Phenytoin belongs to the group of medications known as anti-epileptics. Phenytoin is used to manage and prevent certain types of seizures, as well as to prevent and treat seizures that occur during or following neurosurgery. It works on the brain to reduce the number and severity of seizures.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

The recommended dose of phenytoin varies according to individual needs. The recommended starting dose for adults is 100 mg taken 3 times a day. Your doctor will then adjust the dose based on how well it works and is tolerated. The usual adult dose is 300 mg to 400 mg daily taken by mouth in divided doses. An increase to 600 mg taken in divided doses may be needed in some cases. Once the most appropriate dose has been determined for you, and your seizures are controlled, your doctor may suggest that you take the medication in a single daily dose.

The children's dose is based on age and body weight. The recommended starting dose is 5 mg per kilogram of body weight per day, which is then divided into 3 equal doses.

The level of phenytoin in your blood can be checked through laboratory testing. This test helps your doctor determine the dose of phenytoin that best suits you.

Phenytoin may be taken with or after meals. Do not use capsules that are discoloured.

Many things can affect the dose of a medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.

It is important to take this medication exactly as prescribed by your doctor. If you miss a dose, take it as soon as possible and continue with your regular schedule. If it is almost time for your next dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.

Store all forms of this medication at room temperature, protect it from light and moisture, and keep it out of the reach of children.

Do not use phenytoin if you:

• are allergic to phenytoin or any ingredients of the medication
• are allergic to other medications in the class of medications called hydantoins (e.g., fosphenytoin)
• are taking the anti-viral medication delavirdine
• have heart disorders such as slow heart rate, heart block, or abnormal heart rhythms

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

• body and facial hair growth
• changed sense of taste
• constipation
• difficulty sleeping
• dizziness (mild)
• drowsiness (mild)
• headache
• nausea
• nervousness
• vomiting

Although most of these side effects listed below don't happen very often, they could lead to serious problems if you do not seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• bone pain or fractures
• bleeding, tender, or enlarged gums
• changes to facial features
• changes in muscle movements or coordination
• confusion
• dizziness
• fast, slow, or irregular heartbeat
• “pins and needles” sensation on the skin
• rash
• sensation of spinning
• signs of anemia (low red blood cells; e.g., dizziness, pale skin, unusual tiredness or weakness, shortness of breath)
• signs of bleeding (e.g., unusual nosebleeds, bruising, blood in urine, coughing blood, bleeding gums, cuts that don't stop bleeding)
• signs of infection (symptoms may include fever or chills, severe diarrhea, shortness of breath, prolonged dizziness, headache, stiff neck, weight loss, or listlessness)
• signs of liver problems (e.g., nausea, vomiting, diarrhea, loss of appetite, weight loss, yellowing of the skin or whites of the eyes, dark urine, pale stools)
• slurred speech
• symptoms of high blood sugar (e.g., frequent urination, increased thirst, excessive eating, unexplained weight loss, poor wound healing, infections, fruity breath odour)
• symptoms of psychosis (e.g., seeing, hearing, or believing things that aren’t real)
• unusual eye movement

Stop taking the medication and seek immediate medical attention if any of the following occur:

• seizures
• suicidal thoughts or behaviour
• symptoms of a serious allergic reaction (such as fever, swollen glands, yellowing of skin or eyes, or flu-like symptoms with skin rash or blistering)
• symptoms of a serious skin reaction (such as skin rash; red skin; blistering of the lips, eyes, or mouth; skin peeling; fever; or joint pain)

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Allergy: Some people who are allergic to carbamazepine or barbiturates also experience allergic reactions to phenytoin. Before you take phenytoin, inform your doctor about any previous adverse reactions you have had to medications, especially other medications for seizures. Contact your doctor at once if you experience signs of an allergic reaction, such as skin rash, itching, difficulty breathing, or swelling of the face and throat.

Blood counts: This medication can decrease the number of neutrophils (a type of white blood cell that helps fight infection), red blood cells (which carry oxygen), and platelets (which help your blood to clot). Your doctor will do blood tests to monitor this. If you notice any signs of infection (e.g., fever, chills, or sore throat) or unusual bleeding or bruising, contact your doctor immediately.

