Conflict of interest: none declared. Financial support: none declared.

• 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.