Banned for athletes - and farmers
Clenbuterol is used for chronic breathing disorders such as asthma.
It is on the World Anti-Doping Agency's banned substance list because it can stimulate the beta-2 receptors, which in turn help an athlete to build muscle by allowing the body to release and burn more stored fat.
This is why it's used by livestock farmers to enhance lean pork.
The most high-profile case was three-time Tour de France champion Alberto Contador, who tested positive during the 2010 Tour.
He returned a minute concentration level of 50pg/ml. His adviser claimed he would have needed 180 times that amount to benefit his performance.
In January, the Spanish Cycling Federation (RFEC) proposed a one-year ban, but it subsequently accepted Contador's appeal and cleared him of all charges.
However, the UCI and WADA appealed to the Court of Arbitration for Sport and his case will be heard from November 21 after being deferred from June.
Last month Wada withdrew its appeal to the Court of Arbitration for Sport regarding the Mexican Football Association's decision not to sanction five footballers who tested positive for clenbuterol during the Concacaf Gold Cup in June.
The players blamed the positive samples on meat they ate at a training camp before the tournament.
Clenbuterol is strongly prohibited in Spain and other countries of the European Union, which outlawed the drug for animal use in 1996 and systematically monitors farms to ensure the ban is enforced.
Spain has tested 19,431 samples and none were positive.
In Mexico's case, WADA subsequently received compelling evidence from a Fifa study at the U17 World Cup indicating a serious problem in Mexico of meat being contaminated with clenbuterol - in other words, a public health issue rather than doping.