Important: This page is general information, not medical advice. It does not recommend or prescribe any treatment. Always consult your doctor or pharmacist before starting, stopping or changing any medicine.
Quick facts
- Drug class
- Anti-epileptic Agent [EPC], Cytochrome P450 1A2 Inducers [MoA], Cytochrome P450 2B6 Inducers [MoA], Cytochrome P450 2C8 Inducers [MoA]
- How it is given
- oral
- Type
- human prescription drug
- RxNorm ID
- 1313885
We do not publish dosing. How much of this medicine to take, how often, and for how long depends on your age, weight, kidney and liver function, other conditions and other medicines. That is a decision for the person who prescribes or dispenses it. Follow the directions on your own pack and ask your pharmacist if anything is unclear.
What it is used to treat
Phenytoin chewable tablets are indicated for the treatment of generalized tonic-clonic (grand mal) and complex partial (psychomotor, temporal lobe) seizures and prevention and treatment of seizures occurring during or following neurosurgery. Phenytoin chewable tablets are indicated for the treatment of generalized tonic-clonic (grand mal) and complex partial (psychomotor, temporal lobe) seizures and prevention and treatment of seizures occurring during or following neurosurgery. ( 1 )
About this medicine
Phenytoin is related to the barbiturates in chemical structure, but has a five-membered ring. The chemical name is 5,5-diphenyl-2,4 imidazolidinedione, having the following structural formula: Each phenytoin chewable tablet, USP for oral administration, contains 50 mg phenytoin, USP. Also contains: Artificial banana flavor, compressible sugar, D&C yellow No. 10 aluminum lake, FD&C yellow No. 6 aluminum lake, hypromellose 2208, lactose monohydrate, magnesium stearate, saccharin sodium, and talc. Chemical Structure
How it works
12.1 Mechanism of Action The precise mechanism by which phenytoin exerts its therapeutic effect has not been established but is thought to involve the voltage-dependent blockade of membrane sodium channels resulting in a reduction in sustained high-frequency neuronal discharges.
Who should not take it
Phenytoin chewable tablets are contraindicated in patients with: • A history of hypersensitivity to phenytoin, its inactive ingredients, or other hydantoins [see Warnings and Precautions (5.5) ] . Reactions have included angioedema. • A history of prior acute hepatotoxicity attributable to phenytoin [see Warnings and Precautions (5.8) ] . • Coadministration with delavirdine because of the potential for loss of virologic response and possible resistance to delavirdine or to the class of non-nucleoside reverse transcriptase inhibitors. • Hypersensitivity to phenytoin, its ingredients, or other hydantoins ( 4 , 5.5 ) • A history of prior acute hepatotoxicity attributable to phenytoin ( 4 , 5.8 ) • Coadministration with delavirdine ( 4 )
Warnings and cautions
• Withdrawal Precipitated Seizure: May precipitate status epilepticus. Dose reductions or discontinuation should be done gradually. ( 5.1 ) • Suicidal Behavior and Ideation: Monitor patients for the emergence or worsening of depression, suicidal thoughts or behavior, and/or any unusual changes in mood or behavior. ( 5.2 ) • Serious Dermatologic Reactions: Discontinue phenytoin chewable tablets at the first sign of a rash, unless the rash is clearly not drug-related. If signs or symptoms suggest SJS/TEN, use of this drug should not be resumed and alternative therapy should be considered. ( 5.3 ) • Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS)/Multiorgan Hypersensitivity: If signs or symptoms of hypersensitivity are present, evaluate the patient immediately. Discontinue if an alternative etiology cannot be established. ( 5.4 ) • Cardiac Effects: Bradycardia and cardiac arrest have been reported. ( 5.6 ) • Angioedema: Discontinue immediately if symptoms of angioedema such as facial, perioral, or upper airway swelling occur. ( 5.7 ) • Hepatic Injury: Cases of acute hepatotoxicity have been reported with phenytoin chewable tablets. If this occurs, immediately discontinue. ( 4 , 5.8 ) • Hematopoietic Complications: If occurs, follow-up observation is indicated and an alternative antiepileptic treatment should be used. ( 5.9 ) 5.1 Withdrawal Precipitated Seizure, Status Epilepticus Abrupt withdrawal of phenytoin in epileptic patients may precipitate status epilepticus. When, in the judgment of the clinician, the need for dosage reduction, discontinuation, or substitution of alternative anticonvulsant medication arises, this should be done gradually. However, in the event of an allergic or hypersensitivity reaction, more rapid substitution of alternative therapy may be necessary. In this case, alternative therapy should be an anticonvulsant not belonging to the hydantoin chemical class. 5.2 Suicidal Behavior and Ideation Antiepileptic drugs (AEDs), including phenytoin chewable tablets, increase the risk of suicidal thoughts or behavior in patients taking these drugs for any indication. […]
