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J Clin Pharmacol. 1999 Sep;39(9):936-40.

Inhibition of caffeine metabolism by estrogen replacement therapy in postmenopausal women.

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Geriatric Psychopharmacology Program, Western Psychiatric Institute and Clinic, Pittsburgh, PA 15213, USA.


This study was conducted to investigate the effect of therapeutic estrogen on cytochrome P450 1A2-mediated metabolism in postmenopausal women using caffeine as a model substrate. Twelve healthy postmenopausal women underwent estrogen replacement therapy in the form of estradiol (Estrace). Estradiol was initiated at a dose of 0.5 mg a day and titrated to achieve a steady-state plasma concentration of 50 to 150 pg/ml. Caffeine metabolic ratios (CMR; paraxanthine/caffeine) were assessed both before and after 8 weeks of estrogen replacement. For the 12 subjects, there was a mean reduction in CMR of -29.2 +/- 25.0 (p = 0.0019). Consistent with previous results found in younger women, these results indicate that exogenous estrogen in older women may inhibit CYP1A2-mediated caffeine metabolism.

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