Phenytoin is used to prevent and control seizures (also called an anticonvulsant or antiepileptic drug). It works by reducing the spread of seizure activity in the brain.

This section contains uses of this drug that are not listed in the approved professional labeling for the drug but that may be prescribed by your health care professional. Use this drug for a condition that is listed in this section only if it has been so prescribed by your health care professional.

This drug may also be used for certain types of irregular heartbeats.

Read the Medication Guide provided by your pharmacist before you start taking phenytoin and each time you get a refill. If you have any questions, ask your doctor or pharmacist.

Shake this medication well before each dose. Take this medication by mouth as directed by your doctor, usually 2 or 3 times a day. This product is not recommended for use once a day. You may take it with food if stomach upset occurs.

Carefully measure the dose using a special measuring device/spoon/syringe. Do not use a household spoon because you may not get the correct dose.

Use this medication regularly in order to get the most benefit from it. It is important to take all doses on time to keep the amount of medicine in your body at a constant level. Remember to use it at the same times each day. The dosage is based on your medical condition, lab tests, and response to treatment. Children's dosage is also based on weight.

Antacids and nutritional tube-feeding (enteral) products may decrease the absorption of phenytoin. Do not take these products at the same time as your phenytoin dose. Separate liquid nutritional products at least 1 hour before and 1 hour after your phenytoin dose, or as directed by your doctor.

Do not stop taking this medication without consulting your doctor. Seizures may become worse when the drug is suddenly stopped. Your dose may need to be gradually decreased.

Tell your doctor if your condition does not get better or if it gets worse.

Headache, nausea, vomiting, constipation, dizziness, feeling of spinning, drowsiness, trouble sleeping, or nervousness may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly.

Phenytoin may cause swelling and bleeding of the gums. Massage your gums and brush and floss your teeth regularly to minimize this problem. See your dentist regularly.

Remember that this medication has been prescribed because your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.

Tell your doctor right away if you have any serious side effects, including:

• slow heartbeat
• unusual eye movements
• loss of coordination
• trouble speaking
• confusion
• muscle twitching
• double/blurred vision
• tingling of the hands/feet
• facial changes (such as swollen lips, butterfly-shaped rash around the nose/cheeks)
• excessive hair growth
• increased thirst/urination
• unusual tiredness
• bone/joint pain
• easily broken bones

A small number of people who take anticonvulsants for any condition (such as seizure, bipolar disorder, pain) may experience depression, suicidal thoughts/attempts, or other mental/mood problems. Tell your doctor right away if you or your family/caregiver notice any unusual/sudden changes in your mood, thoughts, or behavior including signs of depression, suicidal thoughts/attempts, thoughts about harming yourself.

Rarely, males may have a painful or prolonged erection lasting 4 or more hours. If this occurs, stop using this drug and get medical help right away, or permanent problems could occur.

Get medical help right away if you have any very serious side effects, including:

• uncontrolled muscle movements
• signs of liver problems (such as nausea/vomiting that doesn't stop, stomach/abdominal pain, yellowing eyes/skin, dark urine)
• easy bruising/bleeding

A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including:

• fever
• swollen lymph nodes
• rash
• itching/swelling (especially of the face/tongue/throat)
• severe dizziness
• trouble breathing

This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.

In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch.

In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

Before taking phenytoin, tell your doctor or pharmacist if you are allergic to it; or to other anti-seizure medications (such as carbamazepine, ethosuximide, ethotoin, fosphenytoin, oxcarbazepine, phenobarbital, primidone, trimethadione); or if you have any other allergies. This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details.

Before using this medication, tell your doctor or pharmacist your medical history, especially of:

• alcohol use
• certain blood conditions (porphyria)
• diabetes
• liver disease (including liver disease caused by past phenytoin use)
• lupus
• folate or vitamin B-12 deficiency (megaloblastic anemia)

This drug may make you dizzy or drowsy. Alcohol or marijuana (cannabis) can make you more dizzy or drowsy. Do not drive, use machinery, or do anything that needs alertness until you can do it safely. Limit alcoholic beverages. Talk to your doctor if you are using marijuana (cannabis). Alcohol may also affect your blood levels of this drug.

