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What is phenytoin?

Phenytoin is an anticonvulsant drug used to control seizure activity in the brain. It is only available with a prescription.

The FDA approved phenytoin to treat seizures in 1953. It is also used to prevent seizures.

Phenytoin is a generic drug. It is also sold under the brand names Dilantin and Phenytek.

Generic phenytoin is available in tablet, capsule, or liquid form.

The drug works by decreasing abnormal electrical activity in the brain. Your brain has billions of neurons — cells that send out electrical signals between different parts of your body. Seizures can occur when the signals are sent out abnormally.

Phenytoin is thought to work by binding to a receptor in the brain. This reduces nerve activity and prevents seizures.

What is phenytoin used to treat?

Phenytoin is an anticonvulsant medication used to prevent and treat seizures.

Your doctor may prescribe phenytoin for the following types of seizure:

  • Epilepsy

  • Complex partial seizures

  • Generalized tonic-clonic seizures

  • Status epilepticus

How do you take phenytoin?

Following your doctor’s instructions carefully is very important when taking phenytoin.

The drug is usually taken one to four times a day. However, the exact phenytoin dosage will depend on the severity of your seizures and your medical history.

Take phenytoin exactly as prescribed with or without food. Be sure to swallow the entire tablet or capsule without breaking or chewing it.

Do not skip doses or stop taking phenytoin without medical guidance. Doing so could worsen your seizures.

Seeing results

It can take four weeks¹ or longer to notice the medication’s full effects. Your doctor may start you on a low dose and increase it slowly to minimize side effects.

Potential side effects of phenytoin

Phenytoin is generally well-tolerated, but like all medications, it can cause side effects.

Common side effects

Some of the common side effects of phenytoin include:

  • Drowsiness

  • Fatigue

  • Dizziness

  • Loss of coordination

  • Confusion

  • Nausea

  • Vomiting

  • Constipation

  • Headache

  • Blurred vision

  • Excess hair growth

  • Slurred speech

These side effects should ease, but see your doctor if they worsen or persist.

Serious side effects

Phenytoin can also cause serious side effects that may be life-threatening. Serious phenytoin side effects include:

  • Allergic reactions, such as purple or red skin rash, blisters, itching, difficulty breathing, or swelling of the face, lips, tongue, or throat

  • Liver damage with symptoms such as nausea, vomiting, yellowing of the skin or eyes, pain in the upper right side of the abdomen, or decreased appetite

  • Bone marrow suppression, with symptoms such as fatigue, bruising or bleeding, increased infections, or pale skin

  • Heart problems, with symptoms such as chest pain, irregular heartbeat, shortness of breath, or fainting

Seek medical attention immediately if you develop any of these symptoms.

Long-term use of phenytoin

Long-term use of phenytoin can lead to osteoporosis, a condition where the bones weaken and become brittle.

While measures like exercising and not smoking help, your doctor may prescribe calcium and vitamin D supplements to avoid osteoporosis.

DRESS (drug reaction with eosinophilia and systemic symptoms) is another possible effect of long-term phenytoin use.

Missed doses

If you miss a dose, take it as soon as possible. If it is almost time for your next dose, skip the missed dose and continue with your normal schedule. Do not take two doses at once.

Set a reminder to take phenytoin at the same time each day.


A phenytoin overdose can occur if you intentionally or accidentally take more than your prescribed dose.

Symptoms of overdose can include:

  • Loss of coordination

  • Fatigue

  • Slurred speech

  • Vomiting

  • Uncontrollable shaking

  • Unconsciousness

  • Shortness of breath

Seek emergency medical help right away if you believe you or someone else has taken too much phenytoin, or if you develop these symptoms.

Allergy information

Phenytoin can cause serious hypersensitivity reactions in some people. While mild skin rash is the most common reaction, some people have developed more serious reactions, including diffuse skin rash, fever, and organ damage.

Contact your doctor immediately if you develop a skin rash or other reactions while taking phenytoin.

What to discuss with your doctor before taking phenytoin

Before taking phenytoin, tell your doctor if:

  • You are allergic to phenytoin

  • You have a mental health condition

  • You have liver, kidney, or heart disease

  • You have a history of suicidal thoughts

  • You are pregnant or plan to become pregnant

  • You are breastfeeding

  • You are taking any type of herbal medicine or vitamin supplement

  • You have a history of addiction or you are addicted to alcohol or other drugs

Phenytoin may not be right for you if you have certain medical conditions. Tell your doctor about all your medical conditions to make sure phenytoin is safe for you.

Stopping phenytoin

Only stop taking phenytoin if your doctor tells you to. They may taper your dosage to prevent withdrawal effects.

