NSAIDs Pharmacology - USMLE High-Yield Review

1. Mechanism of Action

All NSAIDs (aspirin included) work by inhibiting cyclooxygenase (COX), the enzyme that converts arachidonic acid into prostaglandins (PGE2, PGI2/prostacyclin) and thromboxane A2 (TXA2). Arachidonic acid itself is liberated from membrane phospholipids by phospholipase A2 (the enzyme blocked by corticosteroids).

COX-1 ("housekeeping" enzyme): constitutively expressed everywhere - gastric mucosa (cytoprotective PGE2/PGI2), platelets (TXA2 for aggregation), kidney (afferent arteriolar dilation via PGE2/PGI2).
COX-2: inducible at sites of inflammation (macrophages, synoviocytes) but also constitutive in kidney and vascular endothelium (PGI2 - antithrombotic).

"NSAID anti-inflammatory activity is mediated chiefly through inhibition of prostaglandin biosynthesis... Selectivity for COX-1 versus COX-2 is variable and incomplete for the older NSAIDs, but drugs that are selective in vitro COX-2 inhibitors have been synthesized." - Katzung's Basic and Clinical Pharmacology, p. 1007

Below is the arachidonic acid cascade showing where NSAIDs, aspirin, and lipoxygenase-pathway drugs act:

Prostanoid biosynthetic pathway showing COX and lipoxygenase branches

Key distinction - Aspirin vs. other NSAIDs:

Aspirin irreversibly acetylates COX-1 and COX-2 (covalent modification) - this is why its antiplatelet effect lasts the lifetime of the platelet (~7-10 days, since platelets lack nuclei and cannot resynthesize COX).
Non-aspirin NSAIDs (ibuprofen, naproxen, indomethacin, diclofenac) are reversible, competitive COX inhibitors.
COX-2 selective agents (celecoxib) spare platelet COX-1, so they do not inhibit platelet aggregation.

2. Four Core Pharmacologic Effects

Effect	Mechanism
Anti-inflammatory	Decreased PGE2/PGI2 → less vasodilation, less sensitization of nociceptors, decreased leukocyte chemotaxis
Analgesic	Decreased peripheral sensitization of nerve endings to bradykinin/other mediators; some central effect
Antipyretic	Decreased PGE2 in hypothalamus (blocks pyrogen-induced fever)
Antiplatelet	Decreased TXA2 (irreversible with aspirin only)

3. Pharmacokinetics (General)

Most NSAIDs are weak organic acids, well absorbed orally, highly protein-bound (~98%, mostly albumin) - this matters for drug interactions (displacement of warfarin, sulfonylureas, methotrexate).
Hepatic metabolism (CYP2C9/CYP3A families) with renal excretion; significant enterohepatic recirculation contributes to lower-GI irritation.
Accumulate in synovial fluid, providing sustained joint effect even for short-half-life drugs. (Katzung, p. 1006)

4. Individual Agents - USMLE Essentials

Aspirin

Dose-dependent effects: low dose (<300 mg/day) = antiplatelet; intermediate (~2-4 g/day) = analgesic/antipyretic; high dose (4-8 g/day) = anti-inflammatory.
Toxicity/overdose (salicylism): tinnitus, vertigo, hyperventilation. Classic acid-base picture = early respiratory alkalosis (direct stimulation of the medullary respiratory center) → mixed metabolic acidosis + respiratory alkalosis as toxicity progresses (uncoupling of oxidative phosphorylation → lactic acid, ketoacid accumulation).
Reye syndrome: aspirin given to children/adolescents during a viral illness (especially influenza B, varicella) is linked to acute hepatic failure with cerebral edema - "aspirin and other salicylates are contraindicated in children and young adults less than 20 years of age with viral illness-associated fever." - Goodman & Gilman's Pharmacological Basis of Therapeutics, p. 3615. This is why acetaminophen is preferred for pediatric fever.
Aspirin-exacerbated respiratory disease (Samter's triad: asthma, nasal polyps, aspirin/NSAID sensitivity) - due to shunting of arachidonic acid toward the leukotriene pathway when COX is blocked.

Ibuprofen, Naproxen - nonselective, standard first-line OTC NSAIDs; naproxen has longer half-life (twice daily dosing) and the most favorable cardiovascular safety profile among NSAIDs.

Indomethacin - potent nonselective NSAID; USMLE-classic dual use: closes a patent ductus arteriosus (inhibits PGE-mediated ductal patency) in premature infants, and treats acute gout. Notable for higher GI and CNS side effects (headache).

