The concept of therapeutic index refers to the relationship between toxic and therapeutic dose. The therapeutic index of a drug is the ratio of the dose that produces toxicity to the dose that produces a clinically desired or effective response in a population of individuals.
Where: TD50 is the dose of drug that causes a toxic response in 50% of the population and ED50 is the dose of drug that is therapeutically effective in 50% of the population. Both ED50 and TD50 are calculated from quantal dose response curves, which represent the frequency with which each dose of drug elicits the desired response or toxic effect in the population.

Dose of drug in plasma is plotted in the horizontal axis while the percentage of individuals (animals or humans) that responds or shows a toxic effect is represented in the vertical axis. The dose required to cause a therapeutic effect (positive response) in 50% of a population is the ED50.
This pharmacodynamic parameter is relevant to clinical practice because it determines how safe (or toxic) a drug is. Some examples of positive responses include: relief of headache for an antimigraine drug, increase in heart rate of at least 20 bpm for a cardiac stimulant, or 10 mmHg fall in diastolic blood pressure for an antihypertensive.

Unlike graded dose-response graphs, data for quantal dose-response curves is obtained from many individuals.

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