How is the therapeutic index defined?

The therapeutic index is an important pharmacological concept that quantifies the safety of a drug. It is defined as the ratio of the toxic dose to the effective dose. More specifically, the therapeutic index is calculated using the formula TD50/ED50, where TD50 is the dose at which 50% of the population experiences toxic effects, and ED50 is the dose at which 50% of the population experiences the desired therapeutic effect.

A high therapeutic index indicates a wide margin of safety, meaning there is a substantial difference between effective and toxic doses. This is crucial in drug development and clinical applications because it provides insight into how safely a drug can be administered.

In contrast, other choices do not accurately reflect the definition of the therapeutic index. For example, the first option suggests that the effective dose is divided by the toxic dose, which is the reverse of the correct definition. The second option references the maintenance dose, which is not relevant to the therapeutic index, and the fourth option introduces the concept of a loading dose, which also does not pertain to the calculation of the therapeutic index. Thus, the correct formulation of TD50 divided by ED50 succinctly embodies the concept of therapeutic index, making it fundamental for evaluating the safety and efficacy