Interaction between phenytoin and norethisterone in the rhesus monkey.

The effects of phenytoin treatment on plasma concentrations of norethisterone (NET) was studied in 7 healthy female rhesus monkeys. Before phenytoin treatment was started each monkey received an oral dose of 0.5 mg NET and 2 weeks later the same dose was given intravenously. On both occasions, plasma concentrations of NET were measured at frequent intervals during 24 hours. The monkeys were given phenytoin daily orally for more than 4 weeks. The absorption of phenytoin was confirmed by plasma determinations of phenytoin. While still on phenytoin treatment, the monkeys were again given 0.5 mg NET orally and 2 weeks later i.v. and blood sampling was repeated. During phenytoin treatment, plasma NET concentrations were always below those found before treatment. Both at iv and oral administration, the areas under the plasma concentration vs time curve (AUC) were reduced to 55% of the pretreatment AUCs. No difference in plasma half-life was found during phenytoin treatment. The results of this study may suggest that women on antiepileptic treatment with phenytoin should be given an oral contraceptive with a comparatively high dose of the steroids to provide contraceptive efficacy.