Which ratio best represents the therapeutic index of a drug?
- A ED50 / LD50
- B LD50 / ED50 ✓
- C EC50 / ED50
- D LD50 / EC50
Correct answer: B. LD50 / ED50
Explanation
The therapeutic index measures the margin of safety for a drug, defined as the ratio of the lethal dose for 50% of the population (LD50) to the effective dose for 50% (ED50). A higher ratio indicates greater safety. Option A inverts the ratio, incorrectly implying that a lower value is safer. The index relies on LD50 rather than lethal dose for 100% because population variability makes the median more statistically reliable for comparison.
Reference: Katzung Basic and Clinical Pharmacology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.