Which measure expresses the safety margin of a drug by comparing lethal and effective doses?

• A. Potency
• B. Lethal dose
• C. Effective dose
• D. Therapeutic index

Answer: (D)

The safety margin of a drug is conveyed by comparing the dose that produces a desired effect with the dose that can cause death. This is captured by the therapeutic index, which is the ratio of the lethal dose to the effective dose (often LD50/ED50). A larger therapeutic index means a wider safety margin—the drug can be used with less risk of serious toxicity relative to its therapeutic benefit. For example, if the effective dose is 20 units and the lethal dose is 600 units, the therapeutic index is 30, indicating a relatively wide safety margin. In contrast, a small therapeutic index signals a narrow margin and greater risk if dosing is off.

Potency is about how much drug is needed to achieve a given effect, not about safety margins. Lethal dose and effective dose are the individual doses themselves, not the combined ratio that expresses safety. The therapeutic index combines these two doses to express safety.