It suppresses gonadal androgen production and hence circulating androgen levels due to its antigonadotropic effects; it stimulates hepatic sex hormone-binding globulin (SHBG) production, thereby increasing circulating levels of SHBG and decreasing the free fraction of testosterone and dihydrotestosterone (DHT) in the circulation; and it may have direct cytotoxic effects in the testes and prostate gland. DES has also been found to decrease DNA synthesis at high doses.DES is a long-acting estrogen, with a nuclear retention of around 24 hours. Antigonadotropic effects
Due to its estrogenic activity, DES has antigonadotropic effects. That is, it exerts negative feedback on the hypothalamic–pituitary–gonadal axis (HPG axis), suppresses the secretion of the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), and suppresses sex hormone production as well as gamete production or maturation in the gonads. A study of ovulation inhibition in women found that 5 mg/day oral DES was 92% effective, with ovulation occurring in only a single cycle. DES consistently suppresses testosterone levels in men into the castrate range (<50 ng/dL) within 1 to 2 weeks at doses of 3 mg/day and above. Conversely, a dosage of 1 mg/day DES is unable to fully suppress testosterone levels into the castrate range in men, which instead often stabilize at just above castrate levels (>50 ng/dL). However, it has also been reported that 1 mg/day DES results in approximately 50% suppression of testosterone levels, albeit with wide interindividual variability. It has been said that doses of DES of less than 1 mg/day have no effect on testosterone levels.
The FDA also removed prevention of miscarriage as an indication for DES use and added pregnancy as a contraindication for DES use. On February 5, 1975, the FDA ordered 25 mg and 100 mg tablets of DES withdrawn, effective February 18, 1975. The number of persons exposed to DES during pregnancy or in utero during the period of 1940 to 1971 is unknown, but may be as high as 2 million in the United States. DES was also used in other countries, most notably France, the Netherlands, and Great Britain. From the 1950s through the beginning of the 1970s, DES was prescribed to prepubescent girls to begin puberty and thus stop growth by closing growth plates in the bones. Despite its clear link to cancer, doctors continued to recommend the hormone for "excess height".In 1960, DES was found to be more effective than androgens in the treatment of advanced breast cancer in postmenopausal women.