he structural formulas for the active hormones are: HO Ethinyl Estradiol [19-Norpregna-1,3,5(10)-trien-20-yne-3,17-diol, (17a)-] 0 II 0 ,..CCH, CH l .0-= CH 0 Norethindrone Acetate [19-Norpregn-4-en-20-yn-3-one, 17-(acetyloxy)-, (17a)-] CLINICAL PHARMACOLOGY Combination oral contraceptives act by supp