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Elimination half‐life is denoted as t_½ and reported in a unit of time (such as minutes or hours). It is an important pharmacokinetic parameter that helps to understand the rate of drug elimination from the body. Half‐life can be defined as ‘the time it takes for the plasma (or blood or serum) drug concentration to reduce by half’. Drug elimination from the body is non‐linear and follows first‐order kinetics for most drugs; therefore, the elimination phase of the pharmacokinetic curve can be explained by the drug's half‐life. Half‐life is a concentration‐independent property; therefore, it can be determined at any point in the elimination phase of the plasma drug concentration–time profile.

Figure 2.8 shows a first‐order pharmacokinetic profile following intravenous drug administration on a log‐linear scale. The profile shows that the drug concentration reduces to half every two hours; hence, the drug's elimination half‐life (t_½) is two hours.

Half‐life, can be used to calculate how long it will take for a drug to be completely removed from the body following a dose, often referred to as the washout period. Typically, it takes three to five half‐lives for most of the drug to be eliminated from the body.

This would mean that a drug with t_½ = 2 h will take about 6 to 10 hours to eliminate from the body (Box 2.4). The drug concentration in the body after five half‐lives does not reach a mathematical zero but is so little that five half‐lives principle is usually acceptable in pharmacokinetic studies. The wash‐out period is important in clinical research and drug development to plan studies and to generate the cleanest possible data set.