Zuclomiphene Citrate is the cis isomer of clomiphene which exhibits weak estrogen agonist activity evaluated for antineoplastic activity against breast cancer. The individual isoforms are not available commercially, but Repros Therapeutics (The Woodlands, TX, USA) has separated them and is using enclomiphene citrate (ENC) in clinical trials in men with secondary hypogonadism who wish to preserve their fertility. Zuclomiphene, possessing no oestrogen antagonism at physiological concentrations, appears to have a longer biological half-life than enclomiphene, and thus may persist for long periods in the body. At high concentrations zuclomiphene can act as an oestrogen agonist. Clomiphene citrate (CC) is often used ‘off-label’ in men who have low testosterone to raise levels, it is also useful for the restoration of sperm counts in men. CC is approved by FDA and widely used in women for induction of ovulation for several conditions. CC is a mixture of two diastereoisomers, a cis isomer, zuclomiphene citrate (ZUC, 38%) and a trans isomer, ENC (62%). The two clomiphene isomers have mixed estrogenic and antiestrogenic effects that vary among species.