Phenytoin level

Monitoring phenytoin level ensures treatment is both safe and effective.

Phenytoin level Overview

Reviewed: April 22, 2014

Phenytoin is used for the management of tonic-clonic (grand mal) and complex partial seizures, and for prevention of seizures following head trauma or neurosurgery.

Phenytoin level is measured in microgram/milliliters (mcg/mL). 

The therapeutic range for phenytoin is 10–20 mcg/mL.

  • Toxic level: 25-50 mcg/mL
  • Lethal level: >100 mcg/mL


Blood draw

What the results mean

A level that is higher than normal is usually due to a change in formulation or drug interaction. A high phenytoin level may require a reduction in dose. Some symptoms of toxicity include: ataxia, dizziness, nystagmus, diplopia, and paradoxical seizures.

A low phenytoin level may require an increase in phenytoin dose.  A low phenytoin level may be caused by an absorption problem. This becomes important in young infants (younger than 3 months) or in patients given phenobarbital, charcoal, or antacids at the same time as phenytoin. A low phenytoin level may be associated with noncompliance.