Medically reviewed by Philip Thornton, DipPharm. Last updated on May 2, 2021.
What is phenytoin?
Phenytoin is used to control seizures. It does not treat all types of seizures, and your doctor will determine if it is the right medicine for you.
You should not use phenytoin if you also take delavirdine (Rescriptor), or if you are allergic to ethotoin (Peganone), fosphenytoin (Cerebyx), or mephenytoin (Mesantoin). If you are pregnant, DO NOT START TAKING this medicine unless your doctor tells you to. Phenytoin may cause harm to an unborn baby, but having a seizure during pregnancy could harm both the mother and the baby. If you become pregnant while taking this medicine, DO NOT STOP TAKING the medicine without your doctor's advice. Seizure control is very important during pregnancy and the benefits of preventing seizures may outweigh any risks posed by using phenytoin.
Report any new or worsening symptoms to your doctor, such as: mood or behavior changes, depression, anxiety, or if you feel agitated, hostile, restless, hyperactive (mentally or physically), or have thoughts about suicide or hurting yourself.
Before taking this medicine
You should not use phenytoin if you are allergic to it, or if you have ever had:
liver problems caused by phenytoin;
if you currently take delavirdine (Rescriptor).
To make sure phenytoin is safe for you, tell your doctor if you have ever had:
suicidal thoughts or actions;
a vitamin D deficiency or any other condition that causes thinning of the bones;
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 this medicine).
Some people have thoughts about suicide while taking this medicine. Your doctor will need to check your progress at regular visits. Your family or other caregivers should also be alert to changes in your mood or symptoms.
Seizure control is very important during pregnancy. Do not start or stop taking phenytoin without your doctor's advice if you are pregnant. Phenytoin may harm an unborn baby, but having a seizure during pregnancy could harm both mother and baby. Tell your doctor right away if you become pregnant while taking this medicine.
If you are pregnant, your name may be listed on a pregnancy registry to track the effects of phenytoin on the baby.
If you have taken this medicine 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.
It may not be safe to breastfeed while using this medicine. Ask your doctor about any risk.
How should I take phenytoin?
Take phenytoin exactly as prescribed by your doctor. Follow all directions on your prescription label and read all medication guides or instruction sheets. Your doctor may occasionally change your dose.
Swallow an extended-release capsule whole and do not crush, chew, break, or open it.
Phenytoin chewable tablets are not for once-per-day dosing. You must take them 2 or 3 times per day. Follow your doctor's dosing instructions very carefully.
Shake the oral suspension (liquid) before you measure a dose. Use the dosing syringe provided, or use a medicine dose-measuring device (not a kitchen spoon).
You may need frequent blood tests. You may also need a blood test when switching from one form to another. Visit your doctor regularly.
Tell your doctor if phenytoin does not seem to work as well in controlling your seizures. Do not stop using this medicine suddenly, even if you feel fine. Stopping suddenly may cause increased seizures. Follow your doctor's instructions about tapering your dose.
In case of emergency, wear or carry medical identification to let others know you have seizures.
Phenytoin can cause swelling in your gums. Brush and floss your teeth and visit your dentist regularly to help prevent this problem.
Store at room temperature away from moisture, light, and heat.
Usual Adult Dose for Seizures:
Oral (except suspension) Loading dose: Only when indicated for inpatients.
1 g orally divided in 3 doses (400 mg, 300 mg, 300 mg) given at 2 hour intervals. Then normal maintenance dosage started 24 hours after loading dose.
Initial dose: 100 mg extended release orally 3 times a day.
Maintenance dose: 100 mg orally 3 to 4 times a day. If seizure control is established with divided doses of three 100 mg capsules daily, once-a-day dosage with 300 mg of extended release phenytoin sodium may be considered. Alternatively, the dosage may need to be increased up to 200 mg orally 3 times a day, if necessary.
Suspension: Patients who have received no previous treatment may be started on 125 mg (one teaspoonful) of the suspension three times daily, and the dose is then adjusted to suit individual requirements. An increase to five teaspoonfuls daily may be made, if necessary.
IV: Do not exceed the infusion rate of 50 mg/min.
Loading dose: 10 to 15 mg/kg IV slowly.
Maintenance dose: 100 mg IV every 6 to 8 hours.
IM: Avoid the IM route due to erratic absorption.
