A larger TI implies a wider safety margin between therapeutic and toxic doses.

• A. Closer together
• B. Wider apart
• C. Equal
• D. Cannot determine

Answer: (B)

A larger Therapeutic Index means a wider safety margin between doses that produce a therapeutic effect and those that cause toxicity. The TI is the ratio of the toxic (or lethal) dose to the effective dose, so a bigger ratio means the toxic dose sits much higher than the dose needed for benefit. This wider gap reduces the risk of toxicity from normal dosing variations or small interactions, making the drug safer in practice. For example, if the effective dose is 50 mg and the toxic dose is 500 mg, the TI is 10, indicating a tenfold margin between benefit and harm. If the margin is small, say the toxic dose is only twice the effective dose, there’s a higher chance that a small dosing error could cause toxicity. Therefore, a larger TI corresponds to a wider separation between therapeutic and toxic doses.