Paracetamol relieves pain and fever effectively but has negligible anti-inflammatory activity compared with aspirin. The accepted explanation is that paracetamol:
- A Is rapidly conjugated in the liver before reaching inflamed tissues
- B Acts only on a central COX isoform and is inhibited by the high peroxide tone present at sites of inflammation ✓
- C Irreversibly acetylates COX-1 but not COX-2
- D Blocks phosphodiesterase rather than cyclooxygenase in peripheral tissues
Correct answer: B. Acts only on a central COX isoform and is inhibited by the high peroxide tone present at sites of inflammation
Explanation
Paracetamol inhibits cyclooxygenase weakly and reversibly, with its main action thought to be on a central COX variant, explaining antipyresis and analgesia. At inflammatory foci, high concentrations of peroxides generated by leukocytes inactivate the reduced form of COX required by paracetamol, so peripheral prostaglandin suppression is poor and anti-inflammatory effect is minimal. It does not irreversibly acetylate COX, which distinguishes it from aspirin.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
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