Aspirin
Is It Right for You?
A complete guide to Aspirin — what it treats, how it works, dosages, side effects, and when a clinician may prescribe it following an online consultation.
What Is Aspirin?
Aspirin (acetylsalicylic acid) is one of the oldest and most versatile drugs in clinical medicine — a non-steroidal anti-inflammatory drug (NSAID) with analgesic, antipyretic, anti-inflammatory, and antiplatelet properties. In the UK, aspirin is available in multiple formulations: standard tablets (75 mg and 300 mg), dispersible tablets (75 mg and 300 mg), and gastro-resistant (enteric-coated) tablets.
At low doses (75 mg), aspirin is used as an irreversible antiplatelet agent for the long-term secondary prevention of cardiovascular events (myocardial infarction, stroke, and peripheral arterial events) in patients with established cardiovascular disease. At higher doses (300–900 mg), it provides analgesic and antipyretic effects. At full anti-inflammatory doses (3–6 g/day), it has historically been used in rheumatic disease, though this is now uncommon given the availability of safer NSAIDs.
In the UK, low-dose aspirin 75 mg is prescription-only when used for secondary prevention. Higher dose formulations are available over the counter. A clinician must assess the cardiovascular indication, risk of GI bleeding, and suitability before prescribing long-term antiplatelet therapy.
What Conditions Is Aspirin Used For?
Aspirin is Used for:
long-term antiplatelet therapy to prevent recurrent MI following a first event
prevention of recurrent thrombotic stroke following an ischaemic event or transient ischaemic attack
part of dual antiplatelet therapy following ACS or coronary revascularisation
antiplatelet therapy to reduce cardiovascular events
300 mg loading dose at presentation
300–900 mg for mild to moderate pain and fever
The role of low-dose aspirin in primary cardiovascular prevention (in people who have not yet had a heart attack or stroke) is no longer routinely recommended, as evidence suggests the bleeding risks may outweigh the cardiovascular benefits in this setting. Aspirin for primary prevention should only be used under explicit clinician advice.
How Does Aspirin Work?
Aspirin irreversibly inhibits the cyclo-oxygenase (COX) enzymes — both COX-1 and COX-2 — by covalent acetylation of a serine residue at the active site. This permanently inactivates the enzyme for the lifetime of the affected cell.
Antiplatelet mechanism — COX-1 in platelets synthesises thromboxane A2 (TXA2), a potent promoter of platelet aggregation and vasoconstriction. Because platelets lack nuclei and cannot synthesise new COX-1 enzyme, aspirin's irreversible inhibition of platelet COX-1 abolishes TXA2 synthesis for the entire platelet lifespan (seven to ten days). Daily low-dose aspirin therefore maintains continuous antiplatelet effect with each new platelet being inhibited as it enters the circulation.
Analgesic and antipyretic mechanism — at higher doses, inhibition of COX-2 at sites of inflammation and in the hypothalamus reduces prostaglandin synthesis, producing analgesic, antipyretic, and anti-inflammatory effects through the same mechanism as other NSAIDs.
The irreversibility of aspirin's COX inhibition distinguishes it from other NSAIDs (which are reversible inhibitors), explaining its unique long-duration antiplatelet effect at low doses.
Dosages & Administration
The correct dose depends on the type and severity of infection, age, weight, and kidney function. Always follow your clinician's instructions. The table below is for general reference only.
Administration Tips
Take low-dose aspirin at the same time each day — once-daily dosing is sufficient for antiplatelet effect given aspirin's irreversible mechanism.
Take with or after food to reduce gastric irritation. Enteric-coated formulations may further reduce gastric discomfort, though evidence for significantly lower GI bleeding risk with enteric coating is limited.
Do not crush or chew enteric-coated tablets — this destroys the protective coating. Dispersible tablets, however, can be dissolved in water.
For acute MI, chew a 300 mg tablet (aspirin is absorbed buccally) or dissolve a dispersible tablet in water for rapid absorption; do not wait for a full glass of water.
Do not stop low-dose aspirin without consulting your clinician — sudden cessation in patients with cardiovascular disease can cause a rebound increase in platelet aggregability and thrombotic risk.
Side Effects of Aspirin
Most side effects are mild and resolve on their own. Serious side effects are rare but require immediate medical attention.
