Phenytoin | Side Effects, Dosage, Uses & More
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Generic Name:

phenytoin, Oral capsule

Generic Name:
Dilantin,Phenytek

phenytoin, Oral capsule

All Brands

  • Dilantin
  • Phenytek
SECTION 1 of 5

Highlights for phenytoin

Oral capsule
1

Phenytoin is an oral drug that’s used to treat certain types of seizures.

2

Your dose will depend on your age, weight, form of the drug, or brand of the medication. Your doctor will decide the dose that’s right for you.

3

Common side effects include problems with walking and coordination, slurred speech, confusion, dizziness, trouble sleeping, nervousness, tremor, headache, nausea, vomiting, constipation, and rash.

4

This drug may cause thoughts of self-harm or suicide. Call your doctor if this happens, or if you have any unusual changes in your mood or behavior.

5

Phenytoin can cause serious rashes that may require you to be hospitalized. These rashes may even be fatal. This life-threatening rash can occur at any time, but it is most likely to happen within the first 28 days of starting this medication. Your doctor will have you stop taking phenytoin at the first sign of rash, unless the rash isn't drug related.

IMPORTANT INFORMATION

Risk of birth defects

Using this drug during pregnancy can cause serious birth defects in your baby. You shouldn’t take this drug if you’re pregnant or a woman of childbearing age, unless your doctor says it’s necessary. You should use effective birth control while you’re taking this drug.

Severe allergic reaction

This is called DRESS (drug reaction with eosinophilia and systemic symptoms). This reaction can occur 2–6 weeks after starting to take this drug and may be fatal (cause death). Symptoms can include:

  • skin rash
  • fever
  • swollen lymph glands
  • organ damage, including liver failure

Low vitamin D levels

This drug can decrease the levels of vitamin D in your body. This can lead to a vitamin D deficiency, as well as low calcium and phosphate levels in your body. Having lower amounts of these nutrients can put you at increased risk for bone softening. This can lead to broken bones, osteopenia, or osteoporosis. Talk to your doctor about whether you should take a vitamin D supplement.

Drug features

This drug is a prescription drug. It’s available in these forms: extended-release oral capsule, chewable tablet, and oral suspension.

This drug is available as a generic drug. Generic drugs usually cost less. In some cases, they may not be available in every strength or form as the brand. Talk to your healthcare provider to see if the generic will work for you.

This drug may be taken as part of a combination therapy with other seizure medications.

Why it's used

This drug is used to treat tonic-clonic and complex partial seizures in people with epilepsy. It can be used alone or in combination with other anti-seizure or antiepileptic drugs.

How it works

This drug belongs to a class of drugs called antiepileptic drugs (AEDs). A class of drugs refers to medications that work similarly.

More Details

How it works

This drug belongs to a class of drugs called antiepileptic drugs (AEDs). A class of drugs refers to medications that work similarly. They have a similar chemical structure and are often used to treat similar conditions.

This drug works by blocking the spread of seizure activity in the brain. It also helps to keep the neurons in your brain from becoming too active. This reduces how often you have seizures.

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SECTION 2 of 5

phenytoin Side Effects

Oral capsule

Most Common Side Effects

The most common side effects that occur with phenytoin include:

  • problems with walking and coordination

  • slurred speech

  • confusion

  • dizziness

  • trouble sleeping

  • nervousness

  • tremors

  • headache

  • nausea or vomiting

  • constipation

  • skin rash

If these effects are mild, they may go away within a few days or a couple of weeks. If they’re more severe or don’t go away, talk to your doctor or pharmacist.

Serious Side Effects

If you experience any of these serious side effects, call your doctor right away. If your symptoms are potentially life-threatening or if you think you’re experiencing a medical emergency, call 9-1-1.

  • serious skin rashes. Symptoms can include:

    • itchiness
    • skin redness
    • blistering or peeling of your skin
    • hives
    • painful sores in your mouth or around your eyes

    These rashes typically occur during the first 28 days of treatment with this drug, but can occur later. These rashes can include toxic epidermal necrolysis (TEN) or Stevens-Johnson syndrome (SJS), which can be fatal.

