Phenytoin | Side Effects, Dosage, Uses, And More - Healthline

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Important warnings

Birth defects warning: Using this drug during pregnancy can cause serious abnormalities. You shouldn’t take this drug if you’re pregnant or able to have kids unless your doctor says it’s necessary. Use effective birth control while you’re taking this drug.
Severe allergic reaction warning: This is reaction is called DRESS, which stands for “drug reaction with eosinophilia and systemic symptoms.” It can occur 2–6 weeks after starting this drug and may be fatal (cause death). Symptoms can include:
- skin rash
- fever
- swollen lymph glands
- organ damage, including liver failure
Serious skin reactions warning: This drug may cause life-threatening allergic reactions. These are called Stevens-Johnson syndrome, or SJS, and toxic epidermal necrolysis, or TEN. Either can cause severe damage to your skin or internal organs and can lead to death. Call your doctor right away if you have any of these symptoms while taking this drug:
- skin rash
- hives
- sores in your mouth
- blistering or peeling of your skin
Increased seizure warning: Do not stop taking this drug unless your doctor tells you to. Doing so may cause you to have more severe seizures. If your doctor is stopping your treatment with phenytoin, they will lower your dosage slowly to reduce your risk of more seizures
Low vitamin D levels warning: 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. 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 while taking this drug.
Thyroid warning: This drug can affect your thyroid hormone levels. If you have thyroid disease, discuss this condition with your doctor before taking phenytoin.
Angioedema (swelling under the skin) warning: This drug can cause dangerous and sometimes deadly swelling of your face, lips, throat, or tongue. That can make it hard to breathe or swallow. If you develop these symptoms, stop taking this medication and seek medical help immediately.
Suicidal thoughts or actions warning: This drug may increase your risk of suicidal thoughts or actions. Your risk may be higher if you already have a mood disorder, such as depression or anxiety. Call your doctor right away if you have any changes in mood or behavior.
Heart problems warning: This drug can cause heart problems, such as a slow heartbeat, that can lead to death. Call your doctor right away if you have any of these symptoms:
- dizziness
- tiredness
- feeling like your heart is beating slowly or skipping beats
- chest pain

What is phenytoin?

Phenytoin is a prescription drug. It comes in three oral forms: extended-release capsule, chewable tablet, and suspension. It also comes as an injectable medication that’s given by a healthcare provider.

Phenytoin oral capsule is available as the brand-name drugs Phenytek and Dilantin. It’s also available as a generic drug. Generic drugs usually cost less than the brand-name version. In some cases, they may not be available in every strength or form as the brand-name drug.

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

Why it’s used

Phenytoin oral capsule 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.

It is also used to treat and prevent seizures during and after brain surgery.

How it works

Phenytoin belongs to a class of drugs called antiepileptic drugs (AEDs). A class of drugs is a group of medications that work in a similar way. These drugs are often used to treat similar conditions.

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

Phenytoin side effects

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

Phenytoin can also cause other side effects.

More common side effects

The more common side effects that can occur with phenytoin include:

problems with walking and coordination
slurred speech
confusion
dizziness
trouble sleeping
nervousness
tremors
headache
nausea or vomiting
constipation
skin rash
irregular movement of the eye

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

Call your doctor right away if you have serious side effects. Call 911 if your symptoms feel life-threatening or if you think you’re having a medical emergency. Serious side effects and their symptoms can include the following:

Serious skin rashes. These rashes typically occur during the first 28 days of treatment with this drug, but can occur later. They can include toxic epidermal necrolysis (TEN) or Stevens-Johnson syndrome (SJS), which can be fatal. Your doctor will have you stop taking phenytoin at the first sign of rash, unless the rash isn’t drug-related. Symptoms of these rashes can include:
- itchiness
- skin redness
- blistering or peeling of your skin
- hives
- painful sores in your mouth or around your eyes
Suicidal thoughts or behaviors. Symptoms can include:
- thoughts about suicide or dying
- attempts to commit suicide
- new or worsened depression or anxiety
- feeling agitated or restless
- panic attacks
- trouble sleeping
- new or worsened 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 (can lead to broken bones, osteopenia, or osteoporosis)
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 dosage or stop your 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
Angioedema (swelling under the skin). Symptoms can include:
- swelling of your face, lips, throat, or tongue
- trouble breathing, talking, or swallowing

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.

