1 A 'low-dose' combined oral contraceptive steroid (OCS) preparation containing 30 microgram ethinylo-estradiol and 150 microgram levonorgestrel was found to reduce significantly antipyrine clearance in a group of women acting as their own controls. 2 An OCS preparation containing only a progestogen (75 microgram norgestrel) did not reduce antipyrine clearance in a second group of women. 3 The evidence suggesting that the oestrogen component of combined OCS preparations could be responsible for the reduction in antipyrine clearance is discussed.
PIP: This study investigates the ability of a single preparation of either the low-dose combined or progestogen-only type of oral contraceptives (OCs) to alter antipyrine elimination in 2 groups of women. 6 patients aged 22-28 were given a low-dose combined OC preparation containing 30 mcg ethinyl estradiol and 150 mcg levonorgestrel; 12 patients aged 18-41 were given a preparation containing only a progestogen, 75 mcg norgestrel. Antipyrine elimination kinetics was setermined during the menstrual cycle, and at 11 weeks for the 1st group, and at between 11-14 weeks for the 2nd group, in saliva samples. There was a significant reduction of 29% in antipyrine clearance in the 1st group, and no significant change in antipyrine elimination in the 2nd group. These results are in agreement with previous published results; it is possible that the estrogen component of combined OC preparations could be responsible for the reduction in antipyrine clearance. If it is confirmed that progestogen-only OCs are without effect on antipyrine elimination, then it does imply that the concern that the elimination of any drug which is metabolized by liver microsomal enzymes will be impaired when the drug is administered with an OC is not applicable to progestogen-only preparations.