  • thoughts or behaviors of self-harm or suicide. Symptoms can include:

    • thoughts about suicide or dying
    • attempts to commit suicide
    • new or worse depression or anxiety
    • feeling agitated or restless
    • panic attacks
    • trouble sleeping
    • new or worse irritability
    • acting aggressive, angry, or violent
    • acting on dangerous impulses
    • extreme increases in activity and talking
    • other unusual changes in mood or behavior
  • multiorgan hypersensitivity. Symptoms can include:

    • skin rash
    • fever, swollen glands (lymph nodes), or sore throat that doesn’t go away or comes and goes
    • yellowing of your skin or the whites of your eyes
    • increased bruising or bleeding
    • severe tiredness or weakness
    • severe muscle pain
    • frequent infections or an infection that doesn’t go away
    • loss of appetite
    • nausea or vomiting
    • decrease in urine output
  • allergic reactions. Symptoms can include:

    • rash
    • hives
    • swelling of your lips, tongue, face
    • trouble breathing
  • low vitamin D levels. Symptoms can include:

    • low levels of calcium and phosphate
    • softening of your bones. This can lead to broken bones, osteopenia, or osteoporosis.

    Talk to your doctor about whether you should take a vitamin D supplement.

  • severe confusion. If your blood levels of phenytoin are above the recommended range, you may have severe confusion known as delirium, psychosis, or encephalopathy (brain disorder). Your doctor may lower your dose or stop treatment with phenytoin.

  • overgrowth of your gums. Brushing and flossing your teeth and seeing a dentist regularly can help prevent this from happening.

  • lowered blood cell counts. Symptoms can include:

    • increased bruising or bleeding
    • severe tiredness
    • frequent infections, or an infection that doesn’t go away
  • worsening of porphyria, an inherited blood disorder

Pharmacist's Advice
Healthline Pharmacist Editorial Team

This drug may cause drowsiness. You shouldn’t drive, use machinery, or do other tasks that require alertness until you know how this drug affects you. It can slow your thinking and motor skills.

Disclaimer: Our goal is to provide you with the most relevant and current information. However, because drugs affect each person differently, we cannot guarantee that this information includes all possible side effects. This information is not a substitute for medical advice. Always discuss possible side effects with a healthcare provider who knows your medical history.
SECTION 3 of 5

phenytoin May Interact with Other Medications

Oral capsule

Phenytoin can interact with other medications, herbs, or vitamins you might be taking. That’s why your doctor should manage all of your medications carefully. If you’re curious about how this drug might interact with something else you’re taking, talk to your doctor or pharmacist.

Note: You can reduce your chances of drug interactions by having all of your prescriptions filled at the same pharmacy. That way, a pharmacist can check for possible drug interactions.

Alcohol interaction

You shouldn’t drink alcohol while taking phenytoin. Alcohol may change your blood levels of phenytoin, which could cause serious problems.

Medications that might interact with this drug

Heart or cardiovascular drugs
  • amiodarone
  • ticlopidine

These drugs can increase the level of phenytoin in your body. This may lead to more side effects.

Cholesterol drugs
  • fluvastatin

Using this drug with phenytoin can increase the levels of both drugs in your body. This can lead to more side effects.

Antiseizure drugs
  • ethosuximide
  • felbamate
  • oxcarbazepine
  • methsuximide
  • topiramate

These drugs can increase the level of phenytoin in your body. This may lead to more side effects.

  • carbamazepine
  • vigabatrin

These drugs can lower the level of phenytoin in your body. This means that it may not work as well to control your seizures.

  • phenobarbital
  • sodium valproate or valproic acid

These drugs may increase or decrease the levels of phenytoin in your body. This means that you may have more side effects, or the phenytoin may not work as well.

Antifungal drugs
  • ketoconazole
  • itraconazole
  • miconazole
  • voriconazole
  • fluconazole

These drugs can affect the level of phenytoin in your body. Ketoconazole, itraconazole, miconazole, and voriconazole can increase it. This can lead to more side effects. Fluconazole can lower the level of phenytoin in your body. This can keep the phenytoin from working well.

Cancer drugs
  • capecitabine
  • fluorouracil

These drugs can increase the level of phenytoin in your body. This may lead to more side effects.

  • bleomycin
  • carboplatin
  • cisplatin
  • doxorubicin
  • methotrexate

These drugs can lower the level of phenytoin in your body. This means that it may not work as well to control your seizures. 

