SIDE EFFECTS: Headache, nausea, vomiting, constipation, dizziness, feeling of spinning, drowsiness, trouble sleeping, or nervousness may occur. If any of these effects persist or worsen, 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 your doctor has prescribed this medication because he or she 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 any of these unlikely but serious side effects occur: unusual eye movements, loss of coordination, slurred speech, confusion, muscle twitching, double or blurred vision, tingling of the hands/feet, facial changes (e.g., swollen lips, butterfly-shaped rash around the nose/cheeks), excessive hair growth, increased thirst or urination, unusual tiredness, bone or 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. For males, in the very unlikely event you have a painful or prolonged erection lasting 4 or more hours, stop using this drug and seek immediate medical attention, or permanent problems could occur. Get medical help right away if any of these rare but very serious side effects occur: 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.

PRECAUTIONS: Before taking phenytoin, tell your doctor or pharmacist if you are allergic to it; or to other anti-seizure medications (e.g., carbamazepine, ethotoin, phenobarbital, ethosuximide, 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 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. Alcohol may also affect your blood levels of this drug. Tell your doctor you are using phenytoin prior to surgery or any procedure that makes you unable to take it by mouth. If you have diabetes, this drug may increase your blood sugar levels. Check your blood (or urine) glucose level frequently, as directed by your doctor. Promptly report any abnormal results as directed. Your medicine, exercise plan, or diet may need to be adjusted. 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 breast-feeding.

DRUG INTERACTIONS: 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: azapropazone, darunavir, delavirdine, dofetilide, etravirine, nisoldipine, rilpivirine, colesevelam, molindone, orlistat, pyridoxine (vitamin B6), sucralfate, telithromycin. Other medications can affect the removal of phenytoin from your body, which may affect how phenytoin works. Examples include amiodarone, azole antifungals (such as itraconazole), macrolide antibiotics (such as erythromycin), estrogens, isoniazid, rifamycins (such as rifabutin), St. John's wort, other anti-seizure medicines (such as valproic acid), 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), felodipine, quetiapine, quinidine, suvorexant, 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 additional reliable 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 product can affect the results of certain lab tests. Make sure laboratory personnel and your doctors know you use this drug.

OVERDOSE: 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 their local poison control center at 1-800-222-1222. Canada residents can call a provincial poison control center. Symptoms of overdose may include: severe mental/mood changes, severe drowsiness, loss of consciousness, slowed breathing.