In the Netherlands an estimated 20,000 people use anabolic androgenic steroids (AAS). The use of AAS is particularly common in regular visitors to gyms and fitness centres. AAS are usually synthetic derivatives of testosterone with both an anabolic and an androgenic effect. AAS have many side effects like liver damage (oral use) or infections (intramuscular use), which can be explained partly by the androgenic effect and partly by the manner of use. Many of these side effects are only reported in case studies and have not been systematically investigated.