Testosterone is the male sex hormone responsible for masculine secondary sexual characteristics as well as the promotion of muscle growth. Males produce testosterone by chemical reactions in the testes. If an athlete takes anabolic (building up) steroids to enhance his or her performance, the natural reactions that produce testosterone will slow down or stop. The tissues in which these reactions take place will atrophy and there is a serious question as to whether the tissues will fully recover when the athlete stops taking steroids. Large doses of anabolic steroids over time can result in shrinking testes, enlarged prostate, impotence, fluid retention, high blood pressure and heart attack. Women lose secondary female sexual characteristics and suffer masculinization. Both males and females show mood changes (often violent) and increased aggressiveness.