Phenytoin may interact with other medications

Phenytoin oral capsule can interact with other medications, vitamins, or herbs you may be taking. An interaction is when a substance changes the way a drug works. This can be harmful or prevent the drug from working well.

To help avoid interactions, your doctor should manage all of your medications carefully. Be sure to tell your doctor about all medications, vitamins, or herbs you’re taking. To find out how this drug might interact with something else you’re taking, talk to your doctor or pharmacist.

Examples of drugs that can cause interactions with phenytoin are listed below.

Heart or cardiovascular drugs

Taking these drugs with phenytoin can increase the level of phenytoin in your body. This may lead to more side effects. Examples of these drugs include:

amiodarone
ticlopidine

Cholesterol drug

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

Anti-seizure drugs

Taking these anti-seizure drugs with phenytoin can increase the level of phenytoin in your body. This may lead to more side effects. Examples of these drugs include:

ethosuximide
felbamate
oxcarbazepine
methsuximide
topiramate

Taking these anti-seizure drugs with phenytoin can lower the level of phenytoin in your body. This means that it may not work as well to control your seizures. Examples of these drugs include:

carbamazepine
vigabatrin

Taking these anti-seizure drugs with phenytoin can increase or decrease the levels of phenytoin in your body. This means that you may have more side effects, or that phenytoin may not work as well. Examples of these drugs include:

phenobarbital
sodium valproate or valproic acid

Antifungal drugs

Taking certain antifungal drugs with phenytoin can increase the level of phenytoin in your body. This can lead to more side effects. Examples of these drugs include:

ketoconazole
itraconazole
miconazole
voriconazole
fluconazole

Taking fluconazole with phenytoin can lower the level of phenytoin in your body. This can keep phenytoin from working well.

Antibiotics

Taking certain antibiotics with phenytoin can increase the level of phenytoin in your body. This may lead to more side effects. Examples of these drugs include:

chloramphenicol

Cancer drugs

Taking certain cancer drugs with phenytoin can increase the level of phenytoin in your body. This may lead to more side effects. Examples of these drugs include:

capecitabine
fluorouracil

Taking other cancer drugs with phenytoin can lower the level of phenytoin in your body. This means that it may not work as well to control your seizures. Examples of these drugs include:

bleomycin
carboplatin
cisplatin
doxorubicin
methotrexate

Diabetes drugs

Taking certain diabetes drugs with phenytoin can increase the level of phenytoin in your body. This may lead to more side effects. Examples of these drugs include:

tolbutamide

Oral birth control pills

Taking phenytoin with oral birth control pills that contain estrogen can make those pills less effective. This means they won’t work as well to prevent pregnancy. You should use a backup form of birth control if taking these drugs together.

Menopause drugs

Taking phenytoin with oral pills that contain estrogen can make those pills less effective. This means they won’t work as well to prevent symptoms of menopause, such as hot flashes.

Stomach acid drugs

Taking certain stomach acid drugs with phenytoin can affect the level of phenytoin in your body. This may lead to more side effects, or make the phenytoin not work as well. Examples of these drugs include:

cimetidine
omeprazole

Tuberculosis drug

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

Mental health drugs

Taking certain mental health drugs with phenytoin can increase the level of phenytoin in your body. This may lead to more side effects. Examples of these drugs include:

chlordiazepoxide
disulfiram
fluoxetine
fluvoxamine
sertraline
trazodone
methylphenidate
phenothiazines, such as fluphenazine, chlorpromazine, or perphenazine

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

Blood thinner

Taking warfarin with phenytoin 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

Taking these drugs with phenytoin can increase the level of phenytoin in your body. This may lead to more side effects. Examples of these drugs include:

sulfamethizole
sulfaphenazole
sulfadiazine
sulfamethoxazole-trimethoprim

Respiratory drug

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

HIV drugs

Taking certain HIV drugs with phenytoin can lower the level of phenytoin in your body. This means that it may not work as well to control your seizures. Examples of these drugs include:

fosamprenavir
nelfinavir
ritonavir

Do not take delavirdine together with phenytoin. Taking these medications together will prevent delavirdine from working and treating HIV.

