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Phenytoin is used to treat certain types of seizures. Common side effects include drowsiness, confusion, slurred speech, trouble with balance or coordination, and uncontrollable eye movements. Brush and floss your teeth daily to help prevent your gum tissue from growing too much. Call your health care provider right away if you get a rash, unusual bleeding, sore throat, fever, vomiting, or changes in your mood or behavior. Do not stop this medicine or start or stop any other medicine without talking with your health care provider.

Uses

What is phenytoin used for?

Phenytoin is commonly used to treat certain types of seizures. It is also used to prevent and treat seizures during or after neurosurgery, such as surgery on your brain, spinal cord, or nerves.

Phenytoin may also be used for other conditions as determined by your health care provider.

How does phenytoin work (mechanism of action)?

Seizures happen when too many nerve signals are sent between brain cells too quickly. Phenytoin is thought to work by slowing down overactive brain signals and making seizures less likely.

How is phenytoin supplied (dosage forms)?

Brands and other names

  • Dilantin
  • Phenytek

Dosage forms and strengths

  • 50 mg chewable tablets
  • 30 mg, 100 mg, 200 mg, 300 mg extended oral capsules
  • 125 mg/5 mL oral suspension

How should I store phenytoin?

Phenytoin should be stored at room temperature, between 68 F and 77 F (20 C and 25 C). It can be exposed to temperatures between 59 F and 86 F (15 C and 30 C) for shorter periods of time, such as when transporting it. Store in a cool, dry place. Protect from light and moisture. Do not freeze phenytoin liquid.

Side Effects

What are the most common side effects of phenytoin?

The most common side effects of phenytoin are listed below. Tell your health care provider if you have any of these side effects that bother you.

  • Uncontrollable eye movements
  • Trouble with balance or coordination
  • Slurred speech
  • Drowsiness
  • Confusion

There may be other side effects of phenytoin that are not listed here. Contact your health care provider if you think you are having a side effect of a medicine. In the U.S., you can report side effects to the FDA at www.fda.gov/medwatch or by calling 800-FDA-1088 (800-332-1088). In Canada, you can report side effects to Health Canada at www.health.gc.ca/medeffect or by calling 866-234-2345.

What are the serious side effects of phenytoin?

While less common, the most serious side effects of phenytoin are described below, along with what to do if they happen.

Suicidal Thoughts and Actions. Phenytoin may cause suicidal thoughts or actions in a small number of people. If you are about to harm yourself, call 911 or call or text 988, the Suicide & Crisis Lifeline. Call your health care provider right away if you have any of the following symptoms.

  • New or increased thoughts of suicide or death
  • Suicide attempt
  • New or increased feelings of anxiety, depression, or other unusual changes in mood or behavior

Severe Skin Reactions. Phenytoin can cause severe skin reactions, called Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN), that can lead to death if not treated. If you develop a rash, call your health care provider right away. Get emergency help if you have any of the following symptoms of SJS or TEN.

  • Painful red or purple skin that looks burned and peels off
  • Flat red rash or blisters on your skin, mouth, nose, and genitals
  • Red, painful, watery eyes

Severe Allergic Reactions. Phenytoin may cause allergic reactions, including a specific type of allergic reaction called DRESS. DRESS stands for drug reaction with eosinophilia and systemic symptoms. It is also sometimes called multiorgan hypersensitivity. This is a reaction that can affect multiple parts of the body including your liver, kidneys, and heart. Get help right away if you have any of the following symptoms of a serious allergic reaction.

  • Breathing problems or wheezing
  • Racing heart
  • Itching, skin rash, or pale red bumps on the skin called hives
  • Fever or general ill feeling
  • Swollen lymph nodes
  • Swelling of the face, lips, mouth, tongue, or throat
  • Shortness of breath, trouble swallowing, or throat tightness
  • Swelling in your feet, ankles, or legs
  • Nausea or vomiting
  • Dizziness, feeling lightheaded, or fainting
  • Stomach cramps
  • Joint pain
  • Dark-colored pee
  • Your skin or the whites of your eyes turning yellowish in color (also called jaundice)

Heart Rate Changes. Phenytoin can make your heart beat slower than normal (bradycardia). In rare cases, this may cause the heart to suddenly stop beating (cardiac arrest). Some people have a higher risk of this, including people with existing heart disease or who take other medicines that affect the heart. Call your health care provider right away if you have any of the following symptoms.

