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Aspirin
Acetylsalicylic acid (ASA) | OTC NSAID + irreversible platelet inhibitor
Aliases (9)
Overview
What is Aspirin?
Aspirin (acetylsalicylic acid, ASA) is the prototypical NSAID and the only commonly-used one that inhibits cyclooxygenase irreversibly via covalent acetylation of a serine residue. FDA-approved 1900 (Bayer); now an OTC commodity drug. Defined by dual-face pharmacology: low-dose (75-100 mg) for permanent platelet inhibition lasting the platelet lifespan (~10 days); higher doses for analgesic, antipyretic, and anti-inflammatory effect.
Key Benefits
Gold-standard secondary cardiovascular prevention (~25% RRR for major vascular events post-MI/stroke); life-saving in acute MI when chewed at symptom onset; possible long-term colorectal cancer chemoprevention with ≥5 years exposure; preeclampsia prevention in high-risk pregnancy. Primary CV prevention role substantially narrowed post-2018 (ARRIVE/ASPREE/ASCEND) and 2022 USPSTF update.
Mechanism of Action
Aspirin covalently acetylates Ser530 on COX-1 and Ser516 on COX-2, permanently blocking arachidonic acid access and prostaglandin/thromboxane synthesis. Anucleate platelets cannot resynthesize the dead enzyme, so a single dose blocks platelet TXA2 production for the platelet lifespan (~10 days). Nucleated cells regenerate COX within hours, requiring sustained dosing for systemic anti-inflammatory effect. The salicylate metabolite (after rapid deacetylation) adds NF-κB inhibition and mitochondrial uncoupling at higher doses.
Pharmacokinetics
Research Indications
COX-1
serine residue at position 530 (Ser530)
COX-2
serine residue at position 516 (Ser516)
Research Protocols
Disclaimer: These are commonly discussed research protocols and not medical advice.
Peptide Interactions
secondary CV prevention combination is standard. No PD interaction.
dual antiplatelet therapy (DAPT) for post-stent/post-ACS. Markedly raises bleed risk.
secondary prevention combinations common; mild renal interaction concern.
secondary prevention combinations standard.
reversible NSAIDs compete with aspirin for COX-1 binding and can prevent the irreversible acetylation if dosed before aspirin, blunting cardioprotection. *An…
bleeding risk multiplies. Combinations used clinically (e.g., warfarin + aspirin in some valve patients) but with explicit risk acceptance and tight monitoring.
additive GI ulcer risk. Standard practice to add PPI for gastroprotection if combined.
modest antiplatelet effect of SSRIs (via serotonin-mediated platelet activation suppression) compounds aspirin's effect; small but real GI/bleed risk increas…
modest antiplatelet contribution; theoretical bleed risk amplification. Magnitude small but worth flagging.
antiplatelet effect; case reports of bleeding when combined with aspirin.
fibrinolytic/proteolytic supplements; theoretical bleed risk amplification.
mild antiplatelet effect; theoretical concern.
Quality Indicators
USP / NF Verified
Look for USP or NF designation on the label, ensuring purity and dissolution standards meet pharmacopoeial requirements.
Recognizable Brand or Major Generic
Bayer, Ecotrin, Bufferin, or store-brand generic from a national pharmacy chain. Aspirin is a true commodity drug — no benefit from premium pricing.
Vinegar Smell on Old Tablets
Aspirin hydrolyzes over time to salicylic acid + acetic acid. Strong vinegar odor indicates degradation; potency reduced and GI irritation increased. Discard.
Discoloration or Crumbling
Yellowing, brown spots, or crumbling tablets indicate moisture exposure and breakdown. Discard; do not consume.
Past Expiration by >1 Year
Aspirin is one of the few drugs with measurable potency loss past expiration; degradation products are also more GI-irritating. Replace bottles annually.
What to Expect
- Week 1Tolerability and dose-response.
- Week 2-4Early effect window.
- Week 4-8Peak benefit assessment.
- Week 8+Cycle decision point.
Side Effects & Safety 6
Side Effects
- 1GI irritation — heartburn, dyspepsia, nausea. Mostly local (acidic salicylate on gastric mucosa) plus systemic (PGE2 suppression of gastric mucus + bicarbonate secretion). Buffered or enteric-coated formulations reduce but do not eliminate this — the mucosal protective effect of PGE2 is prostaglandin-dependent and gone at any therapeutic dose.
- 2Easy bruising — antiplatelet effect manifests as visible bruising from minor trauma.
- 3Prolonged bleeding from cuts — small wounds bleed longer; soft-tissue injection sites may oze longer.
- 4Asymptomatic GI mucosal damage — endoscopic studies show ~60% of chronic aspirin users develop visible erosions or ulcers; most are silent. About 1-2% per year develop a clinically significant GI bleed on chronic low-dose aspirin.
- 5Tinnitus — early sign of salicylate accumulation; common at >2 g/day, rare at 81 mg/day.
- 6Bronchospasm in aspirin-sensitive asthma (AERD) — affects ~5-10% of asthmatics; classic Samter's triad (asthma + nasal polyps + aspirin sensitivity).
When to Stop
- Major GI bleed — annual incidence ~0.5-1% on chronic 81 mg/day in average-risk adults; ~2-4% in older adults or those with additional risk factors (H. pylori, prior ulcer, concurrent NSAID, anticoagulant). Can be fatal. The dominant harm signal in primary-prevention trials.
