Paracetamol has analgesic and antipyretic actions comparable to aspirin but negligible anti-inflammatory activity at usual doses. The accepted explanation is:

• A It does not cross the blood brain barrier
• B It is rapidly conjugated by hepatic glucuronyl transferase
• C It antagonizes bradykinin at B2 receptors in peripheral tissues
• D High peroxide concentrations at inflammatory sites abolish its COX inhibition ✓

Explanation

Paracetamol is a weak reversible inhibitor of COX whose activity is highly dependent on the local peroxide tone. Inflamed tissues are rich in peroxides generated by neutrophils, which inactivate the reduced form of the enzyme paracetamol acts on, so it cannot inhibit prostaglandin synthesis peripherally. The brain has much lower peroxide levels, allowing central COX inhibition and hence analgesia and antipyresis. Poor CNS penetration is false; it crosses well.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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