Medically reviewed by Drugs.com. Last updated on Jun 8, 2022.
Commonly used brand name(s)
In the U.S.
- Dilantin Infatabs
Available Dosage Forms:
- Tablet, Chewable
- Capsule, Extended Release
Therapeutic Class: Anticonvulsant
Chemical Class: Hydantoin (class)
Uses for phenytoin
Phenytoin is used to control seizures (convulsions), including tonic-clonic (grand mal) and psychomotor (temporal lobe) seizures, in the treatment of epilepsy. It is also used to prevent and treat seizures that occur during brain surgery. Phenytoin is an anticonvulsant that works in the brain tissue to stop seizures.
Phenytoin is available only with your doctor's prescription.
Before using phenytoin
In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For phenytoin, the following should be considered:
Tell your doctor if you have ever had any unusual or allergic reaction to phenytoin or any other medicines. Also tell your health care professional if you have any other types of allergies, such as to foods, dyes, preservatives, or animals. For non-prescription products, read the label or package ingredients carefully.
Appropriate studies performed to date have not demonstrated pediatric-specific problems that would limit the usefulness of phenytoin in children.
Appropriate studies performed to date have not demonstrated geriatric-specific problems that would limit the usefulness of phenytoin in the elderly. However, elderly patients are more likely to have age-related liver, kidney, or heart problems, which may require an adjustment in the dose for patients receiving phenytoin.
There are no adequate studies in women for determining infant risk when using this medication during breastfeeding. Weigh the potential benefits against the potential risks before taking this medication while breastfeeding.
Interactions with medicines
Although certain medicines should not be used together at all, in other cases two different medicines may be used together even if an interaction might occur. In these cases, your doctor may want to change the dose, or other precautions may be necessary. When you are taking phenytoin, it is especially important that your healthcare professional know if you are taking any of the medicines listed below. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive.
Using phenytoin with any of the following medicines is not recommended. Your doctor may decide not to treat you with this medication or change some of the other medicines you take.
- Tenofovir Alafenamide
Using phenytoin with any of the following medicines is usually not recommended, but may be required in some cases. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.
- Abiraterone Acetate
- Aluminum Carbonate, Basic
- Aluminum Hydroxide
- Aluminum Phosphate
- Calcium Carbonate
- Dabigatran Etexilate
- Dihydroxyaluminum Aminoacetate
- Dihydroxyaluminum Sodium Carbonate
- Doxorubicin Hydrochloride Liposome
- Eslicarbazepine Acetate
- Ethinyl Estradiol
- Irinotecan Liposome
- Magnesium Carbonate
- Magnesium Hydroxide
- Magnesium Oxide
- Magnesium Trisilicate
- Sirolimus Protein-Bound
- St John's Wort
- Vincristine Sulfate Liposome
Using phenytoin with any of the following medicines may cause an increased risk of certain side effects, but using both drugs may be the best treatment for you. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.
- Aripiprazole Lauroxil
- Folic Acid
- Valproic Acid
Interactions with food/tobacco/alcohol
Certain medicines should not be used at or around the time of eating food or eating certain types of food since interactions may occur. Using alcohol or tobacco with certain medicines may also cause interactions to occur. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive.
Using phenytoin with any of the following may cause an increased risk of certain side effects but may be unavoidable in some cases. If used together, your doctor may change the dose or how often you use phenytoin, or give you special instructions about the use of food, alcohol, or tobacco.
- Enteral Nutrition
Other medical problems
The presence of other medical problems may affect the use of phenytoin. Make sure you tell your doctor if you have any other medical problems, especially:
- Bone problems (eg, osteomalacia, osteopenia, osteoporosis) or
- Depression or mental illness, history of or
- Diabetes or
- Heart disease or
- Lymphadenopathy (lymph node problems) or
- Porphyria (an enzyme problem) or
- Vitamin D deficiency—Use with caution. May make these conditions worse.
- Hypoalbuminemia (low albumin in the blood) or
- Kidney disease or
- Liver disease—Use with caution. The effects may be increased because of slower removal from the body.
- Liver problem from previous phenytoin use, acute—Should not be used in patients with this condition.
Proper use of phenytoin
Take phenytoin only as directed by your doctor. Do not take more of it, do not take it more often, and do not take it for a longer time than your doctor ordered.
Phenytoin should come with a Medication Guide. Read and follow these instructions carefully. Ask your doctor if you have any questions.
Phenytoin may be used with other seizure medicines. Keep using all of your seizure medicines unless your doctor tells you otherwise.
Do not change brands or dosage forms of phenytoin without first checking with your doctor. Different products may not work the same way. If you refill your medicine and it looks different, check with your pharmacist.
Swallow the tablet whole or chew it thoroughly before swallowing it together with a glass of water.
Measure the oral suspension with a marked measuring spoon, oral syringe, or medicine cup. Rinse the dosing spoon or cup with water after each use.
If you are receiving tube feeding preparations, it is best to take phenytoin before or after a feeding.
The dose of phenytoin will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of phenytoin. If your dose is different, do not change it unless your doctor tells you to do so.
The amount of medicine that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine.
