What is therapeutic index?

The therapeutic index is specifically defined as the ratio between the toxic dose and the therapeutic dose of a drug. This concept is critical in pharmacology because it provides insight into the safety of a drug—indicating how much greater the effective dose is compared to the dose that causes adverse effects. A higher therapeutic index suggests a larger margin of safety, meaning that there is a wider range of dosages that can be given before reaching a toxic level.

Understanding the therapeutic index helps healthcare professionals to determine safe and effective dosing regimens. For example, if a drug has a narrow therapeutic index, it means that there is a fine line between its therapeutic effects and toxic effects, requiring close monitoring of patient response and possible dose adjustments.

In contrast, the other options describe different concepts relevant to pharmacology but do not accurately represent the definition of therapeutic index:

• The range of dosage that achieves the desired drug effect refers to the effective range of dosing rather than the balance of safety and toxicity.
• The percentage of patients who experience a side effect relates to drug tolerability rather than the safety margin indicated by the therapeutic index.
• The ratio of drug absorption to excretion addresses pharmacokinetics, not the relationship between effective and toxic doses.