[Effect of levonorgestrel and ethinyl estradiol and their combination on biotransformation reactions]

Caffeine is mainly metabolized by 3-methylcholanthreneinducible cytochrome P-450, whereas metamizol (Analgin) is probably mainly metabolized by the phenobarbital inducible cytochrome P-450 family. Therefore the elimination of caffeine from serum and the elimination of the main metabolites of metamizol in urine reflect the activity of these two cytochrome P-450 families. Sex hormones can influence the activity of cytochrome P-450. Intake of levonorgestrel (0.125 mg) daily for 14 days reduced the metabolism of caffeine slightly, but the elimination of metamizol-metabolites is not influenced. Longterm administration of levonorgestrel (0.250 mg daily) for three months did not change the metabolisms of both model substances tested. In contrast, ethynylestradiol (0.050 mg) alone and also the combination with levonorgestrel markedly retarded the elimination rate of caffeine and metamizol-metabolites. This fact should be taken into consideration in drug therapy.