Which statement describes the therapeutic index?

• A. The ratio of the drug's toxic dose to its effective (therapeutic) dose
• B. The ratio of the drug's effective dose to its toxic dose
• C. The difference between the toxic and effective doses
• D. The half-life of the drug

Answer: (A)

Therapeutic index measures the safety margin of a drug by comparing the dose that produces toxicity with the dose that produces the desired therapeutic effect. It is expressed as the ratio of the toxic dose (usually TD50) to the effective therapeutic dose (ED50). This means a larger TI indicates a greater safety buffer between beneficial and harmful effects. The other statements don’t describe this safety margin: taking the inverse ratio would misrepresent safety, using the difference between doses isn’t the standard index, and half-life refers to how long the drug stays in the body rather than its safety window.