therapeutic index

The therapeutic index (also known as therapeutic ratio), is a comparison of the amount of a therapeutic agent that causes the therapeutic effect to the amount that causes toxic effects. Quantitatively, it is the ratio given by the dose required to produce the toxic effect divided by the therapeutic dose. A commonly used measure of therapeutic index is the lethal dose of a drug for 50% of the population (LD₅₀) divided by the minimum effective dose for 50% of the population (ED₅₀).
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The ratio of the dose required to produce toxic or lethal effects to the dose required to produce nonadverse or therapeutic response.