Which statement best describes therapeutic index?

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

Answer: (B)

The main idea being tested is the safety margin of a drug, known as the therapeutic index. This tells you how much you must push the dose to reach toxicity compared with the dose needed for a therapeutic effect. In practice, it’s expressed as a ratio of the toxic dose (or concentration) to the therapeutic dose (or concentration). A larger ratio means you can achieve the desired therapeutic effect long before reaching toxic levels, so the drug is considered safer.

That’s why the statement describing a ratio between toxic and therapeutic concentrations, with a higher index indicating a safer drug, is the best answer. It directly captures the safety buffer: the bigger the gap between what’s therapeutic and what’s toxic, the safer the drug.

The other ideas miss the standard way we measure safety. One talks about a ratio of absorbed to excreted, which is about pharmacokinetics, not safety margin. Another suggests the ratio of therapeutic to toxic concentrations, which would invert the typical definition and can lead to confusion about safety. The last talks about the difference between lethal and effective dose but asserts that a lower index is safer, which is opposite to how a wider separation indicates greater safety.