Aspirin – Mechanism of Action

Introduction

Aspirin, also known as acetylsalicylic acid, is a widely used Non-Steroidal Anti-Inflammatory Drug (NSAID). It’s primarily known for its analgesic (pain-relieving), antipyretic (fever-reducing), anti-inflammatory, and antiplatelet actions.

Belonging to the salicylate class, Aspirin is often prescribed for:

Mild to moderate pain
Inflammatory conditions like arthritis
Cardiovascular protection (to prevent clot formation)

Due to its clinical importance and frequent exam presence, understanding its mechanism of action is essential for students preparing for GPAT, NEET-PG, NIPER, MBBS, and NCLEX.

Stepwise Mechanism of Action of Aspirin

Aspirin is administered (usually orally).
It gets absorbed and irreversibly inhibits the cyclooxygenase enzymes – COX-1 and COX-2 by acetylating serine residues in the active site.
This blocks the conversion of arachidonic acid to prostaglandin H₂ (PGH₂), the precursor of prostaglandins and thromboxanes.
As a result:
- Prostaglandin synthesis is reduced, leading to reduced inflammation, fever, and pain.
- Thromboxane A₂ synthesis is inhibited, which reduces platelet aggregation and provides an antiplatelet effect.

Clinical Applications of Aspirin

Analgesic: Relief in headache, myalgia, arthralgia, and other mild to moderate pain
Antipyretic: Reduces fever by acting on the hypothalamic heat-regulating center
Anti-inflammatory: Used in rheumatoid arthritis and other inflammatory conditions
Antiplatelet Agent: Low-dose aspirin is used in myocardial infarction, stroke prevention, and other thrombotic conditions

Adverse Effects

Gastric irritation, peptic ulcers, and GI bleeding (due to COX-1 inhibition)
Tinnitus (especially at high doses)
Hypersensitivity reactions
Reye’s syndrome (in children with viral infections – hence contraindicated)

Aspirin vs. Other NSAIDs (e.g., Ibuprofen):

Feature	Aspirin	Ibuprofen
COX Inhibition	Irreversible (COX-1 >> COX-2)	Reversible (COX-1 and COX-2)
Antiplatelet Effect	Strong and long-lasting	Minimal and reversible
Use in CV protection	Preferred (low dose)	Not used
GI Irritation	Higher (COX-1 inhibition)	Lower
Duration of Action	Longer for antiplatelet (~7–10 days)	Short (~4–6 hours)

MCQs for Practice

Q1. Aspirin acts by inhibiting:
A. Thromboxane synthase
B. Lipoxygenase
C. Cyclooxygenase (COX) enzymes ✅
D. Prostaglandin E synthase

Q2. Which effect of aspirin is due to inhibition of thromboxane A₂?
A. Analgesic
B. Antipyretic
C. Antiplatelet ✅
D. Anti-inflammatory

Q3. Which enzyme is irreversibly inhibited by aspirin?
A. Phospholipase A2
B. Cyclooxygenase ✅
C. Lipoxygenase
D. Thromboxane synthase

Q4. Which of the following is NOT a use of low-dose aspirin?
A. Fever
B. Stroke prophylaxis
C. Myocardial infarction prevention
D. Pain relief ✅

Q5. Which adverse effect is uniquely associated with aspirin use in children?
A. Hepatitis
B. Reye’s Syndrome ✅
C. Gastritis
D. Asthma

Q6. Which of the following explains the prolonged antiplatelet effect of aspirin?
A. It reversibly inhibits thromboxane A₂
B. It irreversibly inhibits COX-2 in liver
C. It irreversibly inhibits COX-1 in platelets ✅
D. It enhances prostacyclin synthesis

Q7. Aspirin is contraindicated in children with viral infections because of the risk of:
A. Guillain-Barré Syndrome
B. Reye’s Syndrome ✅
C. Steven-Johnson Syndrome
D. Toxic epidermal necrolysis

Q8. The antipyretic effect of aspirin is mainly due to:
A. Decreased platelet aggregation
B. Central inhibition of PGE₂ synthesis ✅
C. Blocking leukotriene formation
D. Vasoconstriction in skin vessels

Q9. The analgesic action of aspirin is primarily mediated by:
A. Direct stimulation of opioid receptors
B. Activation of adrenergic pathways
C. Inhibition of peripheral nociceptor sensitization ✅
D. CNS dopamine enhancement

Q10. Which of the following statements is TRUE regarding aspirin’s pharmacokinetics?
A. It is rapidly absorbed but poorly protein-bound
B. It has a long half-life as aspirin itself
C. It undergoes hepatic metabolism to salicylate ✅
D. It is excreted unchanged in feces

FAQs

Q1: Why is aspirin used in heart attack prevention?
A: It inhibits thromboxane A₂, reducing platelet aggregation and preventing clot formation.

Q2: Why should aspirin not be given to children with viral fever?
A: Due to the risk of Reye’s syndrome, a rare but serious condition.

Q3: Is aspirin a selective COX-2 inhibitor?
A: No, it is a non-selective, irreversible COX-1 and COX-2 inhibitor.

References

KD Tripathi – Essentials of Medical Pharmacology
Sparsh Gupta – Review of Pharmacology
Goodman & Gilman – The Pharmacological Basis of Therapeutics
PubMed & NCBI articles on aspirin pharmacodynamics and antiplatelet effects

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Authors

Pharmacy Freak Editorial Team: Author
Pharmacy Freak Editorial Team is the official editorial voice of PharmacyFreak.com, dedicated to creating high-quality educational resources for healthcare learners. Our team publishes and reviews exam preparation content across pharmacy, nursing, coding, social work, and allied health topics, with a focus on practice questions, study guides, concept-based learning, and practical academic support. We combine subject research, structured editorial review, and clear presentation to make difficult topics more accessible, accurate, and useful for learners preparing for exams and professional growth.

Harsh Singh Rajput: Reviewer
Harsh Singh Rajput is a pharmacist currently working at ESIC and holds an MBA in Pharmaceutical Management from NIPER Hyderabad. He has a strong academic record with top ranks in national-level pharmacy exams, including AIR 61 in NIPER 2024 (MS/M.Pharm), AIR 27 in NIPER MBA, AIR 147 in GPAT 2024, AIR 907 in GPAT 2023, and AIR 6 in AIIMS CRE-2025 for Drug Store Keeper. At PharmacyFreak.com, he contributes expert content, exam strategies, and practical guidance for future pharmacists.
Mail- harsh@pharmacyfreak.com