- For seizures:
- For oral dosage form (extended-release capsules):
- Adults—At first, 100 milligrams (mg) 3 times a day or 300 mg once a day. Your doctor may increase your dose as needed. For patients in the clinic or hospital (except with a history of liver or kidney disease), a loading dose of 1000 mg is divided into three doses (400 mg, 300 mg, 300 mg) and given every 2 hours. Then, normal maintenance dose may be started 24 hours after the loading dose.
- Children older than 6 years of age—300 mg per day. Your doctor may adjust your dose as needed.
- Children 6 years of age and younger—Dose is based on body weight and must be determined by your doctor. At first, 5 milligrams (mg) per kilogram (kg) of body weight given in 2 or 3 divided doses per day. The doctor may adjust the dose as needed.
- For oral dosage form (suspension):
- Adults—At first, 5 milliliters (mL) or one teaspoonful 3 times a day. Your doctor may adjust your dose as needed. However, the dose is usually not more than 25 mL per day.
- Children older than 6 years of age—300 mg per day. Your doctor may adjust your dose as needed.
- Children 6 years of age and younger—Dose is based on age and body weight and must be determined by your doctor. At first, 5 milligrams (mg) per kilogram (kg) of body weight given in 2 or 3 divided doses per day. Your doctor may adjust your dose as needed.
- For oral dosage form (tablets):
- Adults—At first, 100 milligrams (mg) 3 times a day. Your doctor may adjust your dose as needed.
- Children older than 6 years of age—300 mg per day, given in 2 or 3 divided doses per day. Your doctor may adjust your dose as needed.
- Children 6 years of age and younger—Dose is based on age and body weight and must be determined by your doctor. At first, 5 milligrams (mg) per kilogram (kg) of body weight given in 2 or 3 divided doses per day. The doctor may adjust the dose as needed.
- For oral dosage form (extended-release capsules):
If you miss a dose of phenytoin, take it as soon as possible. However, if it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not double doses.
Store the medicine in a closed container at room temperature, away from heat, moisture, and direct light. Keep from freezing.
Keep out of the reach of children.
Do not keep outdated medicine or medicine no longer needed.
Ask your healthcare professional how you should dispose of any medicine you do not use.
Precautions while using phenytoin
It is very important that your doctor check your or your child's progress at regular visits while using phenytoin to see if it is working properly and to allow for a change in the dose. Blood and urine tests may be needed to check for any unwanted effects.
Using phenytoin while you are pregnant can harm your unborn baby. Use an effective form of birth control to keep from getting pregnant. If you think you have become pregnant while using the medicine, tell your doctor right away.
Do not use phenytoin if you or your child are also using delavirdine (Rescriptor®). Using these medicines together may cause unwanted effects.
Lymph node problems may occur while using phenytoin. Check with your doctor right away if you or your child have swollen, painful, or tender lymph glands in your neck, armpit, or groin.
Do not suddenly stop using phenytoin without checking first with your doctor. Stopping the medicine suddenly may cause your seizures to return or to occur more often. Your doctor may want you or your child to gradually reduce the amount you are using before stopping it completely.
Phenytoin may cause serious allergic reactions, including angioedema, which can be life-threatening and require immediate medical attention. Tell your doctor right away if you or your child have large, hive-like swelling on the face, eyelids, lips, tongue, throat, hands, legs, feet, or sex organs after using phenytoin.
Serious skin reactions (eg, Stevens-Johnson syndrome, toxic epidermal necrolysis, and acute generalized exanthematous pustulosis) can occur while using phenytoin. Check with your doctor right away if you have blistering, peeling, or loosening of the skin, chills, cough, diarrhea, itching, joint or muscle pain, red irritated eyes, red skin lesions, often with a purple center, sore throat, sores, ulcers, or white spots in the mouth or on the lips, or unusual tiredness or weakness with phenytoin.
Tell your doctor right away if you or your child develop a fever, rash, swollen, painful, or tender lymph glands in the neck, armpit, or groin, unusual bleeding or bruising, or yellow eyes or skin after using phenytoin. These may be symptoms of a serious and life-threatening condition called drug reaction with eosinophilia and systemic symptoms (DRESS).
Phenytoin may cause heart problems, including a slow heartbeat. Check with your doctor right away if you have chest pain, dizziness, or tiredness.
Phenytoin may cause liver problems. Check with your doctor right away if you have pain or tenderness in the upper stomach, pale stools, dark urine, loss of appetite, nausea, vomiting, or yellow eyes or skin.
Phenytoin can temporarily lower the number of white blood cells in your blood, increasing the chance of getting an infection. It can also lower the number of platelets, which are necessary for proper blood clotting. If this occurs, there are certain precautions you can take, especially when your blood count is low, to reduce the risk of infection or bleeding:
- If you can, avoid people with infections. Check with your doctor immediately if you think you are getting an infection or if you get a fever or chills, cough or hoarseness, lower back or side pain, or painful or difficult urination.
- Check with your doctor immediately if you notice any unusual bleeding or bruising, black, tarry stools, blood in the urine or stools, or pinpoint red spots on your skin.
