Drug testing has become mandatory in most professional sports leagues. By testing for both illegal and performance enhancing drugs, many leagues feel that they are able to prevent their athletes from being involved in reckless behavior and detracting from the sports field they are a part of. Each league has its regulations at this point as only performance enhancing drugs are regulated by the WADA regulations.
Marijuana drug testing typically involves giving a urine sample that is then tested for levels of THC. Urine is a good type of drug test for marijuana because the kidneys retain the THC until you urinate; the substance is only expelled with the urine. The amount of THC found in the urine is dependent on how much and how often the subject uses Marijuana as well as the rate of metabolism. Research indicates that heavy smokers of the drug have tested positive as much as years later because it can sometimes be retained in body fat.
Hair testing is also being employed to determine marijuana use but this method is for detecting past use as it takes at least 2 weeks for the THC to be found in the hair follicle.
Athletes have high rates of marijuana use according to a 2005 survey and so it makes sense that athletes would be tested for illegal drugs and performance enhancing drugs. Add to this the impairments on concentration, eye-hand coordination, motor coordination and increase of fatigue that the drug imparts to users and you can see why Marijuana is a drug that many leagues test for.