How is the therapeutic index defined?

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Multiple Choice

How is the therapeutic index defined?

The therapeutic index is a safety measure that compares how much of a drug could cause toxicity with how much is needed to achieve a therapeutic effect. It is defined as the ratio of the toxic dose (TD50) to the effective dose (ED50), or equivalently LD50/ED50. A larger ratio means a wider safety margin between beneficial effects and toxic effects, making the drug safer to use. For example, if the toxic dose is 200 mg and the effective dose is 10 mg, the TI is 20, indicating a generous safety buffer. A small TI implies that small changes in dose or patient factors could push a person from benefit to toxicity. The option describing this ratio correctly captures the concept. The other ideas—habitually referring to half-life, or measuring a difference between doses, or inverting the ratio—do not describe the safety margin in the standard way.

The therapeutic index is a safety measure that compares how much of a drug could cause toxicity with how much is needed to achieve a therapeutic effect. It is defined as the ratio of the toxic dose (TD50) to the effective dose (ED50), or equivalently LD50/ED50. A larger ratio means a wider safety margin between beneficial effects and toxic effects, making the drug safer to use. For example, if the toxic dose is 200 mg and the effective dose is 10 mg, the TI is 20, indicating a generous safety buffer. A small TI implies that small changes in dose or patient factors could push a person from benefit to toxicity. The option describing this ratio correctly captures the concept. The other ideas—habitually referring to half-life, or measuring a difference between doses, or inverting the ratio—do not describe the safety margin in the standard way.

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