Excessive exercise can negatively alter energy balance in the body and affect the reproductive system. When energy demand exceeds dietary energy intake, a negative energy balance may occur and may result in hypothalamic dysfunction and alterations in gonadotropin-releasing hormone (GnRH) pulsality, leading to menstrual abnormalities.
There are gender-specific side effects of anabolic steroids. Development of breast tissue in males, a condition called gynecomastia (which is usually caused by high levels of circulating estrogen), may arise because of increased conversion of testosterone to estrogen by the enzyme aromatase. Reduced sexual function and temporary infertility can also occur in males. Another male-specific side effect which 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 anabolic steroid use as normal production of sperm resumes. Even after prolonged use of extremely high doses of anabolic steroids, sperm production can return to normal rates for bodybuilders who stopped the consumption of anabolic steroids over 4 months ago.