How is the therapeutic index defined?

The therapeutic index is defined as the range between the therapeutic dose and the toxic dose of a drug. This measure is crucial for understanding the safety and efficacy of a medication. A high therapeutic index indicates that there is a wide margin between the dose that produces the desired therapeutic effect and the dose that can cause harmful side effects, making the drug safer for use. Conversely, a low therapeutic index suggests that the difference between effective and toxic doses is narrow, requiring careful monitoring to avoid adverse effects.

In the context of the other choices, while the ratio of highest to lowest dose touches on aspects of dosing, it does not precisely encapsulate the concept of therapeutic index. The measure of a drug's efficacy does not directly relate to safety margins, and the number of patients who respond positively is unrelated to the definitions of therapeutic index, which focuses more on the relationship between doses rather than patient outcomes.