Androgenic-anabolic steroids (AAS) are synthetic derivates of testosterone. AAS have been used as performance enhancing agents since the 1930`s. They were put on the IOC`s doping list in 1974, but are still widely used. Today AAS are used mostly by adolescents who are not top athletes, but who use AAS to improve performance and appearance. In Norway about 2% has tried these substances, and the number seems to be increasing. The wanted effects of AAS are increased strength, bodyweight and muscle size. Unfortunately, AAS also have numerous side effects. Some are transient, like acne, oligo-/azoospermia in men, icterus, changes in serum cholesterol levels, hypertension, and elevated hemoglobin levels. Some persists, like striae, peliosis hepatitis, facial hair, clitoris hypertrophy and a deeper voice in women, and decreased final height if AAS is used before puberty. Many general practitioners are unaware of early signs of use, adverse effects and how to do a proper examination, and are thus unable to educate young athletes on this topic. This assignment reviews the physiology of testosterone, what androgenic anabolic steroids are, the adverse effects on the endocrine system, the liver, the blood, the muscles and skeleton, the skin, and the risk of infections. Finally it mentions important questions to ask a possible user, how to examine, and which tests the GP can order to confirm use and follow up side effects.