doctor if you have ever had:

• liver or kidney disease;

• diabetes;

• porphyria (a genetic enzyme disorder that causes symptoms affecting the skin or nervous system); or

• if you are of Asian ancestry (you may need a special blood test to determine your risk for having a skin reaction to phenytoin).

Tell your doctor if you are pregnant. Phenytoin may cause harm to an unborn baby, but having a seizure during pregnancy could harm both mother and baby. The benefit of preventing seizures may outweigh any risks to the baby.

If you have received a phenytoin injection during pregnancy, be sure to tell the doctor 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 are pregnant, your name may be listed on a pregnancy registry to track the effects of phenytoin on the baby.

Phenytoin can make birth control pills less effective. Ask your doctor about using a non-hormonal birth control (condom, diaphragm with spermicide) to prevent pregnancy.

It may not be safe to breast-feed while using this medicine. Ask your doctor about any risk.

In an emergency, you may not be able to tell caregivers if you are pregnant or breast feeding. Make sure any doctor caring for your pregnancy or your baby knows you received this medicine.

How is phenytoin given?

Phenytoin is injected into a muscle, or given as an infusion into a vein. A healthcare provider will give you this injection if you are unable to take the medicine by mouth.

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

Tell your caregivers if you feel any burning, pain, or swelling around the IV needle when phenytoin is injected.

While receiving phenytoin, you may need frequent blood tests. You may also need a blood test when switching from the injection form to the oral form of phenytoin.

You should not stop using phenytoin suddenly, even if you feel fine. Stopping suddenly may cause increased seizures. Follow your doctor's instructions about tapering your dose.

Phenytoin can cause swelling in your gums. Pay special attention to your dental hygiene while receiving this medicine. Brush and floss your teeth regularly.

What happens if I miss a dose?

Because you will receive phenytoin injections in a clinical setting, you are not likely to miss a dose.

What happens if I overdose?

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222. An overdose of phenytoin can be fatal. Overdose symptoms may include twitching eye movements, slurred speech, loss of balance, tremor, muscle stiffness or weakness, nausea, vomiting, feeling light-headed, fainting, and slow or shallow breathing.

What should I avoid while taking phenytoin?

Avoid driving or hazardous activity until you know how this medicine will affect you. Your reactions could be impaired.

Avoid drinking alcohol while you are taking phenytoin. Alcohol use can increase your blood levels of phenytoin and may increase side effects. Daily alcohol use can decrease your blood levels of phenytoin, which can increase your risk of seizures.

Ask a doctor or pharmacist before using over-the-counter medicines such as cimetidine, omeprazole, St. John's wort, or vitamins and mineral supplements that contain folic acid.

What other drugs will affect phenytoin?

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.

Many drugs can interact with phenytoin. Not all possible interactions are listed here. TELL YOUR DOCTOR ABOUT ALL OTHER MEDICINES YOU USE, and any you start or stop using during treatment with phenytoin. This includes prescription and over-the-counter medicines, vitamins, and herbal products.

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.

Medical Disclaimer

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