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Phenytoin information from DrugsUpdate  

See Available Brands of Phenytoin in India

P - Contraindicated in pregnancy
L - Caution when used during lactation
FI - Food *
LI - Lab *

Phenytoin is used to control certain type of seizures, and to treat and prevent seizures that may begin during or after surgery to the brain or nervous system. Phenytoin is in a class of medications called anticonvulsants. It works by decreasing abnormal electrical activity in the brain.



Phenytoin acts as an anticonvulsant by increasing efflux or decreasing influx of sodium ions across cell membranes in the motor cortex during generation of nerve impulses; thus stabilising neuronal membranes and decreasing seizure activity. It acts as an antiarrhythmic by extending the effective refractory period and suppressing ventricular pacemaker automaticity, shortening action potential in the heart.

Slow but almost complete from the GI tract (oral); much slower absorption (IM).


Widely distributed. Protein-binding: 90%.

Extensively hepatic; converted to inactive metabolites.

Via urine as hydroxylated metabolite; elimination half life at steady state: 22 hours.

Phenytoin Indications / Phenytoin Uses

Information Not Available

Phenytoin Adverse Reactions / Phenytoin Side Effects

Hypersensitivity, lack of appetite, headache, dizziness, tremor, transient nervousness, insomnia, GI disturbances (e.g. nausea, vomiting, constipation), tenderness and hyperplasia of the gums, acne, hirsutism, coarsening of the facial features, rashes, osteomalacia. Phenytoin toxicity as manifested as a syndrome of cerebellar, vestibular, ocular effects, notably nystagmus, diplopia, slurred speech, and ataxia; also with mental confusion, dyskinesias, exacerbations of seizure frequency, hyperglycaemia. Solutions for inj may cause local irritation or phlebitis. Prolonged use may produce subtle effects on mental function and cognition, especially in children.

Potentially Fatal: Toxic epidermal necrolysis, Stevens-Johnson syndrome.


Information Not Available

Special Precautions

Cardiovascular disease, e.g. sinus bradycardia, heart blocks; DM; hepatic impairment; hypoalbuminemia; porphyria; seizures (may increase frequency of petit mal seizures); debilitated patients; elderly. Caution in IV admin in hypotension, heart failure or MI, monitor BP and ECG during therapy. IV must be given slowly (too rapid admin may cause hypotension, CNS depression, cardiac arrhythmias and impaired heart conduction). Extravasation and intra-arterial admin must be avoided. Do not discontinue abruptly (may increase seizure frequency), unless safety concerns require a more rapid withdrawal. May impair ability to drive or operate machinery.

Other Drug Interactions

Effects with other sedative drugs or ethanol may be potentiated. Enhances toxic effects of paracetamol, lithium. Increased risk of osteomalacia with acetazolamide. Decreased serum levels/effects with acyclovir, antineoplastics, benzodiazeines, ciprofloxacin, CYP2C9 inducers (e.g. carbamazepine), CYP2C19 inducers (e.g. rifampin), folic acid, vigabatrin. Increased serum concentrations with allopurinol, capecitabine, cimetidine, CYP2C9 inhibitors (e.g. fluconazole), CYP2C19 inhibitors (e.g. delavirdine), disulfiram, methylphenidate, metronidazole, omeprazole, SSRI, trazodone, trimethoprim. Increases metabolism of antiarrhythmics, anticonvulsants, antipsychotics, beta-blockers, calcium channel blockers, chloramphenicol, corticosteroids, doxycycline, oestrogens, HMG-CoA reductase inhibitors, methadone, theophylline, TCAs. Decreases levels/effects of clozapine, ciclosporin, tacrolimus, CYP2B6 substrates (e.g. bupropion, selegiline), CYP2C8 substrates (e.g. amiodarone), CYP2C9 substrates (e.g. celecoxib), CYP2C19 substrates (e.g. citalopram), CYP3A4 substrates (e.g. benzodiazepines), digoxin, itraconazole, levodopa, neuromuscular-blocking agents, thyroid hormones, topiramate. Increases levels/effect of dopamine, ticlopidine. Valproic acid may displace phenytoin from binding sites; and affect phenytoin serum concentrations. Transiently increases the hypothrombinaemia response to warfarin initially, followed by an inhibition of the response.

Potentially Fatal: Enhances the hypotensive properties of dopamine and the cardiac depressant properties of lidocaine.

Other Interactions

Food Interaction
Avoid or limit ethanol (interferes with metabolism of phenytoin). Serum concentrations may be altered if taken with food. May decrease calcium, folic acid, and vitamin D levels. Avoid evening primrose (seizure threshold decreased); valerian, St John's wort, kava kava, gotu kola (may increase CNS depression).


Adult: Initially, 3-4 mg/kg daily as single dose or in divided doses. Alternatively, 150-300 mg daily increased gradually to 600 mg daily if necessary. Maintenance: 200-500 mg daily.
Child: Initially, 5 mg/kg daily in 2-3 divided doses. Maintenance: 4-8 mg/kg daily in divided doses. Max dose: 300 mg daily.

Tonic-clonic status epilepticus
Adult: Adjunctive therapy with a benzodiazepine (e.g. diazepam): 10-15 mg/kg by slow injection or intermittent infusion at a max rate of 50 mg/min. Maintenance: 100 mg IV (or orally) given every 6-8 hours.
Child: Neonates: 20 mg/kg as a loading dose, then 2.5-5 mg/kg bid; 1 month-12 years: 18 mg/kg as a loading dose, then 2.5-5 mg/kg bid; >12 years: 18 mg/kg as a loading dose, then up to 100 mg 3-4 times daily.


Should be taken with food. (When administering to patients on nasogastric or other enteral feeds, be consistent throughout therapy in relation to feed times. Do not switch dosage forms/ brands without prior consideration.)

List of Contraindications

Phenytoin and Pregnancy

Contraindicated in pregnancy

Category D: There is positive evidence of human foetal risk, but the benefits from use in pregnant women may be acceptable despite the risk (e.g., if the drug is needed in a life-threatening situation or for a serious disease for which safer drugs cannot be used or are ineffective).

Phenytoin and Lactation

Caution when used during lactation

Phenytoin and Children

Information Not Available

Phenytoin and Geriatic

Information Not Available

Phenytoin and Other Contraindications

Pregnancy. IV admin in sinus bradycardia, heart block, or Stokes-Adams syndrome.


Solution for injection: Store at room temperature of 15-30°C.

Cap, tab: Store below 30°C. Protect from light and moisture; Oral suspension: Store at room temperature of 20-25°C, do not freeze, protect from light.

Lab interference

Solution for injection: Store at room temperature of 15-30°C.

Cap, tab: Store below 30°C. Protect from light and moisture; Oral suspension: Store at room temperature of 20-25°C, do not freeze, protect from light.

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