Oral birth control pills
  • pills that contain estrogen

Phenytoin may make pills that contain estrogen less effective. This means they won’t work as well to prevent pregnancy. You should use a backup form of birth control. 

Menopause drugs
  • pills that contain estrogen

Phenytoin may make pills that contain estrogen less effective. This means they won’t work as well to prevent symptoms of menopause, such as hot flashes. 

Stomach acid drugs
  • cimetidine
  • omeprazole

These drugs can affect the level of phenytoin in your body. This may lead to more side effects, or make the phenytoin not work as well.

Tuberculosis drugs
  • isoniazid

This drug can increase the level of phenytoin in your body. This may lead to more side effects.

Mental health drugs
  • chlordiazepoxide
  • disulfiram
  • fluoxetine
  • fluvoxamine
  • sertraline
  • trazodone
  • methylphenidate
  • phenothiazines, such as fluphenazine, chlorpromazine, or perphenazine

These drugs can increase the level of phenytoin in your body. This may lead to more side effects.

  • diazepam

This drug can lower the level of phenytoin in your body. This means that it may not work as well to control your seizures.

Blood thinners
  • warfarin

This drug can increase the level of phenytoin in your body. This may lead to more side effects. Also, phenytoin can affect how the body handles warfarin in several ways. Your doctor will monitor you closely when starting, stopping, or adjusting your phenytoin treatment if you also take warfarin.

Sulfonamides
  • sulfamethizole
  • sulfaphenazole
  • sulfadiazine
  • sulfamethoxazole-trimethoprim

These drugs can increase the level of phenytoin in your body. This may lead to more side effects.

Respiratory drugs
  • theophylline

Taking this drug with phenytoin can lower the levels of both drugs in your body. This means that both drugs may not work as well.

Drugs used to treat human immunodeficiency virus (HIV)
  • fosamprenavir
  • nelfinavir
  • ritonavir

These drugs can lower the level of phenytoin in your body. This means that it may not work as well to control your seizures.

Herbal supplements
  • St. John’s Wort

This drug can lower the level of phenytoin in your body. This means that it may not work as well to control your seizures.

Antacids or certain supplements
  • supplements containing:
    • calcium carbonate
    • aluminum hydroxide
    • magnesium hydroxide

These drugs can decrease how well your body absorbs phenytoin. This can keep it from working well to control your seizures. To prevent this, try to take these supplements at a different time of day than when you take phenytoin.

Disclaimer: Our goal is to provide you with the most relevant and current information. However, because drugs interact differently in each person, we cannot guarantee that this information includes all possible interactions. This information is not a substitute for medical advice. Always speak with your healthcare provider about possible interactions with all prescription drugs, vitamins, herbs and supplements, and over-the-counter drugs that you are taking.
Drug warnings
liver problems
People with liver problems

This drug is processed by your liver. If you liver isn’t working well, more of the drug may stay in your body longer. This puts you at risk for toxicity and side effects.

diabetes
People with diabetes

This drug may increase your blood sugar levels.

People with kidney problems
People with kidney problems

If you have severe kidney disease, your dose may need to be adjusted or monitored more closely.

pregnant women
Pregnant women

This drug is a category D pregnancy drug. That means two things:

  1. Studies show a risk of adverse effects to the fetus when the mother takes the drug.
  2. The benefits of taking the drug during pregnancy may outweigh the potential risks in certain cases.

This drug may cause serious birth defects in your unborn baby. You should use effective birth control while taking this medication.

Tell your doctor if you’re pregnant or plan to become pregnant. This drug should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus.

If you become pregnant while taking this drug, talk to your doctor about registering with the North American Antiepileptic Drug Pregnancy Registry. The purpose of this registry is to collect information about the safety of antiseizure medications used during pregnancy.

breast-feeding
Women who are breast-feeding

This drug may pass into breast milk and cause serious effects in a breastfeeding child. Talk to your doctor about the risks and benefits of using this drug while breastfeeding your child.

seniors
For seniors

As you get older, your body is unable to clear this drug as well, so more of the drug may stay in your body longer. This may cause this drug levels in your body to be higher than normal. This can lead to more side effects.

call the doctor
When to call the doctor

Tell your doctor if your seizures get worse or if you have any new types of seizures.

allergies
Allergies

This drug can cause a severe allergic reaction. Symptoms can include:

  • skin rash
  • hives
  • swelling of the lips, tongue, face
  • trouble breathing

Don’t take this drug again if you’ve ever had an allergic reaction to it, osphenytoin (Cerebyx), or ethotoin (Peganone) before. Taking it a second time after an allergic reaction could be fatal.

