Therapeutic index
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The therapeutic index of a medication is a comparison of the amount that causes the therapeutic effect to the amount that causes toxic effects. Quantitatively, it is the ratio of the dose required to produce the desired therapeutic effect and the toxic dose. A commonly used measure of therapeutic index is the effective dose of a drug for 50% of the population (ED50) divided by the lethal dose for 50% of the population (LD50).
Generally, a drug with a narrow therapeutic index will have its dosage adjusted according to actual measurements of the blood levels achieved in the person taking it: examples of such drugs are digoxin and theophylline. This may be achieved through therapeutic drug monitoring (TDM) protocols.