Track and field athletes are banned from ingesting or using certain substances by governing bodies for the sport, from the national to the international level. The IAAF's constitution incorporates the World Anti-Doping Code among other anti-doping measures. Practices such as blood doping and the use of anabolic steroids, peptide hormones, stimulants, or diuretics can give athletes a physical competitive advantage in track and field. The use of such substances in track and field is opposed on both ethical and medical grounds. Given that the sport functions by measuring and comparing athletes' performances, performance-enhancing substances create an uneven playing field — athletes who do not use doping substances have a disadvantage over rivals who do. Medically, the use of banned substances may have an adverse effect upon athletes' health. However, some exemptions are made for athletes who take banned substances for therapeutic use, and athletes are not sanctioned for usage in these cases, such as Kim Collins' failed drug test due to asthma medication.