Which statement best defines the therapeutic index?

• A. The ratio of the therapeutic dose to the toxic dose
• B. The time to onset of the therapeutic effect
• C. The peak blood level relative to trough level
• D. The ratio of the toxic dose to the therapeutic dose

Answer: (D)

Therapeutic index is about the safety margin of a drug, comparing the dose that produces toxicity with the dose that achieves the desired therapeutic effect. It is typically defined as TD50 (or LD50) divided by ED50. A larger ratio means a bigger buffer between effective dosing and toxic effects, so the drug is safer to use. The statement that best defines this index is the ratio of the toxic dose to the therapeutic dose, because that directly quantifies how much you can increase the dose before toxicity occurs relative to the effective dose. The other topics describe how quickly the drug works or the levels seen in the blood, which relate to onset or pharmacokinetics, not the safety margin between efficacy and toxicity.