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Aspirin

Aspirin: The Timeless Pill That Revolutionized Medicine

Introduction

Few drugs in the history of medicine have had as profound an impact as Aspirin. Often referred to as the "wonder drug," Aspirin has been used for over a century to treat a wide array of conditions—from simple headaches to life-threatening cardiovascular diseases. Originally derived from the bark of the willow tree, Aspirin (acetylsalicylic acid) has earned its place in both first-aid kits and hospital formularies worldwide. Despite being one of the oldest medications still in use, the pharmacological insights, therapeutic innovations, and ongoing clinical relevance of Aspirin make it an evergreen topic in pharmacotherapy.

History of Aspirin: From Nature to Laboratory

The therapeutic use of willow bark dates back to ancient civilizations like the Sumerians and Egyptians, who used it for pain and fever. However, it wasn't until the 19th century that salicin, the active compound, was isolated.

  • 1897: German chemist Felix Hoffmann synthesized acetylsalicylic acid (ASA) at Bayer, aiming to create a better-tolerated derivative of salicylic acid.

  • 1899: Bayer trademarked the name Aspirin and launched it as a commercial drug.

  • It quickly became a staple for fever, pain, and inflammation and evolved into a core cardiovascular agent.

Today, Aspirin is recognized on the WHO Model List of Essential Medicines, signifying its critical role in healthcare.

Chemical and Pharmacological Properties

Property Detail
Generic Name Acetylsalicylic acid (ASA)
Chemical Formula C₉H₈O₄
Drug Class Non-steroidal anti-inflammatory drug (NSAID), Antiplatelet
Molecular Weight 180.16 g/mol
Forms Oral tablets, chewable tablets, enteric-coated tablets, suppositories

Mechanism of Action

Aspirin’s effects stem primarily from its ability to irreversibly inhibit cyclooxygenase (COX) enzymes:

COX Inhibition

  • COX-1: Found in most tissues; regulates platelet aggregation, gastric protection, and renal blood flow.

  • COX-2: Induced during inflammation and responsible for pain and fever.

Aspirin’s Unique Action

  • Irreversibly acetylates COX enzymes, unlike other NSAIDs which act reversibly.

  • In platelets, this leads to permanent inhibition of thromboxane A2 (TXA2) → decreased platelet aggregation.

  • In endothelial cells, it reduces prostaglandins → anti-inflammatory and antipyretic effects.

This dual mechanism allows Aspirin to act as both an analgesic and a cardiovascular protectant.

Pharmacokinetics

Parameter Value
Bioavailability ~50–75% (oral)
Absorption Site Stomach and upper small intestine
Onset of Action 15–30 minutes
Peak Plasma Level 1–2 hours
Half-life 15–20 minutes (ASA), 2–3 hours (salicylate)
Metabolism Hepatic (to salicylic acid)
Excretion Renal, pH-dependent

Enteric-coated tablets delay absorption but reduce gastric irritation.

Therapeutic Indications

Aspirin is used across a spectrum of clinical conditions. Its applications can be broadly categorized as analgesic, antipyretic, anti-inflammatory, and antiplatelet.

1. Pain Relief (Analgesic)

  • Effective in mild to moderate pain: headaches, dental pain, musculoskeletal aches, dysmenorrhea.

  • Often combined with opioids or caffeine for enhanced effect.

2. Fever Reduction (Antipyretic)

  • Inhibits prostaglandin synthesis in the hypothalamus to lower fever.

  • Superseded by paracetamol in pediatric populations due to Reye’s syndrome risk.

3. Inflammatory Conditions

  • Historically used in rheumatoid arthritis, osteoarthritis, and systemic lupus erythematosus.

  • Higher doses (3–6 g/day) required, often limited by GI toxicity.

4. Cardiovascular Disease Prevention (Antiplatelet)

  • Low-dose aspirin (75–325 mg/day) is a mainstay in:

    • Secondary prevention after MI, stroke, or TIA

    • Post-stenting and coronary bypass

    • Peripheral artery disease

  • Primary prevention is more selective due to bleeding risks.

