Phenytoin Uses

Phenytoin is used to treat generalized tonic-clonic (grand mal) seizures and complex partial (psychomotor, temporal lobe) seizures. It is also used to prevent and treat seizures that occur during or after surgery on the brain.

There may be other uses of phenytoin—your pharmacist or doctor can give you further drug information.

Phenytoin Mechanism of Action

Phenytoin decreases abnormal activity in the central nervous system by blocking voltage-dependent sodium channels in the membranes of neurons (brain cells), thereby reducing sustained high-frequency discharges from hyperactive neurons.

Phenytoin Doses

This medicine comes in the form of extended phenytoin sodium capsules (100 mg, 200 mg, and 300 mg), chewable tablets (50 mg), and an oral suspension (125 mg/5 mL).

The usual starting minimum adult dosage of phenytoin in individuals who have not taken this medication before is one 100 mg extended phenytoin sodium capsule three times a day. This can be increased to two capsules (200 mg) three times a day. However, clinically relevant doses for most adults are one capsule 3-4 times a day. Your doctor will request monitoring of phenytoin serum levels before adjusting your dose. Serum phenytoin concentrations of 10-20 mcg/mL are typically clinically effective in patients chronically administered phenytoin.

Once you achieve optimal clinical outcome and stable phenytoin serum levels by taking the medicine in equally divided doses (100 mg three times a day), your healthcare provider may switch you to once-daily dosing (one 300 mg extended phenytoin sodium capsule once daily). 

In children, the initial phenytoin dose is typically 5 mg/kg/day in 2-3 equally divided doses. The maintenance dose typically ranges from 4 mg/kg/day to 8 mg/kg/day. The maximum phenytoin dose in children is 300 mg daily. Children over 6 years of age may require the minimum adult dosage (300 mg/day).