Doping in sports is the use of prohibited substances or methods to gain an unfair physical advantage over competitors. It covers everything from taking drugs that build muscle or boost endurance to manipulating blood samples or using techniques that artificially increase the body’s oxygen-carrying capacity. The World Anti-Doping Agency (WADA) maintains a global list of banned substances and methods that applies across virtually all Olympic and professional sports.
How a Substance Gets Banned
WADA updates its Prohibited List every year, and the criteria for adding something are straightforward. A substance or method only needs to meet two of three conditions: it has the potential to enhance sport performance, it poses a health risk to the athlete, or it violates the “spirit of sport” as defined in the World Anti-Doping Code. That second and third criterion matter because they mean something can be banned even if its performance benefit is uncertain, as long as it’s dangerous or fundamentally undermines fair play.
The 2025 Prohibited List, which took effect in January, includes hundreds of substances organized into categories like stimulants, hormones, diuretics, and masking agents. Recent changes reclassified hydrafinil as a non-specified stimulant and clarified that athletes can donate blood at accredited collection centers without triggering a violation. The list evolves as new drugs and methods emerge.
Anabolic Steroids and Muscle Growth
Anabolic steroids are probably the most well-known category of performance-enhancing drugs. They’re synthetic versions of testosterone, and they work through two main mechanisms. First, they ramp up protein production inside muscle cells by switching on genes related to muscle growth and repair. Second, they block the hormones (glucocorticoids) that normally break down protein in cells. The net effect is that muscles grow larger and recover faster.
For athletes, the practical result is the ability to train harder and more frequently. Steroids reduce the muscle damage caused by intense workouts, which shortens recovery time between sessions. Over weeks and months, this compounds into measurable increases in lean body mass, muscle fiber size, and strength. Studies have shown dose-dependent gains in capillary density within muscles and overall leg mass in athletes using these drugs.
Blood Doping and EPO
Blood doping targets endurance rather than strength. The goal is simple: get more oxygen to working muscles so you can sustain higher effort for longer. There are two main approaches.
The first is blood transfusion, where an athlete receives extra red blood cells, either their own (drawn and stored weeks earlier) or from a donor. This practice dates back to the early 1970s and was widespread in cycling and distance running before testing improved.
The second, more common method involves erythropoietin, or EPO. Your kidneys naturally produce EPO when they detect low oxygen levels in your tissues. EPO travels to your bone marrow and essentially keeps developing red blood cells alive long enough to mature, preventing them from dying off early through a process called programmed cell death. More surviving red blood cells means more oxygen delivery throughout the body. Synthetic EPO does the same thing, just at artificially elevated levels. The result is a higher red blood cell count and significantly greater aerobic capacity, which is why it became the drug of choice in endurance sports.
Other Prohibited Methods
Doping goes well beyond pills and injections. WADA’s Prohibited List also bans methods like gene doping (manipulating gene expression to enhance performance), tampering with samples, and using intravenous infusions above a certain volume outside of legitimate hospital treatment. Diuretics and masking agents are banned not because they boost performance directly, but because they can hide the presence of other drugs in urine tests or rapidly alter body weight in sports with weight classes.
Stimulants form another major category. These range from amphetamines to certain asthma medications at high doses. They can increase alertness, reduce fatigue, and raise heart rate, giving athletes a temporary edge in competition. Some stimulants are only banned during competition, while others are prohibited at all times.
How Athletes Get Caught
Anti-doping testing has become increasingly sophisticated. Traditional urine and blood tests look for specific banned substances or their breakdown products. But one of the biggest advances is the Athlete Biological Passport, which tracks an individual athlete’s biological markers over time. Rather than looking for a specific drug, it flags unusual changes in blood values that suggest something artificial is going on. If an endurance athlete’s red blood cell indicators suddenly spike in a way that doesn’t match their training or altitude exposure, that pattern alone can serve as evidence of doping, even without identifying the exact substance used.
Athletes at the elite level can be tested at any time, including out of competition. They’re required to provide their whereabouts so testers can find them for unannounced sample collection. Missing three tests within a 12-month period can itself result in an anti-doping violation.
Health Consequences
The health risks of doping are broad and serious. Performance-enhancing drug use has been linked to cardiovascular problems, liver damage, kidney disorders, psychiatric effects, hormonal disruption, and an increased risk of death. Anabolic steroids in particular can cause lasting damage to the endocrine system, meaning the body’s ability to produce its own hormones naturally may never fully recover. EPO abuse thickens the blood, raising the risk of blood clots, stroke, and heart attack.
These risks are often dose-dependent, and athletes engaged in doping frequently use substances at levels far above anything studied in clinical settings, sometimes stacking multiple drugs simultaneously. The long-term consequences can take years or decades to surface.
Therapeutic Use Exemptions
Not every use of a banned substance counts as doping. Athletes with legitimate medical conditions can apply for a Therapeutic Use Exemption (TUE), which allows them to use an otherwise prohibited treatment. Common conditions covered include asthma, ADHD, diabetes, adrenal insufficiency, and various cardiovascular conditions. The application requires detailed medical documentation and must be approved before the athlete uses the substance.
TUEs are one of the more contentious areas of anti-doping policy. Critics argue they create a loophole, while supporters point out that barring athletes from necessary medication would be discriminatory. WADA publishes specific checklists for over 20 medical conditions, outlining exactly what evidence a physician must submit to support an application.