SECTION 4 of 5

How to Take phenytoin (Dosage)

Oral capsule

All possible dosages and forms may not be included here. Your dose, form, and how often you take it will depend on:

  • your age
  • the condition being treated
  • how severe your condition is
  • other medical conditions you have
  • how you react to the first dose

What are you taking this medication for?

Seizures (epilepsy)

Brand: Dilantin

Form: Oral capsule, extended-release
Strengths: 30 mg, 100 mg
Form: Chewable tablet (Infatabs)
Strengths: 50 mg
Form: Oral suspension
Strengths: 125 mg/5 mL

Brand: Phenytek

Form: Oral capsule, extended-release
Strengths: 200 mg, 300 mg

Generic: phenytoin

Form: Oral capsule, extended-release
Strengths: 100 mg, 200 mg, 300 mg
Form: Chewable tablet
Strengths: 50 mg
Form: Oral suspension
Strengths: 125 mg/5 mL
Adult dosage (ages 18–64 years)
  • Extended-release capsule (Phenytoin, Dilantin):
    • No previous treatment for seizures:
      • The recommended starting dose is one 100-mg capsule taken by mouth three times per day. Your doctor will adjust your dose based on your response.
      • The maintenance dose is one 100-mg capsule taken 34 times per day. Your doctor may increase your dose to two 100-mg capsules taken three times per day if needed.
  • Extended-release capsule (Phenytek):
    • No previous treatment for seizures:
      • The recommended starting dose is one 100-mg capsule taken by mouth three times per day. Your doctor will adjust your dose based on your response.
      • The maintenance dose is one 100 mg-capsule taken 34 times per day. Your doctor may increase your dose to one 200-mg capsule taken three times per day if needed.
    • Once daily dosing:
      • If your seizures are under control with divided doses of three 100-mg capsules per day, your doctor may switch you to one 300-mg capsule taken once per day.
  • Chewable tablet:
    • The recommended starting dose is two 50-mg tablets taken by mouth three times per day. Your doctor will adjust your dose based on your response.
    • The maintenance dose is 6–8 50-mg chewable tablets taken in divided doses as instructed by your doctor. Your doctor may increase your dose to twelve 50-mg chewable tablets per day if needed.
  • Oral suspension:
    • The recommended starting dose is one teaspoon (5 mL) taken by mouth three times per day. Your doctor will adjust your dose based on your response.
    • Your doctor may increase your dose to 5 teaspoons taken in divided doses as instructed by your doctor.
Child dosage (ages 0–17 years)
  • Extended release capsule:
    • The recommended starting dose is 5 mg/kg per day taken by mouth in 2–3 equally divided doses.
    • The maximum dose is 300 mg per day.
    • The maintenance dose is 4–8 mg/kg per day.
    • If you’re between the ages of 6–17 years, you may need an adult dose of 300 mg per day.
  • Chewable tablet:
    • The recommended starting dose is 5 mg/kg per day taken by mouth in 2–3 equally divided doses.
    • The maximum dose is 300 mg per day.
    • The maintenance dose is 4–8 mg/kg per day.
    • If you’re between the ages of 6–17 years, you may need an adult dose of 300 mg per day. If doses cannot be divided equally, you should take the larger dose before bedtime.
  • Oral suspension:
    • The recommended starting dose is 5 mg/kg per day taken by mouth in 2–3 equally divided doses.
    • The maximum dose is 300 mg per day.
    • The maintenance dose is 4–8 mg/kg per day.
    • If you’re between the ages of 6–17 years, you may need an adult dose of 300 mg per day.
Senior dosage (ages 65 years and older)

Older adults may process drugs more slowly. A normal adult dose may cause levels of phenytoin to be higher than normal in your body. If you’re a senior, you may need a lower dose or you may need a different schedule.

