Phenobarbitone and serum unconjugated oestriol concentrations in four pregnant women with hypertension

Drug effects on the intestinal absorption of estrogens

It is now recognized that intestinal metabolism is one of the major factors affecting the bioavailability of orally administered, natural and synthetic estrogens. Other drugs and dietary constituents taken concomitantly may modify the intestinal metabolism and enterohepatic circulation of these steroids. Various antibiotics, phenobarbitone and large doses of ascorbic acid have been reported to modify the clinical efficacy of steroidal oral contraceptives. Experimental and other evidence indicate that antibiotics, by modifying the intestinal microflora, affect the intestinal metabolism and enterohepatic circulation of estrogens. Phenobarbitone and ascorbic acid on the other hand probably influence intestinal metabolism through an effect on intestinal mucosal enzymes. Further experimental evidence in rats suggest that such commonly used drugs as analgesic acetylsalicylic acid, antimalarial chloroquine phosphate and alcohol also alter the intestinal mucosal metabolism of estradiol and estrone sulphate and may thus alter the bioavailability of these estrogens. Further studies are necessary to evaluate the clinical significance of these observations.