  • Feeling lightheaded, dizziness, or fainting
  • Confusion
  • Feeling weak or easily tired
  • Slow heartbeat or feeling of skipped heartbeats
  • Chest pain
  • Shortness of breath

Liver Damage. Liver damage, also called hepatotoxicity, can happen when taking phenytoin. Call your health care provider right away if you have any of the following symptoms of liver damage.

  • Nausea or vomiting
  • Stomach or belly pain
  • Fever
  • Weakness or unusual tiredness
  • Itching
  • Loss of appetite
  • Light-colored poop
  • Dark-colored urine (pee)
  • Your skin or the whites of your eyes turning yellowish in color (also called jaundice)

Blood Disorders. Phenytoin can cause blood disorders, such as agranulocytosis, low platelet levels (thrombocytopenia), low white blood cell levels (leukopenia), and pancytopenia. Get help right away if you have any of the following symptoms of blood disorders.

  • Fever
  • Shortness of breath
  • Pale or yellowish skin
  • Easy bruising or bleeding
  • Frequent infection
  • Unusual weakness or tiredness
  • Dizziness, lightheadedness, or feeling like you are about to pass out
  • Headache
  • Fast or abnormal heartbeat

Weak Bones. Using phenytoin for a long time may cause your bones to become weaker, leading to reduced bone density (osteopenia). It may also cause a more serious condition called osteoporosis, which can cause your bones to break (fracture) more easily. Tell your health care provider if you break a bone, and get a bone mineral density test if one is ordered for you.

High Blood Sugar. Phenytoin may increase your blood sugar levels. Talk to your health care provider about how to maintain healthy blood sugar levels. Keep all appointments with your health care provider for blood tests. Call your health care provider if you have the following symptoms of high blood sugar.

  • Thirsty more than usual
  • Peeing more than usual
  • Increase in hunger
  • Tiredness
  • Feeling sick to your stomach
  • Confusion
  • Your breath smelling fruity

Hallucinations or Psychotic-Like Behaviors. Phenytoin may cause hallucinations or psychotic-like behaviors. This is more likely to happen when the dose of phenytoin is too high. Tell your health care provider right away if you have any of the following symptoms or if you have been told by others you have these symptoms.

  • Hearing, seeing, smelling, tasting, or otherwise perceiving things that others do not
  • Delusional thoughts, such as false beliefs that you are being controlled
  • Disorganized thoughts, such as jumping between topics or having a hard time focusing
  • Sudden decline in self-care, such as personal hygiene or academic/work performance
  • Aggressive behavior

Warnings & Precautions

Who should not use phenytoin?

Allergies to Ingredients. People who are allergic to any of the following should not take phenytoin.

  • Dilantin
  • Phenytek
  • Phenytoin
  • Any other medicine known as a hydantoin, such as ethotoin, fosphenytoin, or mephenytoin
  • Any of the ingredients in the specific product dispensed

Your pharmacist can tell you all of the ingredients in the specific phenytoin products they stock.

Liver Problems. People who have had liver damage from taking phenytoin should not take it again.

What should I know about phenytoin before using it?

Do not take phenytoin unless it has been prescribed to you by a health care provider. Take it as prescribed.

If you are taking the chewable tablets, make sure to chew them thoroughly before swallowing.

If you are taking the liquid form, use an accurate measuring device to measure your dose. A household spoon is not an accurate measuring device and may cause you to take the wrong dose. Ask your pharmacist to recommend a right measuring device. Shake well before using.

Taking antacids or supplements with phenytoin can make it not work as well. Talk with your health care provider about the best way to take antacids and supplements while taking phenytoin.

