Abstract Context Cyproterone acetate (CPA) is a competitive inhibitor of the androgen receptor and exerts negative hypothalamic feedback. It is often used in combination with estrogens in trans women to achieve feminization. However, CPA has been associated with side effects such as changes in liver enzyme concentrations and increases in prolactin concentrations. The question is whether the testosterone-lowering effect, as well as these side effects, are dose dependent. Objective To assess the lowest effective dose of CPA in trans women to prevent side effects. Methods This longitudinal study, conducted at gender identity centers in Amsterdam, Ghent, and Florence, is part of the European Network for the Investigation of Gender Incongruence (ENIGI), a multicenter prospective cohort study. Participants were trans women (n = 882) using estrogens only or in combination with 10, 25, 50, or 100 mg CPA daily. The primary outcome measure was the concentration of testosterone at 3 and/or 12 months of hormone therapy. Results Using estrogens only (without CPA) led to testosterone concentrations of 5.5 nmol/L (standard error of the mean [SEM] 0.3). All doses of CPA resulted in testosterone concentrations below the predefined threshold of suppression of 2 nmol/L (10 mg, 0.9 nmol/L, SEM 0.7; 25 mg, 0.9 nmol/L, SEM 0.1; 50mg, 1.1 nmol/L, SEM 0.1; 100 mg, 0.9 nmol/L, SEM 0.7). Higher prolactin and lower high-density lipoprotein concentrations were observed with increasing doses of CPA. No differences in liver enzyme concentrations were found between the doses. Conclusion Compared with higher doses of CPA, a daily dose of 10 mg is equally effective in lowering testosterone concentrations in trans women, while showing fewer side effects.