Extent of urinary excretion of p-hydroxyphenytoin in healthy subjects given phenytoin

Ther Drug Monit. 1985;7(3):283-9. doi: 10.1097/00007691-198507030-00008.


Urinary excretion of p-hydroxyphenytoin and its glucuronide conjugate was measured in eight healthy young adults in a comparative bioavailability study of oral sodium phenytoin (approximately 5 mg/kg/dose). Among these subjects the percentage of the phenytoin dose converted to p-hydroxyphenytoin and appearing in urine was relatively similar (mean 79%, range 67-88%). The great majority of the p-hydroxyphenytoin appeared in urine as conjugates; only 1.4-3.4% of the excreted p-hydroxyphenytoin was in the form of unconjugated metabolite. The proportion of a single phenytoin dose excreted in urine as p-hydroxyphenytoin or its conjugate increased from the first dose (mean +/- SD) 74.9 +/- 4.6% to the second dose, given 2 weeks later 79.3 +/- 4.6% (p less than 0.05). This finding suggests that autoinduction of phenytoin metabolism may occur after relatively brief exposure to the drug.

MeSH terms

  • Adult
  • Biological Availability
  • Female
  • Glucuronates / metabolism
  • Humans
  • Kinetics
  • Male
  • Phenytoin / analogs & derivatives*
  • Phenytoin / metabolism*
  • Phenytoin / urine


  • Glucuronates
  • hydroxyphenytoin
  • Phenytoin