5. Colorectal Cancer Prevention

  • Emerging evidence suggests long-term low-dose aspirin reduces risk of colorectal adenomas and cancer.

6. Obstetric Uses

  • Low-dose aspirin reduces the risk of preeclampsia in high-risk pregnancies when initiated early.

Dosage and Formulations

Indication Dose
Pain/Fever 325–1000 mg every 4–6 hours (max 4 g/day)
Inflammation 3–6 g/day in divided doses
Cardiovascular prevention 75–325 mg/day
Preeclampsia prevention 81 mg/day from 12–28 weeks gestation

Formulations

  • Plain tablets: Quick action, but harsh on the stomach

  • Enteric-coated: Delayed release, gentler on GI tract

  • Chewables: For emergency use in acute coronary syndromes

  • Suppositories: Alternative route for nausea/vomiting patients

Adverse Effects

Despite its benefits, Aspirin is associated with several adverse effects, especially when used chronically or at high doses.

System Adverse Effects
Gastrointestinal Gastritis, ulcers, bleeding, nausea
Hematologic Increased bleeding risk, especially with anticoagulants
Renal Reduced perfusion in volume-depleted patients
Respiratory Bronchospasm in aspirin-sensitive asthma
Neurological Tinnitus (early sign of toxicity), confusion
Hepatic Rare hepatotoxicity at high doses

Reye’s Syndrome

  • A rare but fatal condition involving encephalopathy and liver failure.

  • Associated with aspirin use in viral infections in children.

  • Contraindicated in children <16 years, except in Kawasaki disease.

Contraindications and Precautions

Absolute Contraindications
Active peptic ulcer disease
Hypersensitivity to NSAIDs
Bleeding disorders (e.g., hemophilia)
Children with viral infections
Third trimester of pregnancy
Use with Caution In
Elderly patients
Chronic kidney disease
Asthma
Concurrent use of anticoagulants

Drug Interactions

Aspirin interacts with various drug classes, which may enhance or antagonize its effects.

Drug Interaction
Warfarin, heparin Increased bleeding risk
NSAIDs GI toxicity, competition at COX binding
ACE inhibitors Reduced antihypertensive efficacy
Methotrexate Increased toxicity due to decreased clearance
SSRIs Additive risk of GI bleeding

Aspirin Resistance

Some patients exhibit Aspirin resistance, leading to reduced antiplatelet effects and recurrent cardiovascular events. Mechanisms include:

  • Genetic polymorphisms

  • Poor absorption

  • Drug interactions

  • Non-compliance

Monitoring tools like serum thromboxane B2 levels or platelet aggregometry can assess responsiveness.

Public Health and Aspirin

Aspirin’s widespread availability and low cost have made it a global health staple, especially in resource-limited settings. Its use in mass prophylaxis (e.g., during cardiovascular epidemics) and its inclusion in essential medicine kits reflect its utility.

Emerging and Novel Uses

  1. Cancer Prevention and Treatment

    • Evidence supports its role in reducing colorectal, esophageal, and gastric cancers.

    • Ongoing trials are examining its use in adjuvant cancer therapy.

  2. COVID-19

    • Initial studies suggested possible benefits in reducing thrombotic complications.

    • However, large trials like RECOVERY found no significant mortality benefit.

  3. Neurodegenerative Disorders

    • Aspirin’s anti-inflammatory and antithrombotic effects are being explored in Alzheimer’s and Parkinson’s diseases, though evidence remains inconclusive.

Aspirin vs Other NSAIDs

Feature Aspirin Ibuprofen Naproxen
COX Binding Irreversible Reversible Reversible
Antiplatelet Yes No No
Half-life Short 2–4 hrs 12–17 hrs
Use in MI Yes No No
GI Risk Moderate-High Moderate Moderate

Conclusion

Aspirin remains one of the most versatile, affordable, and time-tested drugs in human history. From treating aches and pains to saving lives by preventing heart attacks and strokes, Aspirin continues to play a vital role in medicine. Its irreversible COX inhibition, wide-ranging applications, and long-standing safety record make it a foundational therapy. However, as with any medication, understanding its risks, drug interactions, and contraindications is key to optimizing outcomes.