Herbal supplement

Taking St. John’s wort with phenytoin can lower the level of phenytoin in your body. This means it may not work as well to control your seizures.

Antacids or certain supplements

Taking certain antacids or supplements with phenytoin 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. Examples of these drugs include supplements containing:

calcium carbonate
aluminum hydroxide
magnesium hydroxide

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.

Phenytoin warnings

This drug comes with several warnings.

Allergy warning

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

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

If you develop these symptoms, call 911 or go to the nearest emergency room.

Don’t take this drug again if you’ve ever had an allergic reaction to it. Taking it again could be fatal (cause death).

Alcohol interaction warning

You shouldn’t drink alcohol while taking phenytoin. Alcohol may change the amount of phenytoin in your body, which could cause serious problems.

Warnings for people with certain health conditions

For people with liver problems: This drug is processed by your liver. If your 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.

For people with diabetes: This drug may increase your blood sugar levels.

For people with kidney problems: If you have severe kidney disease, your dosage may need to be adjusted or monitored more closely.

For people with thyroid disease: This drug can affect your thyroid hormone levels. Discuss your thyroid condition with your doctor before you take phenytoin.

Warnings for other groups

For pregnant women: Phenytoin may cause serious birth defects. 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.

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 anti-seizure medications used during pregnancy.

When to call the doctor

Tell your doctor if your seizures get worse or if you have any new types of seizures while taking this drug.

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

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 the amount of this drug in your body to be higher than normal. This can lead to more side effects.

People with thyroid disease: This drug can affect your thyroid hormone levels. If you have thyroid disease, discuss this condition with your doctor before taking phenytoin.

People of Asian ancestry: People who are Asian or of Asian ancestry with a genetic risk factor have a higher chance of developing SJS and TEN. Your doctor may test you to see if you have the HLA-B*1502 allele. If your results are positive, you may have a higher risk of these serious and sometimes fatal skin reactions. On the other hand, you can still have these reactions without the genetic risk factor.

How to take phenytoin

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

Drug forms and strengths

Generic: Phenytoin

Form: oral capsule, extended-release
Strengths: 100 mg, 200 mg, 300 mg

Brand: Dilantin

Form: oral capsule, extended-release
Strengths: 30 mg, 100 mg

Brand: Phenytek

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

Dosage for seizures (epilepsy)

Adult dosage (ages 18–64 years)

Extended-release capsule (Generic phenytoin, Dilantin)

No previous treatment for seizures:
- Typical starting dosage: One 100-mg capsule taken three times per day. Your doctor will adjust your dosage based on your body’s response.
- Maintenance dosage: One 100-mg capsule taken 3–4 times per day. Your doctor may increase your dosage to two 100-mg capsules taken three times per day if needed.

Extended-release capsule (Phenytek)

No previous treatment for seizures:
Typical starting dosage: One 100-mg capsule taken three times per day. Your doctor will adjust your dosage based on your body’s response.
Maintenance dosage: One 100 mg-capsule taken 3–4 times per day. Your doctor may increase your dosage 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.

Child dosage (ages 0–17 years)

Extended-release capsule

Typical starting dosage: 5 mg/kg per day taken in 2–3 equally divided doses.
Maintenance dosage: 4–8 mg/kg per day in equally divided doses.
Maximum dosage: 300 mg per day.
Note: If your child is between the ages of 6–17 years, they may need an adult dosage of 300 mg per day.

Senior dosage (ages 65 years and older)

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

Special dosage considerations

For people with 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 to check if your dosage needs to be adjusted.