- Be careful when using a regular toothbrush, dental floss, or toothpick. Your medical doctor, dentist, or nurse may recommend other ways to clean your teeth and gums. Check with your medical doctor before having any dental work done.
- Do not touch your eyes or the inside of your nose unless you have just washed your hands and have not touched anything else in the meantime.
- Be careful not to cut yourself when you are using sharp objects such as a safety razor or fingernail or toenail cutters.
- Avoid contact sports or other situations where bruising or injury could occur.
Phenytoin may decrease the density of bones, which can make your bones weak. Talk with your doctor if you have any concerns about this.
Phenytoin may affect blood sugar levels. If you or your child notice a change in the results of your blood or urine sugar tests or if you have any questions, check with your doctor.
Phenytoin may cause some people to be agitated, irritable, or display other abnormal behaviors. It may also cause some people to have suicidal thoughts and tendencies or to become more depressed. Make sure the doctor knows if you have trouble sleeping, get upset easily, have a big increase in energy, or start to act reckless. Also tell the doctor if you have sudden or strong feelings, such as feeling nervous, angry, restless, violent, or scared. If you, your child, or your caregiver notice any of these side effects, tell your doctor right away.
In some patients (usually younger patients), tenderness, swelling, or bleeding of the gums (gingival hyperplasia) may appear soon after phenytoin treatment is started. To help prevent this, brush and floss your teeth carefully and regularly and massage your gums. Also, see your dentist every 6 months to have your teeth cleaned. If you have any questions about how to take care of your teeth and gums, or if you notice any tenderness, swelling, or bleeding of your gums, check with your doctor or dentist.
Before you have any medical tests, tell the medical doctor in charge that you or your child are taking phenytoin. The results of some tests may be affected by phenytoin.
Phenytoin may cause drowsiness, trouble thinking, or trouble in controlling movements. Make sure you know how you or your child react to phenytoin before you drive, use machines, climb in high places, swim, or do anything else that could be dangerous if you are not alert, well-coordinated, or able to think or see well.
Avoid drinking alcohol while you are using phenytoin.
Using phenytoin together with valproate (Depacon®, Depakene®) may increase the risk of having hyperammonemia (too much ammonia in your blood), which may damage the brain and can be life-threatening. Check with your doctor right away if you or your child have nausea or vomiting that continues, an increase in the frequency of seizures, loss of appetite, swelling of the face, tiredness and weakness, or yellow eyes or skin.
Do not take other medicines unless they have been discussed with your doctor. This includes prescription or nonprescription (over-the-counter [OTC]) medicines and herbal or vitamin supplements.
Phenytoin side effects
Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.
Check with your doctor immediately if any of the following side effects occur:
- Decreased coordination
- mental confusion
- slurred speech
- trouble with breathing, speaking, or swallowing
- unsteadiness, trembling, or other problems with muscle control or coordination
- Inability to move the eyes
- increased blinking or spasms of the eyelid
- shakiness and unsteady walk
- shakiness in the legs, arms, hands, or feet
- sticking out of the tongue
- trembling or shaking of the hands or feet
- twitching, twisting, or uncontrolled repetitive movements of the tongue, lips, face, arms, or legs
- uncontrolled twisting movements of the neck, trunk, arms, or legs
- unusual facial expressions
Incidence not known
- Bleeding gums
- blistering, peeling, or loosening of the skin
- blisters, hives, itching, skin rash
- bloating of the abdomen or stomach
- blood in the urine or stools
- bloody, black, or tarry stools
- chest pain or discomfort
- dark urine
- difficulty with moving
- fainting spells
- fever with or without chills
- general feeling of discomfort or illness
- hair loss
- high fever
- irritation in the mouth
- joint or muscle pain
- light-colored stools
- lightheadedness or dizziness
- lower back or side pain
- muscle stiffness
- numbness, tingling, or pain in the hands or feet
- painful or difficult urination
- pale skin
- pinpoint red spots on the skin
- red skin lesions, often with a purple center
- red, irritated eyes
- redness and swelling of the gums
- slow or irregular heartbeat
- sore throat
- sores, ulcers, or white spots on the lips or in the mouth
- stomach pain
- swollen, painful, or tender lymph glands in the neck, armpit, or groin
- unusual bleeding or bruising
- unusual tiredness or weakness
- upper right abdominal or stomach pain
- weight loss
- yellow eyes or skin
Some side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:
- Trouble sleeping
- uncontrolled eye movements
Incidence not known
- Difficulty having a bowel movement
- enlarged lips
- feeling of constant movement of self or surroundings
- increased hair growth on the forehead, back, arms, or legs
- pain of the penis on erection
- sensation of spinning
Other side effects not listed may also occur in some patients. If you notice any other effects, check with your healthcare professional.
Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.
More about phenytoin
- Side effects
- Drug interactions
- Dosage information
- During pregnancy or Breastfeeding
- Reviews (106)
- Drug images
- Compare alternatives
- Pricing & coupons
- Drug class: group I antiarrhythmics
- Drug Information
- Phenytoin injection
- Phenytoin Intravenous (Advanced Reading)
- Phenytoin Chewable Tablets
- Phenytoin Extended-Release Capsules
- Phenytoin Suspension
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