therapeutic index (therapeutic window) The therapeutic usefulness of a drug, expressed as a ratio of the maximum nontoxic dose and the minimum effective dose of the drug. The therapeutic index is commonly used as a measure of the range of the dose considered to be safe. However, due to the variation in individual response to a given drug, the therapeutic index cannot be calculated accurately.

therapeutic index (th'e-ră-pew-tik) n. the ratio of a dose of a therapeutic agent that produces damage to normal cells to the dose necessary to have a defined level of anticancer activity. It indicates the relative efficacy of a treatment against tumours.