Plotting ED50 and LD50 shows what concept?

• A. The therapeutic index
• B. The margin of safety
• C. The toxic dose
• D. The effective dose

Answer: (A)

ED50 and LD50 mark the doses where a drug begins to produce a desired effect in 50% of people and where it becomes lethal in 50% of people. Plotting them together shows how wide or narrow the safety window is between efficacy and toxicity. The therapeutic index is the ratio of the toxic (or lethal) dose to the effective dose, usually LD50/ED50, and it captures that safety margin. A large ratio means a big gap between the dose that works and the dose that can cause serious harm, indicating a safer drug; a small ratio means a narrow margin and greater risk. So this plot fundamentally illustrates how the therapeutic index quantifies drug safety.