Reported side effects
The following serious adverse reactions are described elsewhere in the labeling: • Withdrawal Precipitated Seizure, Status Epilepticus [see Warnings and Precautions (5.1) ] • Suicidal Behavior and Ideation [see Warnings and Precautions (5.2) ] • Serious Dermatologic Reactions [see Warnings and Precautions (5.3) ] • Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS)/Multiorgan Hypersensitivity [see Warnings and Precautions (5.4) ] • Hypersensitivity [see Warnings and Precautions (5.5) ] • Cardiac Effects [see Warnings and Precautions (5.6) ] • Angioedema [see Warnings and Precautions (5.7) ] • Hepatic Injury [see Warnings and Precautions (5.8) ] • Hematopoietic Complications [see Warnings and Precautions (5.9) ] • Effects on Vitamin D and Bone [see Warnings and Precautions (5.10) ] • Exacerbation of Porphyria [see Warnings and Precautions (5.12) ] • Teratogenicity and Other Harm to the Newborn [see Warnings and Precautions (5.13) ] • Hyperglycemia [see Warnings and Precautions (5.14) ] The following adverse reactions associated with the use of phenytoin were identified in clinical studies or postmarketing reports. Because these reactions are reported voluntarily from a population of uncertain size, it is not always possible to reliably estimate their frequency or establish a causal relationship to drug exposure. Body as a Whole: Allergic reactions in the form of rash and rarely more serious forms and DRESS have been observed, as has angioedema [see Warnings and Precautions (5.3 , 5.4 , 5.7) ] . Anaphylaxis has also been reported. There have also been reports of coarsening of facial features, systemic lupus erythematosus, periarteritis nodosa, and immunoglobulin abnormalities. Digestive System: Acute hepatic failure, toxic hepatitis, liver damage, nausea, vomiting, constipation, enlargement of the lips, and gingival hyperplasia. Hematologic and Lymphatic System: Hematopoietic complications, some fatal, have occasionally been reported in association with administration of phenytoin. These have included thrombocytopenia, leukopenia, granulocytopenia, agranulocytosis, and pancytopenia with or without bone marrow suppression. […]
Interactions with other medicines
Phenytoin is extensively bound to plasma proteins and is prone to competitive displacement. Phenytoin is primarily metabolized by the hepatic cytochrome P450 enzyme CYP2C9 and to a lesser extent by CYP2C19, and is particularly susceptible to inhibitory drug interactions because it is subject to saturable metabolism. Inhibition of metabolism may produce significant increases in circulating phenytoin concentrations and enhance the risk of drug toxicity. Monitoring of phenytoin serum levels is recommended when a drug interaction is suspected. Phenytoin is a potent inducer of hepatic drug-metabolizing enzymes. Multiple drug interactions because of extensive plasma protein binding, saturable metabolism and potent induction of hepatic enzymes. ( 7.1 , 7.2 ) 7.1 Drugs that Affect Phenytoin Concentrations Table 2 includes commonly occurring drug interactions that affect phenytoin concentrations. However, this list is not intended to be inclusive or comprehensive. Individual prescribing information from relevant drugs should be consulted. The addition or withdrawal of these agents in patients on phenytoin therapy may require an adjustment of the phenytoin dose to achieve optimal clinical outcome. Table 2: Drugs That Affect Phenytoin Concentrations Interacting Agent Examples Drugs that may increase phenytoin serum levels Antiepileptic drugs Ethosuximide, felbamate, oxcarbazepine, methsuximide, topiramate Azoles Fluconazole, ketoconazole, itraconazole, miconazole, voriconazole Antineoplastic agents Capecitabine, fluorouracil Antidepressants Fluoxetine, fluvoxamine, sertraline Gastric acid reducing agents H 2 antagonists (cimetidine), omeprazole Sulfonamides Sulfamethizole, sulfaphenazole, sulfadiazine, sulfamethoxazole-trimethoprim Other Acute alcohol intake, amiodarone, chloramphenicol, chlordiazepoxide, disulfiram, estrogen, fluvastatin, isoniazid, methylphenidate, phenothiazines, salicylates, ticlopidine, tolbutamide, trazodone, warfarin Drugs that may decrease phenytoin serum levels Antacids Antacids may affect absorption of phenytoin. […]
Use in specific groups
