The therapeutic index (TI) is defined as the ratio of the dose needed to produce a toxic effect to the dose needed to elicit the desired therapeutic response.

The therapeutic index (TI) is defined as the ratio of the dose needed to produce a toxic effect to the dose needed to elicit the desired therapeutic response.

• A. Dose needed to produce toxic effect divided by dose needed to elicit desired therapeutic response
• B. Dose required for therapeutic effect divided by toxic dose required for effect
• C. Clearance divided by volume of distribution
• D. Maximum tolerated dose divided by minimum effective dose

Answer: (A)

The safety margin is defined by comparing the amount of drug that can cause toxicity to the amount needed for the desired effect. The therapeutic index is expressed as the toxic dose (or toxic effect) divided by the dose that produces the therapeutic response. This ratio shows how much larger the toxic dose is compared with the effective dose; a larger ratio means a wider safety margin. So the correct choice matches that concept: dose that produces toxicity divided by the dose that elicits the therapeutic response. If the toxic dose is much higher than the therapeutic dose, the TI is large and the drug is considered safer. Inverting the ratio would misrepresent the safety margin, and using pharmacokinetic parameters or other dose pairs (like maximum tolerated vs minimum effective) isn’t the standard TI.

The safety margin is defined by comparing the amount of drug that can cause toxicity to the amount needed for the desired effect. The therapeutic index is expressed as the toxic dose (or toxic effect) divided by the dose that produces the therapeutic response. This ratio shows how much larger the toxic dose is compared with the effective dose; a larger ratio means a wider safety margin.

So the correct choice matches that concept: dose that produces toxicity divided by the dose that elicits the therapeutic response. If the toxic dose is much higher than the therapeutic dose, the TI is large and the drug is considered safer. Inverting the ratio would misrepresent the safety margin, and using pharmacokinetic parameters or other dose pairs (like maximum tolerated vs minimum effective) isn’t the standard TI.