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The elimination rate constant (k) (also written as k_e or k_el) represents the proportion of the drug in the body that is eliminated in a given time. The k is expressed as the inverse of time, for example, h⁻¹ or min⁻¹. For instance, a drug with k = 0.1 h⁻¹ will mean that 10% of the drug is being eliminated out of the body each hour. The elimination rate constant is closely related to the elimination half‐life and it complements the understanding of drug elimination. The elimination rate constant of a drug can be calculated from the elimination half‐life of the drug from the equation: .

Figure 2.8 Plasma drug concentration–time profile followed by an intravenous (IV) dose exhibiting first‐order kinetics. Figure shows the estimation of drug half‐live at three different time points.