Absorption and metabolism of phenytoin from tablets and capsules.

Crossover studies were conducted to compare plasma levels of phenytoin and urinary throughput of the major metabolite, 5-(p-hydroxyphenyl)-5-phenylhydantoin (HPPH), after ingestion of single 250-mg oral doses as tablets or as capsules. Plasma levels of phenytoin did not differ significantly for the 2 formulations except at 8 and 12 hr after ingestion of drug, when the mean values for the tablets were higher than for the capsules. The normalized area under the curve, however, did not differ significantly for either preparation, indicating that equivalent amounts of each preparation were absorbed. Urinary throughput of HPPH did not differ significantly between the 2 preparations. The results suggest that the 2 preparations provide equivalent therapeutic effects.