Masking agents (such as diuretics) are used to falsify the results of doping controls and to conceal doping. Substances suitable for this purpose are therefore included in the Prohibited List of the WADA.

The substance class of masking agents comprises primarily of diuretics but also includes probenecid and plasma expanders, as well as substances with a similar chemical structure that alter doping samples, modify urine excretion or conceal the presence of other doping agents.

Diuretics affect the function of the kidneys and thus alter the body's fluid and salt balance (electrolytes: potassium, sodium and phosphorus). They affect various transport systems in the kidney and thus increase the excretion of urine. Body weight is reduced and urine is diluted.

⬆ Water excretion

⬆ Electrolyte excretion

Cardiovascular system

Diuretics massively eliminate fluid and minerals from the body. This disturbs the body's water and salt balance. As a result, acute loss of blood pressure, heart rate irregularities and circulatory shock can occur. Additionally, the thickened blood increases the risk of thrombosis.

Gastrointestinal system and kidneys

As a further consequence, diuretics abuse can lead to gastrointestinal problems and kidney damage.

Muscles

Due to the dehydrating effect of diuretics and the loss of electrolytes, severe muscle cramps can occur.

Because of their concealing effect, diuretics and other masking agents can be abused in all sports. In recent years, positive doping controls have emerged in canoeing, taekwondo, football, fencing and cycling.

In sports with weight classes (boxing, wrestling, judo, weightlifting and others), diuretics are used to reduce body weight. Diuretics are also used in disciplines where a particularly low body weight enhances performance. In 2001, the Russian Dmitry Vasilyev became the first doping case in ski jumping. He had reduced his weight with the diuretic furosemide.

Diuretics are also widespread in body building, where they are nicknamed "water pills". They are used before competitions to flush as much water as possible out of the body. The aim is to make the muscles appear more defined.