In the provided dose-equivalence data, 1 mg of olanzapine is equivalent to how many milligrams of the reference drug (chlorpromazine)?

• A. 80 mg
• B. 20 mg
• C. 40 mg
• D. 10 mg

Answer: (C)

Understanding dose equivalence across antipsychotics uses a common reference drug, typically chlorpromazine, to compare potencies. If the data show that 1 mg of olanzapine is equivalent to 40 mg of chlorpromazine, that means olanzapine is considerably more potent on a weight basis—the same clinical effect can be achieved with a much smaller olanzapine dose, or conversely with a larger chlorpromazine dose.

A practical check: 2.5 mg of olanzapine is often cited as equivalent to about 100 mg of chlorpromazine. Dividing both sides by 2.5 gives 1 mg olanzapine ≈ 40 mg chlorpromazine, matching the provided equivalence. Therefore, the chlorpromazine-equivalent dose for 1 mg of olanzapine is 40 mg. The other options would imply different potency relationships that don’t align with this equivalence data.