Dental hygiene: Because phenytoin can cause swollen and bleeding gums, it is important to practice good dental hygiene by flossing, brushing, and visiting your dentist regularly. Talk to your doctor to learn more about how to take care of your mouth, gums, and teeth while taking this medication.

Diabetes: Phenytoin can decrease the release of insulin from the pancreas, resulting in higher-than-normal levels of glucose (sugar) in the blood. It may also have an effect on the action of medications used to lower blood sugar for people with diabetes.

If you have diabetes, or are at risk of developing diabetes, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed. It may be necessary to monitor your blood sugar more closely when starting or stopping phenytoin treatment.

If you experience symptoms of increased blood sugar, such as increased urinary frequency, increased thirst, excessive eating, unexplained weight loss, or a fruity breath odour, contact your doctor.

Drowsiness/reduced alertness: Some people who take phenytoin become drowsy. Avoid activities that require complete mental alertness, judgment, and physical coordination (such as driving a car or performing hazardous tasks) until you establish how you are affected by phenytoin.

Hypersensitivity syndrome: A severe allergic reaction called hypersensitivity syndrome has occurred for some people with the use of phenytoin. Stop taking the medication and get immediate medical attention if you have symptoms of a severe allergic reaction, including fever, swollen glands, yellowing of the skin or eyes, or flu-like symptoms with skin rash or blistering. These reactions may be more frequent in people of Asian origin, those with immune system problems or those who have a history of similar reactions to other seizure medications.

Lactose intolerance: This capsule form of phenytoin contains lactose. If you have galactose intolerance (galactosemia, glucose-galactose malabsorption, or Lapp lactase deficiency), you should not take the capsule form of this medication.

Liver function: Decreased liver function or liver disease can cause phenytoin to build up in the body, causing increased side effects.

If you experience symptoms of liver problems such as fatigue, feeling unwell, loss of appetite, nausea, yellowing of the skin or whites of the eyes, dark urine, pale stools, abdominal pain or swelling, and itchy skin, contact your doctor immediately. If you have decreased liver function or liver disease, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Osteoporosis: Long term use of anti-seizure medications, including phenytoin, may affect the strength of your bones, because these medications reduce the amount of vitamin D available for your body to use. If you have or are at risk for developing osteoporosis, discuss with your doctor whether you need to take additional vitamin D.

Other medications and alcohol: Many medications and alcoholic beverages interact with phenytoin, causing decreased effectiveness or increased side effects. If you are taking phenytoin, consult with your doctor or pharmacist before taking any other medications or drinking alcoholic beverages.

Regular dosing: Remember that when you are taking phenytoin, it is very important to take the medication exactly as prescribed. Inform your doctor of any condition that prevents you from taking the medication as prescribed (e.g., surgery, illness, difficulty swallowing).

Stopping the medication: Do not stop taking phenytoin without consulting your doctor. Stopping the medication suddenly may cause an increase in the number or severity of seizures. If it is necessary to stop taking this medication, talk to your doctor about how to gradually reduce the dose of the medication before completely stopping.

Suicidal thoughts and behaviour: Some people taking anti-seizure medications may have suicidal thoughts or behaviour. If this happens to you or you notice this in a family member who is taking this medication, contact your doctor immediately. You should be closely monitored by your doctor for emotional and behaviour changes while taking this medication.

Pregnancy: When phenytoin is taken during pregnancy, it may cause harm to the developing baby. This medication should not be used during pregnancy unless the benefits outweigh the risks. If you become pregnant while taking this medication, contact your doctor immediately.

Breast-feeding: This medication passes into breast milk. If you are breast-feeding and taking phenytoin, it may affect your baby. Talk to your doctor about whether you should continue breast-feeding.

Seniors: People over the age of 65 are more likely to experience side effects of taking phenytoin. Doses for seniors should generally be lower and increase more slowly than for other adults.

