Therapeutic index

The therapeutic index is a measure of a drug's margin of safety. It is commonly calculated as the dose that causes a toxic effect in half of a population (the TD50) divided by the dose that produces the wanted effect in half of that population (the ED50). In animal studies the dose lethal to half the animals, the LD50, may be used instead. The larger the ratio, the wider the gap between a helpful dose and a harmful one.

Drugs with a wide therapeutic index, such as penicillin and many over-the-counter medicines, are forgiving: a moderate error in dose, or a small difference between brands, is unlikely to cause harm. Drugs with a narrow therapeutic index, such as warfarin, digoxin and lithium, work at concentrations close to those that cause poisoning. Small changes in dose, kidney function, diet or other medicines can tip them into toxicity or make them ineffective, so their blood levels or effects are often monitored.

A related idea is the therapeutic window, the range of blood concentrations between the lowest level that works and the level at which unacceptable side effects begin.