text: When a drug is used therapeutically, it is important to understand the margin of safety that exists between the dose needed for the desired effect and the dose that produces unwanted and possibly dangerous side-effects (measured by the TD, the dose that produces toxicity in 50% of individuals). This relationship, termed the therapeutic index, is defined as the ratio TD:ED. In general, the narrower this margin, the more likely it is that the drug will produce unwanted effects. The therapeutic index emphasizes the importance of the margin of safety, as distinct from the potency, in determining the usefulness of a drug.