Question:medium
Narrow therapeutic index is seen with:
Narrow therapeutic index is seen with:
Show Hint
Which of these drugs needs regular blood-level monitoring because its safe and toxic doses nearly overlap?
Updated On: Jun 24, 2026
- Desipramine
- Lithium
- Penicillin
- Diazepam
Show Solution
The Correct Option is B
Solution and Explanation
The therapeutic index measures how much room there is between a helpful dose and a harmful one. A high index means the drug is forgiving; a low or narrow index means tiny changes in dose can flip a patient from benefit to toxicity. The drugs we worry about most clinically are the narrow-index ones, because they need blood level checks.
Lithium is the standout here. Its useful plasma range is roughly $0.6$ to $1.2$ mEq/L, and toxicity begins not far above that. Dehydration, salt loss, or interacting drugs can push levels into the dangerous zone quickly, producing tremor, confusion, and seizures. For this reason lithium therapy is built around regular serum monitoring, which is the practical signature of a narrow therapeutic index.
Comparing the alternatives, penicillin is one of the safest drugs in medicine with an enormous margin of safety, diazepam is a benzodiazepine that is hard to fatally overdose on by itself, and desipramine, while toxic in overdose, is not the classic monitoring example. Lithium is the answer.
\[\boxed{\text{Lithium}}\]
Lithium is the standout here. Its useful plasma range is roughly $0.6$ to $1.2$ mEq/L, and toxicity begins not far above that. Dehydration, salt loss, or interacting drugs can push levels into the dangerous zone quickly, producing tremor, confusion, and seizures. For this reason lithium therapy is built around regular serum monitoring, which is the practical signature of a narrow therapeutic index.
Comparing the alternatives, penicillin is one of the safest drugs in medicine with an enormous margin of safety, diazepam is a benzodiazepine that is hard to fatally overdose on by itself, and desipramine, while toxic in overdose, is not the classic monitoring example. Lithium is the answer.
\[\boxed{\text{Lithium}}\]
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