Last reviewed 01/2018

The initial dose is 3-4mg/kg daily or 150-300mg daily. This may be in a single dose or divided into two doses. The dose is increased gradually as required (table below).

  • adult daily dose 200-500 mg
  • serum half-life 24-48 hours
  • dose frequency x/day 2

Because the plasma half-life is long (24-48 hrs) phenytoin needs to be given only twice or even once daily. When oral administration is not possible, the drug can be given intravenously (eg 100 to 250 mg at <= 50 mg/minute in an adult). For comprehensive details on the use of this drug, see BNF 4.8.1.

The parenteral preparation is strongly alkaline and may crystallise out in tissues; the intramuscular route is therefore painful and associated with unreliable absorption. Daily doses over 300 to 350 mg should be exceeded only cautiously and with frequent monitoring of serum levels. Increments in the daily dose should not exceed 25-50 mg at this level.