Based on the principle of superposition an expression has been established relating a drug concentration at steady‐state to a concentration after a single dose. This relationship applies for drugs with linear pharmacokinetics given at equal dosage intervals and it is independent of the route of administration. The relationship provides theoretical justification for the single‐point method of dosage prediction reported in the literature. The test conditions in the method can therefore be optimized and the limits of the method defined. The expression can also be used for individualized prediction of maintenance dose after estimation of drug half‐life in the patient with no limit to half‐life values to which it can be applied.