What is the therapeutic index?

The therapeutic index is a critical concept in pharmacology that represents the safety margin of a drug. It is specifically defined as the ratio of the toxic dose of a drug to its therapeutic dose. This measurement provides essential information regarding the safety of a drug; a higher therapeutic index indicates a wider margin between effective and potentially harmful doses, suggesting that the drug can be administered more safely.

For instance, if a drug has a therapeutic index of 10, this means that the toxic dose is ten times higher than the effective dose, indicating a lower risk of toxicity if the drug is used within the recommended therapeutic dose range. Conversely, a drug with a low therapeutic index (for example, a ratio of 2) poses a greater risk of toxicity, as therapeutic and toxic doses are closer together.

The other options address different concepts in pharmacology that don't specifically define the therapeutic index. The ratio of therapeutic to sub-therapeutic doses doesn’t encompass the toxic aspect and thus is not accurate. Measurement of drug bioavailability refers to the proportion of the drug that reaches systemic circulation unchanged and does not relate to the safety and efficacy balance as captured by the therapeutic index. The rate of drug metabolism is another separate concept, focusing on how quickly a drug is processed