Early studies of oral norethisterone in men employing doses of 20 to 50 mg/day observed suppression of 17-ketosteroid excretion, increased estrogen excretion (due to conversion into ethinylestradiol), suppression of spermatogenesis, libido, and erectile function, and incidence of gynecomastia. A dosage of oral norethisterone of 25 mg/day for 3 weeks in men has been reported to suppress testosterone levels by about 70%, to 100 to 200 ng/dL, within 4 or 5 days, as well as to suppress sperm count and to have no effect on libido or erectile function over this short time period.