Ketorolac - potent parenteral NSAID used for short-term moderate-severe pain (post-op), opioid-sparing; limited to ≤5 days use due to GI/renal toxicity risk.

Diclofenac - achiral NSAID, available topically (for OA) and orally; notable hepatotoxicity risk.

Celecoxib - COX-2 selective; spares platelet function and reduces GI ulcer risk, but carries increased cardiovascular thrombotic risk (loses the protective vasodilatory/antithrombotic PGI2 from endothelial COX-2 while TXA2 from platelet COX-1 is unopposed) and is a sulfonamide (contraindicated in sulfa allergy).

5. Adverse Effects (High-Yield for USMLE)

GI toxicity - loss of PGE2/PGI2 cytoprotection (decreased mucus/bicarbonate secretion, decreased mucosal blood flow) → gastritis, peptic ulcers, GI bleeding. Misoprostol (PGE1 analog) is used prophylactically in patients on chronic NSAID therapy who are at high ulcer risk.

Renal toxicity - PGE2/PGI2 normally maintain afferent arteriolar vasodilation, especially when renal perfusion is compromised (CHF, cirrhosis, hypovolemia, elderly). NSAIDs → afferent arteriolar vasoconstriction → decreased GFR → acute kidney injury. Also causes acute interstitial nephritis (often with nephrotic-range proteinuria) and, with chronic use, analgesic nephropathy/papillary necrosis. "Triple whammy": NSAID + ACE inhibitor/ARB + diuretic markedly raises AKI risk.

Cardiovascular - fluid/sodium retention, hypertension (all NSAIDs); COX-2 selective agents and some nonselective agents (diclofenac) increase risk of MI/stroke with chronic use.

Hepatotoxicity - transaminitis, rare fulminant hepatitis (diclofenac has the highest reported risk among common NSAIDs).

Hematologic - GI bleeding risk from antiplatelet effect (especially aspirin, irreversible); reversible platelet dysfunction with other NSAIDs resolves within one drug half-life.

Hypersensitivity - AERD/Samter's triad as above; cross-reactivity between aspirin and other NSAIDs is common in these patients.

6. Drug Interactions

Warfarin: displaces from albumin + additive GI bleeding risk.
ACE inhibitors/ARBs + diuretics: additive nephrotoxicity ("triple whammy"), blunts antihypertensive effect (loss of vasodilatory prostaglandins).
Methotrexate: NSAIDs decrease renal clearance of methotrexate → increased toxicity.
Lithium: NSAIDs decrease renal lithium clearance → lithium toxicity.
SSRIs: additive GI bleeding risk.

7. Special Populations / Contraindications

Pregnancy: avoid in the third trimester - risk of premature closure of the ductus arteriosus and oligohydramnios (decreased fetal renal blood flow/urine output); this is the same PGE-dependent mechanism exploited therapeutically to close a PDA with indomethacin/ibuprofen after birth.
Children with viral illness: avoid aspirin (Reye syndrome).
Renal impairment, heart failure, cirrhosis with ascites, elderly, volume-depleted patients: use NSAIDs cautiously or avoid - these are prostaglandin-dependent states for renal perfusion.
Active peptic ulcer disease / GI bleeding history: relative contraindication; consider COX-2 selective agent + PPI, or avoid entirely.
Established cardiovascular disease: avoid NSAIDs when possible; naproxen has the best relative safety data among nonselective agents; avoid COX-2 selective agents.

Evidence check

A 2026 systematic review (PMID 41230625, Clinical Journal of Pain) on cardiovascular risks of NSAIDs is in press but did not surface data contradicting the standard teaching above (COX-2 selective and diclofenac carry the highest cardiovascular risk; naproxen the lowest among commonly used agents). No practice-changing update was identified beyond established textbook teaching.

Quick USMLE Pearls

Only aspirin is irreversible; that's the one-line differentiator tested repeatedly.
COX-1 = stomach/platelets/kidney (constitutive); COX-2 = inflammation site (inducible) but also kidney/endothelium.
Reye syndrome = aspirin + kids + viral illness.
Indomethacin closes PDA; NSAIDs in pregnancy risk keeping PDA open closed prematurely in utero (opposite direction, same mechanism).
Renal injury from NSAIDs is a hemodynamic, prostaglandin-withdrawal phenomenon, not direct nephrotoxicity, in the acute setting.
Celecoxib = sulfonamide, spares platelets, higher CV risk, lower GI risk.

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