Which pharmacokinetic concept explains why two drugs with the same potency can have different safety profiles?

• A. Volume of distribution
• B. Bioavailability
• C. Therapeutic index
• D. Half-life

Answer: (C)

Two drugs can need similar doses to achieve the same effect, but their safety differs because of the margin between beneficial and toxic effects. The therapeutic index captures this safety margin by comparing the dose that produces toxicity to the dose that produces therapeutic benefit. A large therapeutic index means a wide gap between efficacy and harm, giving a safer profile; a small therapeutic index means a narrow gap, where small increases in exposure can cause toxicity. Pharmacokinetic factors—how the drug is absorbed, distributed, metabolized, and cleared—shape the actual concentrations reached in the body and tissues, influencing where that safety margin lies. Even with the same potency, differences in exposure and in the toxic threshold lead to different safety outcomes.