Special considerations

Liver or kidney disease: Phenytoin is processed by your liver and kidneys. If your liver or kidneys aren’t working well, more of this drug may stay in your body longer. This puts you at risk for side effects. Your doctor may check your phenytoin blood levels to make sure it’s safe for you to take and if your dose needs to be adjusted.

Warnings

Phenytoin tablets, capsules, and oral suspension contain different formulas of the drug. So, you cannot suddenly switch from one form to another. Your doctor would need to adjust your dosage and monitor your body’s response to the change.

Disclaimer: Our goal is to provide you with the most relevant and current information. However, because drugs affect each person differently, we cannot guarantee that this list includes all possible dosages. This information is not a substitute for medical advice. Always to speak with your doctor or pharmacist about dosages that are right for you.
Pharmacist's Advice
Healthline Pharmacist Editorial Team

This drug comes with serious risks if you don’t take it as prescribed.

If you don’t take it at all or stop taking it

Stopping this drug suddenly or not taking it at all may cause serious problems, including having more seizures, or seizures that don’t stop.

If you skip or miss doses

If you skip or miss a dose of this drug, you increase your risk of having seizures.

If you take too much

If you take too much of this drug, you may have the following symptoms:

  • uncontrolled eye movements
  • problems with muscle coordination
  • slurred speech
  • tremor
  • overactive reflexes
  • lack of energy
  • nausea
  • vomiting

You may end up in a coma or have dangerously low blood pressure.

If you take too much this drug, call your doctor or local poison control center right away. If the symptoms are severe, call 9-1-1 or go to the nearest emergency room right away.

How to tell this drug is working

If this drug is working, you should have fewer seizures.

This drug is used for long-term treatment.

Important considerations for taking this drug

This drug must be stored at the right temperature

  • Store phenytoin at room temperature between 68°F (20°C) and 77°F (25°C).
  • Keep this away from light.
  • Keep it away from high temperature.
  • Keep your drugs away from areas where they could get wet, such as bathrooms. Store this drug away from moisture and damp locations.
  • Store it in its original container and keep it tightly closed.
  • Don’t freeze phenytoin suspension.

A prescription for this medication is refillable

You should not need a new prescription for this medication to be refilled. Your doctor will write the number of refills authorized on your prescription.

Travel

When traveling with your medication:

  • Always carry your medication with you, such as in your carry-on bag.
  • Don’t worry about airport x-ray machines. They can’t hurt your medication.
  • You may need to show airport staff your pharmacy’s label to clearly identify the medication. Keep the original prescription label with you when traveling.
  • Don’t leave this medicine in the car, especially when the temperature is hot or freezing.
  • Make sure that you have enough phenytoin before you travel. You don’t want to run out of your seizure medication when you’re traveling.

Clinical monitoring

Before starting and during treatment with perampanel, your doctor may check your:

  • liver function
  • kidney function
  • blood cell counts
  • phenytoin blood levels

Your doctor may also monitor you for the following:

  • seizures. You and your doctor should monitor how often you have seizures.
  • serious skin reactions. Your doctor will monitor you for signs of a serious skin reaction.
  • suicidal thoughts and behaviors. Your doctor will monitor you for thoughts or behaviors of hurting yourself.

Most pharmacies stock the generic form of this drug

Not all pharmacies may have stock of other formulations or brand names, so call ahead.

Insurance

Many insurance companies will require a prior authorization before they approve the prescription and pay for brand name or certain formulations of phenytoin.

Are there any alternatives?

There are other drugs available to treat your condition. Some may be more suitable for you than others. Talk to your doctor about possible alternatives.

What does the pill look like?

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SECTION 5 of 5

How Much Does phenytoin Cost?

Oral capsule

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Lowest price for phenytoin

Walmart $11.62
Sams Club $13.52
Kroger Pharmacy $17.04
These prices represent the lowest priced national pharmacies for phenytoin on GoodRx. They may be lower than your insurance.

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These prices represent the lowest priced national pharmacies for phenytoin on GoodRx. They may be lower than your insurance.

Show Sources

Content developed in collaboration with University of Illinois-Chicago, Drug Information Group

Medically reviewed by Creighton University, Center for Drug Information and Evidence-Based Practice on August 19, 2015

Disclaimer: Healthline has made every effort to make certain that all information is factually correct, comprehensive, and up-to-date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or other healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.
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