Anabolic steroids are synthetic versions of the male hormone testosterone that are used to build muscle mass but can have dangerous side effects. They work by increasing muscle, aggression and cholesterol levels but can also cause acne, baldness, breast growth in men, shrunken testicles, and potentially fatal heart damage or liver cysts. Women risk masculinization including facial hair growth and enlarged clitoris. Both sexes may experience early baldness, stunted growth if taken during teenage years, and decreased fertility.