U.S. Anti-Doping Agency (USADA)

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Half-Life of a Drug

execretion_rateWhy is it so difficult to precisely say how long a substance will take to be completely excreted from the body?

The half-life of a drug, or the time it takes for half of the drug to be metabolized and eliminated from the body, depends on many variables. Published half-life data is usually determined by measuring the decrease in parent drug in serum or plasma. In the anti-doping world, the serum half-life is of limited value in determining how long a substance stays in the body because it does not reflect the presence of metabolites (break-down products from the parent drug). These metabolites are often what is measured in anti-doping tests and the serum half-life does not necessarily reflect urine concentrations which is the main sample of choice in anti-doping testing. Furthermore, the half-life can vary greatly between individuals and is specific for each medication. It can be dose-dependent and affected by other factors such as accumulation in adipose tissue. Marijuana is an example of a drug that is excreted in the urine over a prolonged period that could take weeks or months, depending on the route of administration, to clear completely from an athletes’ body. Aspirin on the other hand, is an example of a rapidly excreted drug, and could clear completely from an athletes’ body within hours.

Athletes are strictly responsible for what is found in their body at the time of a drug test. Understanding clearance times of medications, which is the time it takes for the medication, and its metabolites, to be completely eliminated from the body, becomes essential. This is especially important if the athlete is prescribed a medication that is prohibited only in-competition and they are going to be competing soon. If an athlete uses the medication out-of-competition and it is still present in their body when tested at a competition, they may be held responsible for an anti-doping rule violation. A physician or pharmacist should be able to assist in predicting clearance times for medications. However, USADA cannot predict the clearance time for any medication for any particular individual. If an athlete needs to use a substance prohibited in-competition and they are close to competition time, they are strongly encouraged to contact USADAs Drug Reference Department to learn if they need a Therapeutic Use Exemption (TUE).


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