Do not share phenytoin with other people, even if they have the same condition as you. It may harm them.

Keep phenytoin out of the reach of children.

While phenytoin helps make seizures less likely, it does not cure them. Do not stop taking phenytoin without talking to your health care provider. Stopping phenytoin suddenly may cause an increase in seizure activity, which can be serious.

Phenytoin can affect your alertness or coordination. Do not drive or do other activities that require alertness or coordination until you know how phenytoin affects you.

Your health care provider will likely do blood tests while you are taking this medicine. This is to monitor phenytoin levels in your blood and to make sure the dose is right.

Phenytoin can cause gingival hyperplasia, a condition where the gum tissues surrounding the teeth overgrow. Brushing your teeth and flossing daily helps prevent this from happening. 

What should I tell my health care provider before using phenytoin?

Tell your health care provider about all of your health conditions and any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. This will help them determine if phenytoin is right for you.

In particular, make sure that you discuss any of the following.

Current and Past Health Conditions. Tell your health care provider if you have any of the following.

  • Liver problems
  • Kidney problems
  • Heart problems
  • Blood problems, including porphyria 
  • Mental health problems
  • High blood sugar or diabetes
  • Osteoporosis or osteopenia 
  • Alcohol use disorder

Other Medicines and Supplements. Phenytoin may interact with other medicines and supplements. Before taking phenytoin, tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. See the Interactions section for more details.

Pregnancy. Phenytoin may cause harm to a fetus. Tell your health care provider right away if you are pregnant or plan to become pregnant. Your health care provider will let you know if you should take phenytoin while you are pregnant or trying to get pregnant. If you become pregnant while taking phenytoin, you can sign up for the North American Antiepileptic Drug Pregnancy Registry by calling 888-233-2334 or at www.aedpregnancyregistry.org. This registry collects safety information about the use of seizure medicines during pregnancy.

Breastfeeding. Phenytoin passes into breast milk. Tell your health care provider if you are breastfeeding or plan to breastfeed. Your health care provider will let you know if you should take phenytoin while breastfeeding.

Interactions

Does phenytoin interact with foods or drinks?

There are no known interactions between phenytoin and foods or drinks.

Do not drink alcohol while taking phenytoin. Alcohol can change the amount of phenytoin in your body, which can cause serious problems.

Does phenytoin interact with other medicines (drug interactions)?

Always tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using.

In particular, make sure that you discuss if you are using any of the following before taking phenytoin.

  • Fluconazole (Diflucan), itraconazole (Sporanox and Tolsura), ketoconazole, posaconazole (Noxafil), or voriconazole (Vfend), which are medicines used for fungal infections
  • Ticagrelor (Brilinta), which is a medicine used to prevent blood clots
  • Warfarin, which is a blood thinner
  • Carbamazepine (Carbatrol, Tegretol, and others), which is a medicine commonly used to treat seizures or nerve pain
  • Divalproex sodium, valproate sodium (Depakote and others), and valproic acid, which are medicines used for seizures and other conditions
  • Sulfamethoxazole/trimethoprim, which is an antibiotic used to treat certain infections
  • A medicine for high blood pressure
  • Fluoxetine or sertraline, which are medicines commonly used for anxiety and depression
  • Cimetidine (Tagamet HB) or omeprazole (Prilosec), which are medicines used for heartburn
  • An antacid containing aluminum, calcium, or magnesium

Many other medicines may alter the blood levels of phenytoin, and vice versa. Tell your health care provider about all medicines that you take or have recently taken.

This may not be a complete list of medicines that can interact with phenytoin. Always check with your health care provider.

Does interact with other drugs you are taking?

Enter your medication into the WebMD interaction checker

Overdose/Missed Dose

What should I do if I accidentally use too much phenytoin?

If you or someone else has used too much phenytoin, get medical help right away, call 911, or contact a Poison Control center at 800-222-1222.

What should I do if I miss a dose of phenytoin?

If you miss a dose, take it as soon as you remember. If it is almost time for your next dose, skip the missed dose and only take the next dose. Do not take double or extra doses.