Before having surgery, tell your doctor or dentist about all the products you use (including prescription drugs, nonprescription drugs, and herbal products).

If you have diabetes, this product may make it harder to control your blood sugar. Check your blood sugar regularly as directed and share the results with your doctor. Your doctor may need to adjust your diabetes medication, exercise program, or diet.

Vitamin D supplements may be necessary to prevent weakening of the bones (osteomalacia). Discuss this with your doctor.

During pregnancy, this medication should be used only when clearly needed. It may harm an unborn baby. However, since untreated seizures are a serious condition that can harm both a pregnant woman and her unborn baby, do not stop taking this medication unless directed by your doctor. If you are planning pregnancy, become pregnant, or think you may be pregnant, immediately talk to your doctor about the benefits and risks of using this medication during pregnancy. Since birth control pills, patches, implants, and injections may not work if taken with this medication (see also Drug Interactions section), discuss reliable forms of birth control with your doctor.

Phenytoin passes into breast milk. Consult your doctor before breastfeeding.

See also How to Use section.

Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist. Do not start, stop, or change the dosage of any medicines without your doctor's approval.

Some products that may interact with this drug include:

• colesevelam
• orlistat
• sucralfate

Other medications can affect the removal of phenytoin from your body, which may affect how phenytoin works. Examples include amiodarone, azapropazone, azole antifungals (such as itraconazole), macrolide antibiotics (such as erythromycin), estrogens, isoniazid, rifamycins (such as rifabutin), St. John's wort, among others.

Phenytoin can speed up the removal of other medications from your body, which may affect how they work. Examples of affected drugs include atazanavir, some drugs to treat cancer (such as imatinib, irinotecan), cobicistat, corticosteroids (such as prednisone), doravirine, etravirine, felodipine, nisoldipine, quetiapine, quinidine, rilpivirine, suvorexant, telithromycin, theophylline, vitamin D, among others.

This medication may decrease the effectiveness of hormonal birth control such as pills, patch, or ring. This could cause pregnancy. Discuss with your doctor or pharmacist if you should use reliable backup birth control methods while using this medication. Also tell your doctor if you have any new spotting or breakthrough bleeding, because these may be signs that your birth control is not working well.

This medication may interfere with certain lab tests, possibly causing false test results. Make sure lab personnel and all your doctors know you use this drug.

If someone has overdosed and has serious symptoms such as passing out or trouble breathing, call 911. Otherwise, call a poison control center right away. US residents can call 1-800-222-1222. Canada residents can call 1-844-764-7669. Symptoms of overdose may include: severe mental/mood changes, severe drowsiness, loss of consciousness, slowed breathing.

Do not share this medication with others.

Lab and/or medical tests (such as phenytoin blood levels, liver function, complete blood count) should be done while you are taking this medication. Keep all medical and lab appointments. Consult your doctor for more details.

Do not change from one brand of this product to another, or to another dose form of this drug (such as capsules) without consulting your doctor or pharmacist. Your dosage may have to be adjusted.

If you miss a dose, take it as soon as you remember. If it is within 4 hours of the next dose, skip the missed dose. Take your next dose at the regular time. Do not double the dose to catch up. Check with your doctor if you miss doses for more than 2 days in a row.

Store at room temperature away from light and moisture. Do not freeze. Do not store in the bathroom. Keep all medications away from children and pets.

Do not flush medications down the toilet or pour them into a drain unless instructed to do so. Properly discard this product when it is expired or no longer needed. Consult your pharmacist or local waste disposal company.

Your condition can cause complications in a medical emergency. For information about enrolling in MedicAlert, call 1-888-633-4298 (US) or 1-800-668-1507 (Canada).