Tell me about aspirin
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aspirin pharmacology mechanism clinical use
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I now have excellent, comprehensive information from multiple authoritative textbooks. Here is a thorough answer on aspirin:
Aspirin (Acetylsalicylic Acid)
What It Is
Aspirin is a nonsteroidal anti-inflammatory drug (NSAID) derived from salicylic acid. It is one of the most widely used medications in history, serving roles as an analgesic, antipyretic, anti-inflammatory, and antiplatelet agent depending on the dose. - Henry's Clinical Diagnosis and Management by Laboratory Methods
Mechanism of Action
Aspirin works by irreversibly acetylating a serine residue in the active site of cyclooxygenase (COX) enzymes - both COX-1 and COX-2. This permanently inactivates the enzyme, blocking the conversion of arachidonic acid to prostaglandins and thromboxanes.
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COX-1 inhibition in platelets: blocks thromboxane A2 (TxA2) synthesis, which normally promotes platelet shape change, granule release, and aggregation. Because platelets have no nucleus and cannot synthesize new COX, this inhibition lasts the entire lifespan of the platelet (~7-10 days).
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COX-2 inhibition: reduces prostaglandin synthesis in inflamed tissues, contributing to analgesic and anti-inflammatory effects.
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Unlike other NSAIDs (e.g., ibuprofen, naproxen), aspirin's inhibition is irreversible - this is the pharmacological basis for its unique and prolonged antiplatelet effect.
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Katzung's Basic and Clinical Pharmacology, Morgan & Mikhail's Clinical Anesthesiology, Bradley and Daroff's Neurology in Clinical Practice
Pharmacokinetics
| Parameter | Details |
|---|---|
| Absorption | Rapid in the upper GI tract; peak plasma levels in 20-40 minutes (non-enteric-coated) or 1-2 hours (regular oral) |
| Hydrolysis | Rapidly converted to acetic acid + salicylic acid by plasma esterases and the liver before entering systemic circulation |
| Protein binding | Salicylic acid is 70-90% plasma protein-bound |
| Half-life | Dose-dependent: ~3 hours at 500 mg; up to ~15 hours at 4 g/day |
| Elimination | Hepatic metabolism + urinary excretion of salicylate |
| Antiplatelet duration | Up to 7 days (life of the platelet) |
Enteric-coated formulations delay absorption and should not be used when a rapid antiplatelet effect is needed (e.g., acute coronary syndrome). - Tintinalli's Emergency Medicine
Clinical Uses
1. Analgesia and Antipyresis
- Effective for mild-to-moderate pain: headache, myalgia, neuralgia, arthralgia, and fever.
- Adult analgesic/antipyretic dose: 500 mg as needed, up to 4 g/day.
2. Anti-Inflammatory (Rheumatic Diseases)
- High doses are used for rheumatoid arthritis: 500 mg every 4-6 hours, or up to 1.95 g twice daily.
- Pediatric dose for juvenile rheumatoid arthritis: ~3.5 g/day (with caution due to Reye syndrome risk).
3. Antiplatelet / Cardiovascular
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Acute coronary syndrome (ACS): 325 mg loading dose PO; maintenance 81-162 mg/day.
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Secondary prevention of MI, ischemic stroke, and cardiovascular death: reduces risk of major vascular events by 20-25% (MI) and ~10-15% (stroke). The benefit clearly outweighs bleeding risks in this population.
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Primary prevention: Much more controversial. The FDA reversed its earlier endorsement in 2014 - aspirin for primary prophylaxis in patients without prior MI or stroke is not supported by the data, as benefit is numerically offset by GI bleeding. The 2019 ACC/AHA guidelines suggest considering low-dose aspirin (75-100 mg/day) only in adults aged 40-70 with high atherosclerotic cardiovascular disease risk and no increased bleeding risk.
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Atrial fibrillation: Aspirin is no longer recommended for stroke prevention in AF, as evidence shows no significant stroke reduction vs. placebo. Anticoagulation is preferred.
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Katzung's Basic and Clinical Pharmacology, Goodman & Gilman's, Braunwald's Heart Disease, Tintinalli's Emergency Medicine
4. Other Uses
- Systemic mastocytosis: Aspirin (with pre-treatment antihistamines) can block PGD2-mediated flushing and angioedema in patients with elevated urinary PGD metabolites.
- Niacin-induced flushing: Aspirin inhibits the PGD2-mediated cutaneous flushing associated with high-dose niacin therapy.
- Carotid endarterectomy: Low-dose aspirin (81-325 mg/day) reduces perioperative stroke, MI, and death risk more effectively than higher doses.
- Colorectal cancer: Evidence suggests aspirin reduces risk of recurrent colorectal adenomas.
Adverse Effects
| Adverse Effect | Details |
|---|---|
| GI irritation/bleeding | Most common adverse effect; dose-related. GI hemorrhage occurs in 1-5% of users. Risk factors: advanced age, history of ulcer, H. pylori infection, concomitant glucocorticoids or anticoagulants. Reduced (but not eliminated) by enteric coating, buffering, or antacids. |
| Intracranial hemorrhage | Roughly 2-fold increase in risk even at low doses; must be weighed against cardiovascular benefit. |
| Tinnitus / hearing changes | Classic sign of chronic salicylate toxicity (at higher doses). In children or patients with pre-existing hearing loss, hyperventilation may be the first sign. |
| Reye syndrome | Rare but serious complication in children with viral illness (influenza, varicella). Aspirin is contraindicated in children with febrile illness for this reason. |
| Aspirin-exacerbated respiratory disease | Bronchoconstriction in susceptible individuals with asthma (aspirin-sensitive asthma). |
| Metabolic/acid-base effects | Overdose causes initial respiratory alkalosis (salicylate stimulates respiratory centers), followed by metabolic acidosis. |
Contraindications
- Known hypersensitivity to aspirin or NSAIDs
- Active GI hemorrhage (e.g., bleeding peptic ulcer)
- Children with febrile viral illness (Reye syndrome risk)
- Severe hepatic disease or bleeding disorders (use with caution)
Drug Interactions
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Ibuprofen and naproxen: Compete with aspirin at the COX-1 platelet binding site and can antagonize aspirin's irreversible antiplatelet effect - reducing its cardioprotective benefit if taken before aspirin.
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Anticoagulants: Combined use significantly increases bleeding risk.
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Glucocorticoids: Increase GI ulcer/bleeding risk when combined with aspirin.
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Rheumatology (2-Volume Set), Katzung's Basic and Clinical Pharmacology
Recent Evidence (2024-2026)
A 2026 systematic review (Kanwal et al., PMID 41683716) examined aspirin's potential anti-metastatic role in cancer, finding evidence for aspirin reducing cancer cell migration and metastasis - an area of active investigation. Another 2025 meta-analysis (D'Crus et al., PMID 40452590) explored aspirin's effects on extracellular vesicles in preeclampsia, relevant to its established use in preventing pre-eclampsia in high-risk pregnancies.
In summary: Aspirin is a remarkable drug with a well-defined, irreversible mechanism, a wide range of clinical applications, and an important but nuanced benefit-risk profile that depends heavily on the indication - especially regarding primary vs. secondary cardiovascular prevention.
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