How long does it take for a substance to leave 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).

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Source: USADA