- Intracranial hemorrhage — rare absolutely (~0.05-0.1%/year on aspirin) but disproportionately consequential because outcomes are catastrophic. Risk substantially elevated by trauma — the relevant concern for combat sports athletes.
- Hemorrhagic stroke — small absolute increase on aspirin; in primary-prevention trials, modestly offset by reduction in ischemic stroke, but the *type* of stroke that aspirin prevents (ischemic) is generally less devastating than the type it can cause (hemorrhagic).
- Reye syndrome — acute encephalopathy + fatty liver in children/adolescents (typically <19yo) given aspirin during viral illness, especially influenza or varicella. Mortality ~30-40% historically; near-disappearance after public health campaigns since 1980s. Mechanism involves mitochondrial dysfunction + hepatic toxicity. Absolute contraindication for children/adolescents with viral illness; practical floor for aspirin is age 12+ in most pediatric guidelines, with conservatism urging 16-18+.
- Severe hypersensitivity reactions — anaphylaxis, urticaria, angioedema. Cross-reactivity within NSAID class for AERD patients.
- Stevens-Johnson syndrome / TEN — extremely rare, idiosyncratic.
- Salicylate toxicity (overdose) — tinnitus → nausea/vomiting → hyperventilation (respiratory alkalosis from medullary stimulation) → metabolic acidosis (mitochondrial uncoupling, lactic acid accumulation) → hyperthermia → seizure → coma. Treated with alkalinization (sodium bicarbonate to alkalinize urine and trap salicylate ionic form for renal excretion) ± hemodialysis. Onset is dose-dependent and chronic toxicity can develop at therapeutic-range doses in elderly or renally-impaired patients.
- Acute kidney injury — prostaglandin-mediated mechanism, especially when combined with ACE inhibitor + diuretic ("triple whammy" — ACE-i, NSAID, diuretic).
- Hepatotoxicity — rare at therapeutic doses.
- The 10-day platelet effect is the key issue. A single 325 mg dose 9 days before sparring still meaningfully impairs platelet function during a face/head impact. There is no "wear off in a day" option for aspirin's antiplatelet effect — that's the nature of irreversible inhibition + anucleate platelets.
- Risk of intracranial bleeding with subdural/epidural hematoma after head trauma is materially elevated on aspirin. Even subclinical microbleeds (cumulative microvascular injury) are theoretically worse on antiplatelets.
- Eye injuries — orbital bleeding, retinal hemorrhage from eye/face impact is harder to control on aspirin.
- Ear trauma — auricular hematoma ("cauliflower ear") risk and severity may be increased.
- Recovery from training cuts/scrapes/road rash — bleeds longer, scabs slower.
References
ARRIVE 2018 — Aspirin to Reduce Risk of Initial Vascular Events (Lancet)
31924-X/fulltext) — PMID 30158069. Moderate-risk primary prevention; no CV benefit, doubled GI bleed.
View StudyASPREE 2018 — Effect of Aspirin on Disability-free Survival in the Healthy Elderly (NEJM)
PMID 30221595. ~19,000 healthy 70+ adults; no benefit, increased bleed, paradoxically higher cancer mortality.
View StudyASPREE 2018 — Effect of Aspirin on Cardiovascular Events and Bleeding in the Healthy Elderly (NEJM)
PMID 30221597. CV/bleed companion paper.
View StudyASPREE 2018 — Effect of Aspirin on All-Cause Mortality in the Healthy Elderly (NEJM)
PMID 30221596. Mortality companion paper.
View StudyASCEND 2018 — Effects of Aspirin for Primary Prevention in Persons with Diabetes Mellitus (NEJM)
PMID 30146931. Diabetic primary prevention; small benefit offset by bleed.
View StudyUSPSTF 2022 — Aspirin Use to Prevent Cardiovascular Disease: Recommendation Statement (JAMA)
PMID 35471505. Recommends against initiation in 60+, individualized in 40-59.
View StudyRothwell PM et al. 2010 — Long-term effect of aspirin on colorectal cancer incidence and mortality (Lancet)
61543-7/fulltext) — PMID 20970847. Pooled long-term follow-up; 24% CRC incidence reduction, 35% CRC mortality reduction.
View StudyRothwell PM et al. 2011 — Effect of daily aspirin on long-term risk of death due to cancer (Lancet)
62110-1/fulltext) — PMID 21144578. 20% all-cancer mortality reduction.
View StudyATC 2009 — Antithrombotic Trialists' Collaboration meta-analysis (Lancet)
60503-1/fulltext) — Definitive meta-analysis; secondary prevention 25% RRR for major vascular events.
View StudyISIS-2 1988 — Randomised trial of intravenous streptokinase, oral aspirin, both, or neither among 17,187 cases of suspected acute myocardial infarction (Lancet)
92833-4/fulltext) — Landmark acute-MI trial; aspirin alone reduced 35-day mortality 23%.
View StudyVane JG 1971 — Inhibition of prostaglandin synthesis as a mechanism of action for aspirin-like drugs (Nature New Biology)
Original mechanism paper that won Vane the 1982 Nobel Prize.
View StudyPatrono C 2014 — The multifaceted clinical readouts of platelet inhibition by low-dose aspirin (J Am Coll Cardiol)
Key review of low-dose aspirin pharmacology.
View StudyHow was your experience with this compound?
Anonymous · one vote per session · results below at 5+ votes.
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