There are also sex-specific side effects of AAS. Development of breast tissue in males, a condition called gynecomastia (which is usually caused by high levels of circulating estradiol ), may arise because of increased conversion of testosterone to estradiol by the enzyme aromatase .  Reduced sexual function and temporary infertility can also occur in males.    Another male-specific side-effect that can occur is testicular atrophy , caused by the suppression of natural testosterone levels, which inhibits production of sperm (most of the mass of the testes is developing sperm). This side-effect is temporary; the size of the testicles usually returns to normal within a few weeks of discontinuing AAS use as normal production of sperm resumes.