The concept of therapeutic index refers to the relationship between toxic and therapeutic dose. This pharmacodynamic parameter is relevant to clinical practice because it determines how safe (or toxic) a drug is.

Article outline:

Therapeutic index equation

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 a drug that causes a toxic response in 50% of the population and ED50 is the dose of a drug that is therapeutically effective in 50% of the population.

How therapeutic index is determined

Therapeutic index and quantal dose-response curves

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.

There are some important characteristics of quantal dose-response curves (see image above) that are worth noting:


The graph below shows how ED50 is calculated.

The dose required to cause a therapeutic effect (positive response) in 50% of a population is the ED50.


The dose required to produce a toxic effect in 50% of the studied population is the TD50. For animal studies, LD50 is the dose that results in the death of 50% of the population.

Narrow therapeutic index drugs

The list below mentions some examples of narrow therapeutic index drugs:

References and further reading

Craig, C. Modern Pharmacology With Clinical Applications. 6th ed. LWW, 2003

Golan, D. “Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy”. 2nd ed. LWW, 2008.