Question
Explain the mechanism by which antipyretics reduce fever. Use paracetamol (acetaminophen) as an example and explain the biochemical pathway involved.
Solution — Step by Step
Fever is an elevation of the body’s core temperature set-point above the normal range (~37°C). It is not simply the body overheating — it is a regulated increase.
The mechanism: when pathogens (bacteria, viruses) infect the body, immune cells (macrophages, monocytes) release signalling proteins called pyrogens — specifically cytokines such as interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumour necrosis factor-alpha (TNF-α).
These cytokines reach the hypothalamus (the brain’s thermostat) and stimulate the production of prostaglandin E₂ (PGE₂) via the arachidonic acid cascade. PGE₂ raises the hypothalamic temperature set-point → the body perceives the new, higher set-point as “normal” and actively generates heat (shivering) and reduces heat loss (vasoconstriction) until body temperature reaches the new set-point.
The production of prostaglandins (including fever-causing PGE₂) depends on a critical enzyme: cyclooxygenase (COX), also called prostaglandin H synthase.
The pathway:
- Membrane phospholipids are cleaved by phospholipase A₂ to release arachidonic acid
- COX enzyme (COX-1 or COX-2) converts arachidonic acid to prostaglandin H₂ (PGH₂)
- PGH₂ is further converted to specific prostaglandins (including PGE₂) by specific synthases
COX is the target of most antipyretics and non-steroidal anti-inflammatory drugs (NSAIDs).
Two isoforms:
- COX-1: Constitutively expressed (“always on”). Produces prostaglandins that protect the stomach lining and help platelets aggregate.
- COX-2: Inducible (expressed mainly during inflammation). Produces prostaglandins responsible for fever, pain, and inflammation.
Paracetamol (chemical name: N-acetyl-para-aminophenol or APAP; brand names: Crocin, Dolo) reduces fever and pain but has weak anti-inflammatory action. Its mechanism is more complex and debated than that of ibuprofen/aspirin:
Primary mechanism: Paracetamol inhibits COX activity, but mainly in the central nervous system (CNS) — specifically in the hypothalamus and spinal cord. It inhibits COX-1 and COX-2 by reducing the active ferryl intermediate of the enzyme (a form of the enzyme with oxidised haem). In the CNS, where peroxide concentrations are lower, this inhibition is effective.
In peripheral tissues: Where inflammation causes high peroxide levels, paracetamol’s inhibition is weak — which is why it doesn’t reduce inflammation much (unlike ibuprofen).
Net effect: Reduced PGE₂ synthesis in the hypothalamus → hypothalamic temperature set-point is lowered → body temperature returns to normal.
For comparison:
Ibuprofen: Reversible, competitive COX-1 and COX-2 inhibitor. Reduces fever, pain, AND inflammation because it inhibits peripheral prostaglandin synthesis too.
Aspirin (acetylsalicylic acid): Irreversibly inhibits COX by acetylating a serine residue (Ser530 in COX-1, Ser516 in COX-2) near the active site. Because the inhibition is irreversible, the effect lasts for the enzyme’s lifetime. Aspirin’s irreversible COX-1 inhibition is the basis of its use as an antiplatelet drug (prevents platelet aggregation, reducing clot formation).
Paracetamol vs NSAIDs: Paracetamol is safer for the stomach (doesn’t inhibit stomach-protective COX-1 in the gut) but does not treat inflammation. NSAIDs treat fever + pain + inflammation but can cause gastric irritation/ulcers with long-term use.
At therapeutic doses, paracetamol is metabolised safely in the liver (primarily by conjugation with glucuronide and sulphate). A small fraction (~5%) is oxidised by cytochrome P450 (CYP2E1) to a reactive, toxic metabolite: NAPQI (N-acetyl-p-benzoquinone imine).
At therapeutic doses, NAPQI is rapidly detoxified by glutathione (a cellular antioxidant) → non-toxic mercapturic acid → excreted.
In overdose: glutathione is depleted → NAPQI accumulates → attacks liver cell proteins → hepatotoxicity (liver damage). This is why paracetamol overdose causes liver failure — even though the drug seems “mild” at normal doses.
Treatment of overdose: N-acetylcysteine (NAC) — replenishes glutathione stores, protecting the liver.
Why This Works
Antipyretics work “downstream” — they don’t neutralise the infection or kill pathogens. They block the signalling cascade that tells the hypothalamus to raise the temperature set-point. Without the PGE₂ signal, the hypothalamus reverts to its normal 37°C set-point → fever breaks.
This is why antipyretics need to be re-dosed every 4–6 hours — once the drug is metabolised and cleared, new PGE₂ can be synthesised (from ongoing arachidonic acid metabolism during active infection), and the fever returns until the underlying infection is cleared.
Alternative Method — Comparison Table
| Drug | Mechanism | Antipyretic | Analgesic | Anti-inflammatory | COX inhibition |
|---|---|---|---|---|---|
| Paracetamol | CNS-specific COX inhibition | Yes | Yes | Weak | Central only |
| Ibuprofen | Reversible COX-1/2 inhibition | Yes | Yes | Yes | Peripheral + central |
| Aspirin | Irreversible COX-1/2 inhibition | Yes | Yes | Yes | Peripheral + central |
Common Mistake
Students sometimes say “paracetamol kills the fever by killing bacteria.” Paracetamol has no antibacterial action — it does not kill pathogens at all. It simply reduces the hypothalamic set-point by blocking prostaglandin synthesis. The infection (and immune response) continues; only the fever signal is reduced.
Also, do not confuse “antipyretic” (reduces fever) with “antibiotic” (kills bacteria) or “antiviral” (inhibits viruses). These are completely different drug classes with different mechanisms.
JEE and NEET both test chemistry of medicines. Key facts to memorise: (1) paracetamol = antipyretic + analgesic (not anti-inflammatory); (2) aspirin = irreversible COX inhibitor (acetylation); (3) ibuprofen = reversible COX inhibitor; (4) paracetamol overdose → NAPQI → liver damage → treat with NAC; (5) target enzyme = cyclooxygenase (COX) → produces prostaglandins.