A few progestins have weak estrogenic activity. These include the 19-nortestosterone derivatives norethisterone, noretynodrel, and tibolone, as well as the norethisterone prodrugs norethisterone acetate, norethisterone enanthate, lynestrenol, and etynodiol diacetate. The estrogenic activity of norethisterone and its prodrugs are due to metabolism into ethinylestradiol. High doses of norethisterone and noretynodrel have been associated with estrogenic side effects such as breast enlargement in women and gynecomastia in men, but also with alleviation of menopausal symptoms in postmenopausal women. In contrast, non-estrogenic progestins were not found to be associated with such effects.