Dosage 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 speak with your doctor or pharmacist about dosages that are right for you.

Take as directed

Phenytoin oral capsule is used for long-term treatment. It 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
blurred vision

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

If you take too much of this drug, call your doctor or local poison control center right away. If the symptoms are severe, call 911 or go to the nearest emergency room right away.

How to tell this drug is working: You should have fewer seizures.

Important considerations for taking phenytoin

Keep these considerations in mind if your doctor prescribes phenytoin for you.

General

Do not open, cut, or crush the capsules.

Storage

Store this drug at room temperature between 68°F and 77°F (20°C and 25°C).
Keep this away from light.
Keep it away from high temperature.
Don’t store this medication in moist or damp areas, such as bathrooms.
Store this drug in its original container and keep it tightly closed.

Refills

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. When flying, never put it into a checked bag. Keep it in your carry-on bag.
Don’t worry about airport X-ray machines. They can’t harm your medication.
You may need to show airport staff the pharmacy label for your medication. Always carry the original prescription-labeled container with you.
Don’t put this medication in your car’s glove compartment or leave it in the car. Be sure to avoid doing this when the weather is very hot or very cold.

Clinical monitoring

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

liver function
kidney function
blood cell counts
blood levels of this drug

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 signs of wanting to harm yourself.

Availability

Most pharmacies stock the generic form of this drug, but not all pharmacies may have stock of other formulations or brand names. When filling your prescription, be sure to call ahead to make sure your pharmacy carries the form your doctor prescribed for you.

Prior authorization

Many insurance companies require a prior authorization for this drug. This means your doctor will need to get approval from your insurance company before your insurance company will pay for the prescription.

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.

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.

Phenytoin, oral capsule images

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How we reviewed this article:

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DILANTIN- extended phenytoin sodium capsule. (2021).https://dailymed.nlm.nih.gov/dailymed/fda/fdaDrugXsl.cfm?setid=8848de76-8d74-4620-bcc7-a86a596e5dd9&type=display#S5.14
Dilantin - phenytoin sodiumcapsule, extended release. (2021).http://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=8848de76-8d74-4620-bcc7-a86a596e5dd9
Dilantin infatabs - phenytointablet chewable. (2021).http://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=ca119a89-2394-4d34-8078-cd1fa4e8e2f1
Dilantin-125 - phenytoinsuspension. (2021).http://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=051742b5-b0f0-44a5-8d31-3288527b0638
Phenytek - phenytoin sodiumcapsule, extended release. (2020).http://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=0f94c791-8b07-4b41-927e-c9f28bec5f60
Phenytoin sodium - phenytoinsodium capsule, extended release. (2020).http://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c40f66cc-6f4a-4b29-84ba-d265e4848735
Phenytoin - phenytoin tablet,chewable. (2019).http://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=15d32f84-14f6-49a6-9bad-d7f4b14eaf14
Phenytoin - suspensionsuspension. (2019).http://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=093fd736-5971-47af-b4ef-08c1696cebe8

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Medically reviewed by Femi Aremu, PharmD — Written by University of Illinois — Updated on April 14, 2021

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Important warnings

What is phenytoin?

Why it’s used

How it works

Phenytoin side effects

More common side effects

Serious side effects

Phenytoin may interact with other medications

Heart or cardiovascular drugs

Cholesterol drug

Anti-seizure drugs

Antifungal drugs

Antibiotics

Cancer drugs

Diabetes drugs

Oral birth control pills

Menopause drugs

Stomach acid drugs

Tuberculosis drug

Mental health drugs

Blood thinner

Sulfonamides

Respiratory drug

HIV drugs

Herbal supplement

Antacids or certain supplements

Phenytoin warnings

Allergy warning

Alcohol interaction warning

Warnings for people with certain health conditions

Warnings for other groups

When to call the doctor

How to take phenytoin

Drug forms and strengths

Dosage for seizures (epilepsy)

Special dosage considerations

Dosage warnings

Take as directed