Why is therapeutic drug monitoring important for drugs with narrow therapeutic indices, and provide an example?

• A. To keep drug levels within the therapeutic window to prevent toxicity or lack of efficacy; example: warfarin requires INR monitoring, or vancomycin trough to avoid nephrotoxicity.
• B. It is unnecessary for narrow TI drugs; monitor only symptoms.
• C. It only applies to antibiotics; not to anticoagulants.
• D. It is used to adjust dosing randomly.

Answer: (A)

Drugs with narrow therapeutic indices demand careful control because the line between an effective dose and a toxic one is very thin. Small changes in how a patient absorbs, distributes, metabolizes, or excretes the drug can shift blood levels from therapeutic to harmful or vice versa. Therapeutic drug monitoring uses lab measurements of drug concentration (or pharmacodynamic markers like INR for anticoagulants) to guide dosing so the level stays within a safe, effective range. This approach helps prevent toxicity while ensuring the medication works as intended. A classic example is warfarin, where INR testing guides dose adjustments to maintain anticoagulation in a target range and minimize bleeding risk. Another example is vancomycin, where trough levels are checked to ensure sufficient exposure for efficacy while avoiding nephrotoxicity. Merely watching for symptoms isn’t reliable for these drugs, since toxicity or under-treatment can occur even when symptoms are subtle or absent, and monitoring applies to many drug classes beyond antibiotics. Dosing should be adjusted based on lab results and patient-specific factors, not random changes.