What is the therapeutic index, and why is it important when selecting analgesics for a dog?

• A. TI = toxic dose / effective dose; a larger TI indicates a wider safety margin.
• B. TI = effective dose / toxic dose; a larger TI indicates a narrower safety margin.
• C. TI = dose causing analgesia / dose causing sedation; independent of safety margin.
• D. TI = lethal dose / minimum effective dose; smaller TI is safer.

Answer: (A)

Therapeutic index is a measure of drug safety that compares the dose that could cause toxicity to the dose that provides the desired analgesic effect. It is defined as toxic dose divided by effective dose. A larger ratio means a wider safety margin, because there is a bigger gap between the amount needed for pain relief and the amount that could cause harm. When selecting analgesics for a dog, a higher TI is preferred because it accommodates variations in size, age, organ function, concurrent medications, and potential dosing errors, reducing the risk of toxicity while still providing relief. Defining TI as something like the ratio of analgesia to sedation or swapping the numerator/denominator would not reflect this safety buffer, and a smaller TI would indicate a narrower safety margin and greater risk.