The therapeutic index is a measure used to evaluate the safety and effectiveness of a drug. It is defined as the ratio between the dose that produces a therapeutic effect and the dose that causes toxicity. A higher therapeutic index indicates a wider margin of safety, meaning there is a greater difference between effective and harmful doses. For example, if a medication has a therapeutic index of 10, it means that the effective dose is ten times lower than the dose that could cause adverse effects. This concept is crucial in pharmacology, guiding healthcare professionals in prescribing medications like antibiotics and pain relievers safely.