Some bodybuilders and athletes use trenbolone esters for their muscle-building and otherwise performance-enhancing effects.  Such use is illegal in the United States and many other countries. The DEA classifies trenbolone and its esters as Schedule III controlled substances under the Controlled Substances Act .  Trenbolone is classified as a Schedule 4 drug in Canada  and a class C drug with no penalty for personal use or possession in the United Kingdom .  Use or possession of steroids without a prescription is a crime in Australia . 
A substance can be legal but may be banned in certain circumstances. For instance, an insurance company may test an insurance applicant for nicotine to verify the person's assertion that he or she is not currently using tobacco. In the case of prescription medications, the high addictive potential for certain medications makes drug testing important. In the case of some athletes, many of the prohibited substances are legal but have the potential to affect the athlete's performance. Athletes must exercise caution in their choices of medications as they are ultimately responsible for any substances detected in their bodies.