Usual Adult Dose for Arrhythmias:
1.25 mg/kg IV every 5 minutes. May repeat up to a loading dose of 15 mg/kg, or
250 mg orally 4 times a day for 1 day, then 250 mg twice daily for 2 days
300 to 400 mg/day orally in divided doses 1 to 4 times a day
Usual Adult Dose for Status Epilepticus:
Loading dose: Manufacturer recommends 10 to 15 mg/kg by slow IV administration (at a rate not exceeding 50 mg/minute). Alternatively, generally accepted guidelines suggest 15 to 20 mg/kg by slow IV administration (at a rate not exceeding 50 mg/minute).
Maintenance rate: 100 mg orally or IV every 6 to 8 hours
Maximum rate: 50 mg/minute
Maintenance dose: IV or Oral: 100 mg every 6 to 8 hours
Usual Adult Dose for Neurosurgery:
Neurosurgery (prophylactic): 100 to 200 mg IM at about 4 hour intervals during surgery and the immediate postoperative period. (Note: While the manufacturer recommends IM administration, this route may cause severe local tissue destruction and necrosis. Some clinicians recommend the use of fosphenytoin if IM administration is necessary.) If IM administration is not necessary, accepted protocol has been 100 to 200 mg IV at about 4 hour intervals during surgery and the immediate postoperative period.
Usual Pediatric Dose for Seizures:
Status Epilepticus: Loading Dose:
Infants, Children: 15 to 20 mg/kg IV in a single or divided doses
Anticonvulsant: Loading Dose:
All ages: 15 to 20 mg/kg orally (based on phenytoin serum concentrations and recent dosing history). The oral loading dose should be given in 3 divided doses administered every 2 to 4 hours.
Anticonvulsant: Maintenance Dose:
(IV or oral) (Note: May initially divided daily dose into 3 doses/day, then adjust to suit individual requirements.)
Less than or equal to 4 weeks: Initial: 5 mg/kg/day in 2 divided doses
Usual: 5 to 8 mg/kg/day IV in 2 divided doses (may require dosing every 8 hours).
Greater than or equal to 4 weeks: Initial: 5 mg/kg/day in 2 to 3 divided doses
Usual: (may require up to every 8 hour dosing)
6 months to 3 years: 8 to 10 mg/kg/day
4 to 6 years: 7.5 to 9 mg/kg/day
7 to 9 years: 7 to 8 mg/kg/day
10 to 16 years: 6 to 7 mg/kg/day
Usual Pediatric Dose for Arrhythmias:
Greater than 1 year:
Loading Dose: 1.25 mg/kg IV every 5 minutes. May repeat up to a loading dose of 15 mg/kg.
Maintenance Dose: 5 to 10 mg/kg/day orally or IV in 2 to 3 divided doses
What happens if I miss a dose?
Take the medicine as soon as you can, but skip the missed dose if it is almost time for your next dose. Do not take two doses at one time.
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 to avoid
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.
Avoid driving or hazardous activity until you know how this medicine will affect you. Your reactions could be impaired.
Phenytoin side effects
Get emergency medical help if you have signs of an allergic reaction to phenytoin (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.
Report any new or worsening symptoms to your doctor, such as: mood or behavior changes, anxiety, panic attacks, trouble sleeping, or if you feel impulsive, irritable, agitated, hostile, aggressive, restless, hyperactive (mentally or physically), more depressed, or have thoughts about suicide or hurting yourself.
Call your doctor at once if you have:
slow or uneven heartbeats, chest pain, fluttering in your chest, and dizziness (like you might pass out);
any skin rash, no matter how mild;
fever, chills, sore throat, swollen glands;
red or swollen gums, mouth sores;
easy bruising, unusual bleeding, purple or red spots under your skin; or
liver problems - loss of appetite, upper stomach pain, dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes).
Common phenytoin side effects may include:
abnormal eye movement; or
problems with balance, coordination, 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.
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. This includes prescription and over-the-counter medicines, vitamins, and herbal products.
More about phenytoin
- Side Effects
- During Pregnancy or Breastfeeding
- Dosage Information
- Drug Images
- Drug Interactions
- Compare Alternatives
- Pricing & Coupons
- 100 Reviews
- Drug class: group I antiarrhythmics
- Advanced Reading
- Phenytoin Intravenous (Advanced Reading)
- Phenytoin Chewable Tablets
- Phenytoin Extended-Release Capsules
- Phenytoin Suspension
Related treatment guides
Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use phenytoin only for the indication prescribed.
Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.
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