Question

Narrow therapeutic index is seen with:

• A. Desipramine
• B. Lithium
• C. Penicillin
• D. Diazepam

Hint:

Which of these drugs needs regular blood-level monitoring because its safe and toxic doses nearly overlap?

Answer 1

The Correct Option is B

Step 1: Define therapeutic index (TI). It is the ratio of the dose that produces toxicity to the dose that produces the desired effect, \(TI = \frac{TD_{50}}{ED_{50}}\). A narrow TI means the toxic dose is very close to the effective dose, so small dosing errors are dangerous.
Step 2: Apply this to lithium. Lithium has a tight therapeutic serum range of about 0.6 to 1.2 mEq/L, and levels above 1.5 mEq/L cause toxicity (tremor, ataxia, seizures, renal damage). Because the safe window is so small, lithium requires routine plasma level monitoring, which is the classic marker of a narrow therapeutic index.
Step 3: Rule out the others. Penicillin (C) is extremely safe with a very wide margin between effective and toxic doses, so it has a high TI. Diazepam (D) is a benzodiazepine with a wide safety margin. Desipramine (A) is a tricyclic with overdose risk, but among these choices lithium is the textbook example of a narrow TI drug needing monitoring.
Answer: Option B (Lithium).