What is the therapeutic index and why is it important?

• A. The ratio of absorbed to excreted.
• B. The difference between lethal and effective dose; lower index is safer.
• C. The ratio between toxic and therapeutic concentrations; a higher index indicates a safer drug.
• D. The ratio of therapeutic to toxic concentrations; higher index indicates a safer drug.

Answer: (C)

The therapeutic index measures how wide the safety margin is for a drug by comparing the amount that causes toxicity to the amount that produces the desired therapeutic effect. It’s typically expressed as the ratio of toxic dose to therapeutic dose (for example, TD50/ED50 or LD50/ED50). A higher ratio means you can dose more before toxicity occurs, so the drug is considered safer. In practice, drugs with a large therapeutic index give clinicians more flexibility and a larger cushion for variability between patients; those with a small index require careful dosing and close monitoring.

The statement that reflects this idea correctly says it’s the ratio between toxic and therapeutic concentrations, and that a higher index indicates a safer drug. Other phrasings either invert the concept or describe a different pharmacokinetic relationship (such as absorbed versus excreted), which don’t capture the safety margin the therapeutic index is meant to quantify.