Question

Which of the following drugs requires therapeutic drug monitoring?

• A. Acetaminophen
• B. Amoxicillin
• C. Lithium
• D. Prodrug activated in the liver

Hint:

Therapeutic drug monitoring (TDM) means measuring blood levels of a drug to keep them within a safe and effective range.

Answer 1

The Correct Option is C

Step 1: What is therapeutic drug monitoring (TDM)?
TDM means regularly measuring a drug's blood level to keep it within a safe and effective range. It is needed when a drug has a narrow therapeutic index (the gap between the effective dose and the toxic dose is very small).

Step 2: Evaluate each option.
Acetaminophen (Option 1) has a wide safety margin at normal doses and does not need routine TDM. Amoxicillin (Option 2) is an antibiotic whose effect depends on time above MIC, not on a precise blood level, so TDM is not routine. Option 4 (prodrug activated in liver) is not a real drug name and is a distractor.

Step 3: Focus on Lithium.
Lithium (Option 3) is used to treat bipolar disorder. Its therapeutic range is very narrow (0.6 to 1.2 mEq/L). Levels slightly above this cause tremors, confusion, and kidney damage. Levels below this are ineffective.

Step 4: Why does Lithium need TDM?
Lithium is eliminated entirely by the kidneys and competes with sodium. Any change in hydration, salt intake, or kidney function shifts the blood level quickly. Regular monitoring is the only way to stay safe.

Step 5: Confirm the correct answer.
Among the four choices, only Lithium has a narrow therapeutic index that makes TDM mandatory in clinical practice.

Answer: Option (3) — Lithium