Do not suddenly stop taking phenytoin. Studies show this can increase the frequency or intensity of seizures. It may also cause status epilepticus, a life-threatening condition marked by continuous seizures.

Who should not take phenytoin

Phenytoin is not suitable for everyone. Your doctor may decide phenytoin is unsafe for you if:

  • You are allergic to phenytoin or have had allergic reactions to similar medications

  • You have kidney, heart, or liver problems

  • You have a mental health condition

  • You have a history of suicidal thoughts

  • You have diabetes

  • You have a condition called porphyria

  • You have existing skin conditions, such as contact dermatitis, rash, or blisters

  • You are pregnant or planning a pregnancy

Phenytoin and pregnancy

Phenytoin is a pregnancy category D drug², meaning studies in pregnant humans have shown it can harm a baby.

Some studies have shown phenytoin can increase the risk of certain birth defects. These can include delayed development, heart defects, cleft palate, and certain facial features. Known as fetal hydantoin syndrome, these birth defects have been seen in 10%³ of pregnant women who take phenytoin.

More research is needed to establish the safety of phenytoin in pregnancy, but be sure to tell your doctor if you are pregnant or are planning a pregnancy.

Phenytoin and breastfeeding

Phenytoin is thought to be safe to use while breastfeeding. Because breast milk contains deficient levels of phenytoin, it does not harm a nursing baby. However, talk to your doctor about breastfeeding while taking phenytoin to check if it’s safe to do so.

Interactions with other drugs

Taking phenytoin with other medications and/or alcohol can affect how well each medication works.

The following drugs can affect the level of phenytoin in your body:

  • Amiodarone (Cordarone and Pacerone), a heart medication

  • Antibiotics

  • Cimetidine (Tagamet), a medication used to treat stomach ulcers

  • Diazepam (Valium), an anti-anxiety medication

  • Birth control pills

  • Salicylates, such as aspirin

  • Other seizure medications, such as carbamazepine (Epitol, Carbatrol, and others)

Phenytoin can make these drugs less effective:

  • Corticosteroids

  • Blood thinners, such as warfarin

  • Heart medications, such as digoxin (Digitek and Lanoxin)

  • Antibiotics, such as doxycycline (Acticlate, Doryx, Monodox, and others)

  • Birth control pills

  • Diuretics, such as furosemide (Lasix)

  • Antidepressants

  • Vitamin D

Before starting phenytoin treatment, tell your doctor about all prescription and over-the-counter drugs you are currently taking, and any herbal products or supplements.

Drug approval history

1953: The FDA approves phenytoin to treat seizures under the brand name Dilantin. The 30mg/5ml and 125mg/5ml oral suspension formulations were also approved.

1956: The FDA approves the injectable form of Dilantin. Many phenytoin formulations, including injectables, oral capsules, and suspensions, have been approved by the FDA since 1956.

1998: The FDA approves phenytoin oral capsules and an extended-release formulation to be sold under the brand name Phenytek.

2012: A chewable generic version of Dilantin is approved by the FDA.

Tips and advice for taking phenytoin

The following tips and advice can help you take phenytoin safely and effectively:

  • Store the drug at room temperature, away from light and moisture.

  • Take phenytoin with or without food. Swallow the tablet or capsule whole without breaking or chewing it.

  • Take phenytoin exactly as prescribed by your doctor. Don’t take more or less than the prescribed dose. Don’t stop taking it abruptly.

  • Ask your doctor if you can drink alcohol when taking phenytoin. It is known to change phenytoin levels in the body by making them too low, increasing the risk of seizures, or making them too high, increasing the risk of serious side effects.

  • Do not drive or do anything that requires high concentration until you know how phenytoin affects you. It can cause sleepiness, dizziness, and poor coordination.

  • Phenytoin can make birth control medication less effective. Tell your doctor you take birth control pills before starting phenytoin treatment.

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Here at HealthMatch, we’ve done our best to ensure that the information provided in this article is helpful, up to date, and, most importantly, accurate.

However, we can’t replace the one-to-one advice of a qualified medical practitioner or outline all of the possible risks associated with this particular drug and your circumstances.

It is therefore important for you to note that the information contained in this article does not constitute professional medical or healthcare advice, diagnosis or recommendation of treatment and is not intended to, nor should be used to, replace professional medical advice. This article may not always be up to date and is not exhaustive of all of the risks and considerations relevant to this particular drug. In no circumstances should this article be relied upon without independent consideration and confirmation by a qualified medical practitioner.

Your doctor will be able to explain all possible uses, dosages, precautions, interactions with other drugs, and other potential adverse effects, and you should always talk to them about any kind of medication you are taking, thinking about taking or wanting to stop taking.

Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.