There may be an interaction between phenytoin and any of the following:

• abiraterone
• acetaminophen
• acetazolamide
• alcohol
• aliskiren
• allopurinol
• amiodarone
• antacids (e.g., aluminum hydroxide, calcium carbonate, magnesium hydroxide)
• antineoplastic agents (chemotherapy; e.g., capecitabine, carboplatin, etoposide, irinotecan, paclitaxel, vincristine)
• antipsychotics (e.g., cariprazine, chlorpromazine, clozapine, haloperidol, olanzapine, quetiapine, risperidone)
• apalutamide
• apixaban
• apremilast
• aprepitant
• azole antifungals (e.g., fluconazole, ketoconazole, voriconazole)
• benzodiazepines (e.g., clonazepam, diazepam, lorazepam)
• benzydamine
• birth control pills
• bisoprolol
• bromocriptine
• buprenorphine
• bupropion
• buspirone
• cannabis
• calcium channel blockers (e.g., amlodipine, diltiazem, nifedipine, verapamil)
• calcium supplements
• CF transmembrane regulators (e.g., elexacaftor, ivacaftor, lumacaftor, tezacaftor)
• chlorpheniramine
• cimetidine
• clarithromycin
• clindamycin
• ciprofloxacin
• cobicistat
• corticosteroids (e.g., dexamethasone, hydrocortisone, prednisone)
• cyclosporine
• dabigatran
• dapsone
• darolutamide
• deferasirox
• desmopressin
• diabetes medications (e.g., canagliflozin, linagliptin, repaglinide, saxagliptin)
• diazoxide
• digoxin
• disopyramide
• domperidone
• doxycycline
• dronedarone
• edoxaban
• elagolix
• eliglustat
• eplerenone
• estrogens (e.g., conjugated estrogen, estradiol, ethinyl estradiol)
• everolimus
• fenofibrate
• fesoterodine
• fexofenadine
• finerenone
• flibanserin
• flunarizine
• folic acid
• fostemsavir
• furosemide
• gemfibrozil
• general anesthetics (medications used to put people to sleep before surgery)
• guanfacine
• hepatitis C antiviral medications (e.g., glecaprevir, ledipasvir, velpatasvir, voxilaprevir)
• HIV integrase inhibitors (e.g., cabotegravir, dolutegravir, elvitegravir)
• HIV non-nucleoside reverse transcriptase inhibitors (NNRTIs; e.g., doravirine, efavirenz, etravirine, nevirapine, rilpivirine)
• HIV protease inhibitors (e.g., atazanavir, lopinavir, ritonavir)
• isoniazid
• ivabradine
• ketamine
• lemborexant
• letermovir
• leuprolide
• levodopa
• lidocaine
• lithium
• macitentan
• maraviroc
• maribavir
• mavacamten
• mebendazole
• mefloquine
• meperidine
• methadone
• methotrexate
• metronidazole
• mexiletine
• mifepristone
• milk thistle
• mirabegron
• mirtazapine
• moclobemide
• modafinil
• multiple vitamin/mineral supplements
• naloxegol
• narcotic pain relievers (e.g., codeine, fentanyl, oxycodone, tramadol)
• nirmatrelvir and ritonavir
• ondansetron
• orlistat
• perampanel
• phenobarbital
• phosphodiesterase-5 inhibitors (e.g., sildenafil, vardenafil)
• ponesimod
• progestins (e.g., dienogest, medroxyprogesterone)
• propafenone
• protein kinase inhibitors (e.g., bosutinib, dasatinib, imatinib, nilotinib)
• proton pump inhibitors (e.g., esomeprazole, omeprazole)
• quinine
• ranolazine
• rifabutin
• rifampin
• riociguat
• rivaroxaban
• roflumilast
• romidepsin
• salicylates (e.g., acetylsalicylic acid)
• other seizure medications (e.g., carbamazepine, lamotrigine, phenobarbital, topiramate, valproic acid)
• selective serotonin reuptake inhibitors (SSRIs; e.g., citalopram, fluoxetine, sertraline)
• sirolimus
• sodium oxybate
• solifenacin
• "statin" anti-cholesterol medications (e.g., atorvastatin, lovastatin, simvastatin)
• sucralfate
• sulfadiazine
• sulfamethoxazole
• tacrolimus
• tamoxifen
• tecovirimat
• tenofovir
• thalidomide
• theophyllines (e.g., aminophylline, theophylline)
• thyroid replacements (e.g., desiccated thyroid, levothyroxine)
• ticagrelor
• tofacitinib
• tolterodine
• tolvaptan
• trazodone
• trimethoprim
• upadacitinib
• vitamin D analogues (e.g., alfacalcidol, calcitriol, cholecalciferol)
• warfarin
• zolpidem
• zopiclone

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

• stop taking one of the medications,
• change one of the medications to another,
• change how you are taking one or both of the medications, or
• leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.