- Gastrointestinal irritation — dyspepsia, heartburn, nausea
- GI bleeding — even at low doses; typically minor but may be occult
- Bruising easily — antiplatelet effect prolongs bleeding time
- Gastrointestinal ulceration and major GI bleeding — black tarry stools, haematemesis, severe abdominal pain; risk is dose-dependent and increases with concurrent NSAIDs, corticosteroids, or anticoagulants
- Haemorrhagic stroke — aspirin increases the risk of intracranial bleeding; a significant consideration in elderly patients and those with uncontrolled hypertension
- Reye's syndrome — rare but serious; aspirin must not be given to children under 16 years of age due to the risk of this potentially fatal encephalopathy
- Salicylate toxicity (overdose) — tinnitus, hyperventilation, confusion, hyperthermia; can be fatal in overdose
- Bronchospasm — in aspirin-sensitive asthma; potentially severe
Even low-dose aspirin (75 mg) significantly increases the risk of major GI bleeding, which can be life-threatening, particularly in older patients. A proton pump inhibitor (PPI) should be co-prescribed in patients aged over 70, those with a history of peptic ulcer disease, or those taking concurrent NSAIDs, corticosteroids, or anticoagulants.
Drug Interactions
Always disclose all prescription drugs, over-the-counter medicines, vitamins, and supplements to your clinician or pharmacist before starting Asprin
additive bleeding risk; combination increases the risk of major haemorrhage substantially. Dual therapy is sometimes clinically necessary (e.g. post-ACS with AF) but requires careful benefit-risk assessment and co-prescription of a PPI.
concurrent NSAIDs may antagonise aspirin's irreversible antiplatelet effect and substantially increase GI bleeding risk; avoid concurrent use.
aspirin may attenuate the natriuretic effect of spironolactone.
may reduce absorption of aspirin; separate doses.
Important Warnings
Aspirin must never be given to children or adolescents under the age of 16 (except under specialist supervision for specific conditions such as Kawasaki disease) due to the risk of Reye's syndrome — a rare but frequently fatal combination of acute encephalopathy and hepatic failure.
Aspirin and other NSAIDs are contraindicated in patients with aspirin-sensitive asthma — a syndrome involving nasal polyps, sinusitis, and aspirin-triggered bronchospasm. Up to 20% of adult asthmatics may be aspirin-sensitive.
Low-dose aspirin nearly doubles the risk of major upper GI bleeding compared to placebo. PPI co-prescription should be considered in all patients over 70 or those with GI risk factors on long-term low-dose aspirin.
Aspirin is usually continued during minor surgical procedures due to haemostatic compensatory mechanisms. For major surgery, your surgical team will advise whether to continue or temporarily withhold aspirin.
Speak to a Clinician About Treatment
Getting advice no longer means sitting in a waiting room. Through The GP Service, you can consult with a licensed clinician in minutes — from home, on your lunch break, or wherever works for you. If treatment is clinically appropriate, your clinician can issue a prescription during the consultation. A consultation does not guarantee a prescription.
The treatment you need, when you need it.
Aspirin FAQs
Current UK and international guidelines no longer recommend aspirin for primary prevention of cardiovascular disease in most healthy adults, as the bleeding risks — particularly GI bleeding and haemorrhagic stroke — are judged to outweigh the small cardiovascular benefit. Aspirin for primary prevention should only be taken on explicit medical advice.
No — ibuprofen and other NSAIDs can interfere with aspirin's irreversible antiplatelet effect by competitively blocking access to the COX-1 active site. If you need pain relief while taking low-dose aspirin, paracetamol is the preferred choice.
Enteric coating prevents dissolution in the stomach, reducing direct gastric mucosal contact. However, aspirin causes GI bleeding primarily through its systemic antiplatelet and prostaglandin-inhibiting effects, not solely through local gastric contact. Enteric coating does not reliably prevent major GI bleeding — a proton pump inhibitor offers more robust GI protection.
In patients with coronary artery disease or previous stent implantation, sudden cessation of antiplatelet therapy can trigger a rebound prothrombotic state, increasing the risk of acute MI or stent thrombosis. Always discuss any planned changes to antiplatelet therapy with your clinician before stopping.
Seek immediate medical attention or call 999 if symptoms of salicylate toxicity occur — including ringing in the ears, rapid breathing, confusion, vomiting, or drowsiness. Aspirin overdose can be serious or fatal.