• Pregnancy: Prenatal exposure to phenytoin may increase the risks for congenital malformations and other adverse developmental outcomes. ( 5.13 , 8.1 ) • Renal and/or Hepatic Impairment or Hypoalbuminemia: Monitor unbound phenytoin concentrations in these patients. ( 8.6 ) 8.1 Pregnancy Pregnancy Exposure Registry There is a pregnancy exposure registry that monitors pregnancy outcomes in women exposed to antiepileptic drugs (AEDs), such as phenytoin, during pregnancy. Physicians are advised to recommend that pregnant patients taking phenytoin enroll in the North American Antiepileptic Drug (NAAED) Pregnancy Registry. This can be done by calling the toll free number 1-888-233-2334, and must be done by patients themselves. Information on the registry can also be found at the website http://www.aedpregnancyregistry.org/. Risk Summary In humans, prenatal exposure to phenytoin may increase the risks for congenital malformations and other adverse developmental outcomes. Prenatal phenytoin exposure is associated with an increased incidence of major malformations, including orofacial clefts and cardiac defects. In addition, the fetal hydantoin syndrome, a pattern of abnormalities including dysmorphic skull and facial features, nail and digit hypoplasia, growth abnormalities (including microcephaly), and cognitive deficits has been reported among children born to epileptic women who took phenytoin alone or in combination with other antiepileptic drugs during pregnancy [see Data ] . There have been several reported cases of malignancies, including neuroblastoma, in children whose mothers received phenytoin during pregnancy. Administration of phenytoin to pregnant animals resulted in an increased incidence of fetal malformations and other manifestations of developmental toxicity (including embryofetal death, growth impairment, and behavioral abnormalities) in multiple species at clinically relevant doses [see Data ] . In the U.S. general population, the estimated background risk of major birth defects and of miscarriage in clinically recognized pregnancies is 2 to 4% and 15 to 20%, respectively. […]
If too much is taken
The lethal dose in pediatric patients is not known. The lethal dose in adults is estimated to be 2 grams to 5 grams. The initial symptoms are nystagmus, ataxia, and dysarthria. Other signs are tremor, hyperreflexia, lethargy, slurred speech, blurred vision, nausea, and vomiting. The patient may become comatose and hypotensive. Bradycardia and cardiac arrest have been reported [see Warnings and Precautions (5.6) ] . Death is caused by respiratory and circulatory depression. There are marked variations among individuals with respect to phenytoin serum levels where toxicity may occur. Nystagmus, on lateral gaze, usually appears at 20 mcg/mL, ataxia at 30 mcg/mL; dysarthria and lethargy appear when the serum concentration is over 40 mcg/mL, but as high a concentration as 50 mcg/mL has been reported without evidence of toxicity. As much as 25 times the therapeutic dose has been taken to result in a serum concentration over 100 mcg/mL with complete recovery. Irreversible cerebellar dysfunction and atrophy have been reported. Treatment: Treatment is nonspecific since there is no known antidote. The adequacy of the respiratory and circulatory systems should be carefully observed and appropriate supportive measures employed. Hemodialysis can be considered since phenytoin is not completely bound to plasma proteins. Total exchange transfusion has been used in the treatment of severe intoxication in pediatric patients. In acute overdosage the possibility of other CNS depressants, including alcohol, should be borne in mind.
Storing it safely
16.2 Storage and Handling Store at 20° to 25°C (68° to 77°F) [see USP Controlled Room Temperature]. Protect from moisture.
Before you take anything new
Tell your pharmacist or doctor about every medicine you take — including ones bought without a prescription, herbal products and supplements. Interactions are one of the most common avoidable causes of harm from medicines, and a pharmacist can check your full list in a couple of minutes.
This page is built from the openFDA record of the U.S. Food and Drug Administration's approved labelling for this product, effective 29 April 2026. Label text has been shortened and reformatted for readability; safety wording has not been altered in meaning. FDA labelling describes products approved in the United States — brand names, strengths, formulations and approved uses may differ in your country. Always read the leaflet that comes with your own pack. PocketsInfo is not affiliated with, and is not endorsed by, the FDA. Retrieved 2026-07-30.