Step 1: Understanding the Concept:
The question asks which drug does not require therapeutic drug monitoring, the practice of regularly checking blood drug levels to avoid toxicity or under-treatment.
Step 2: Key Concept or Approach:
TDM is reserved for drugs with a narrow therapeutic index, where the effective dose and the toxic dose are close together, or where blood levels are hard to predict from the dose alone.
Step 3: Detailed Explanation:
Digitoxin can cause dangerous heart rhythm problems if levels climb even a little too high, so it needs monitoring.
Phenytoin's blood level does not rise in a simple straight line with dose, so small dose changes can push it into a toxic range, requiring monitoring.
Gentamicin can harm the kidneys and hearing at high levels, so peak and trough levels are tracked.
Acetaminophen, used at normal recommended doses, has a comfortably wide gap between an effective dose and a toxic one, and its behavior in the body is predictable, so routine blood level checks are not part of standard use.
Step 4: Final Answer:
Acetaminophen does not require therapeutic drug monitoring.