Medication · Anticoagulants, antiplatelets & haematology · full clinical detail
Aspirin
Also known as: acetylsalicylic acid, ASA, low-dose aspirin
A low-dose antiplatelet that stops platelets clumping into clots, used long term after heart attacks and strokes; at higher doses it is a painkiller and anti-inflammatory.
A small daily dose of aspirin stops platelets, the tiny cell fragments that start clots, from sticking together. That makes a clot in a narrowed heart or brain artery less likely, which is why it is taken after a heart attack or stroke. Bigger doses also block the chemicals that cause pain, fever and inflammation.
Route
Oral (dispersible, enteric-coated and standard tablets); rectal suppositories
Prescription aspirin for symptoms of rheumatoid arthritis and other inflammatory conditions; non-prescription aspirin for pain, fever and, on medical advice, to reduce the risk of heart attack and stroke
Not recommended in current UK guidance: bleeding outweighs benefit for most healthy people
Licensed indications come from the product licence in the named country; guideline-supported and off-label uses are cited to the guideline that supports them. Licensing differs between countries.
How it works
A small daily dose of aspirin stops platelets, the tiny cell fragments that start clots, from sticking together. That makes a clot in a narrowed heart or brain artery less likely, which is why it is taken after a heart attack or stroke. Bigger doses also block the chemicals that cause pain, fever and inflammation.
Irreversibly blocks the enzyme COX-1 in platelets so they cannot make thromboxane, the signal that makes them clump. Because platelets have no nucleus they cannot make new enzyme, so one small daily dose keeps them quiet for their whole 7-10 day life. Higher doses also block COX-2 in tissues, reducing prostaglandins that cause pain, fever and inflammation.
Mechanism of action
Aspirin acetylates a serine residue in the active site of cyclo-oxygenase, inhibiting it irreversibly. In platelets, which cannot synthesise new enzyme, this abolishes thromboxane A2 production for the platelet's lifespan (7-10 days), reducing aggregation; because the effect is irreversible, a single low daily dose (75-100 mg) is enough and the effect persists for days after stopping. At analgesic doses (300-900 mg) aspirin also inhibits COX-2 in inflamed tissue and the hypothalamus, reducing prostaglandin-mediated pain sensitisation, inflammation and fever. Inhibition of gastric COX-1 removes the prostaglandins that protect the mucosa, the basis of dyspepsia, ulceration and gastrointestinal bleeding; reduced renal prostaglandins can impair kidney perfusion. Aspirin is rapidly hydrolysed to salicylate, which is the main circulating species at higher doses.
Stomach ulcers and gastrointestinal bleeding (risk rises with age, alcohol, NSAIDs and anticoagulants)
Bleeding in the brain (rare)
Reye syndrome in children with viral illness: not for under-16s except on specialist advice
Bronchospasm in aspirin-sensitive asthma
Source: NHS: Aspirin. Frequencies follow the source's categories; no percentages are invented.
Serious safety information
Special warning. Gastrointestinal bleeding: aspirin can cause stomach and duodenal ulcers and bleeding, with risk rising with age, alcohol, previous ulcers, and combination with NSAIDs, anticoagulants, SSRIs or corticosteroids. UK information advises urgent help for black or bloody stools, vomiting blood or severe stomach pain. UK
Special warning. Aspirin-sensitive asthma: aspirin and NSAIDs can trigger severe bronchospasm in a minority of people with asthma, nasal polyps or previous reactions. UK
Warnings and precautions
Contraindication. Children under 16: aspirin is not given to children except on specialist advice (for example Kawasaki disease) because of the risk of Reye syndrome, a rare but often fatal brain and liver illness after viral infection. UK
Precaution. Stopping before surgery: aspirin's effect lasts the life of the platelet, so decisions about stopping it before operations are made by the surgical team; stopping it on one's own after a stent can be dangerous. UK
Pregnancy. High (pain-relief) doses are not recommended in pregnancy, particularly in the third trimester; low-dose aspirin for pre-eclampsia prevention is prescribed from 12 weeks when indicated. UK
"Absolute" and "relative" follow the wording of the cited source; where the source does not classify, the cell is blank.
Interactions
Check interactions
Taking Aspirin with other medicines? Add them to the checker to see what the official sources say about each pair: the mechanism, general management and monitoring, never a bare "safe".
Interactions documented in the official sources cited on each card. The list covers the medicines represented on this site and is not exhaustive; how the interaction data is compiled.
Contraindicated or avoidOfficial guidance: avoid the combination
Ibuprofen and other NSAIDs add to aspirin's stomach irritation and bleeding risk, and ibuprofen taken before low-dose aspirin can block its protective antiplatelet effect.
Aspirin with warfarin substantially increases bleeding, including gastrointestinal and intracranial bleeding.
Why it can occur
additive bleeding risk
What official information says
UK information lists blood thinners such as warfarin and apixaban among medicines to tell a doctor about; the BNF advises avoiding the combination unless specifically indicated.
What to discuss with a clinician
Official guidance: combined only for specific indications (for example mechanical valves) under specialist direction, usually with gastroprotection.
BNF: Apixaban interactions (increased risk of bleeding); NHS apixaban: only if prescribed
Sources
NHS: Aspirin(Low-dose aspirin · Taking it with other medicines and herbal supplements)UK BNF: Apixaban interactions(Interactions)UK NHS: Apixaban(Taking it with other medicines and herbal supplements · Medicines that interact with apixaban)UK
Review
Facts checked against the cited sources; not yet clinically reviewed · last reviewed 12 September 2026
Aspirin reduces the kidney's excretion of methotrexate, raising its levels and toxicity (bone-marrow suppression, mucositis, liver injury).
Why it can occur
renal clearance
What official information says
UK information lists methotrexate among medicines to tell a doctor about; the BNF advises avoiding aspirin with high-dose methotrexate and monitoring with low-dose.
What to discuss with a clinician
Official guidance: avoided with high-dose methotrexate; with weekly low-dose methotrexate the combination is used only with monitoring.
SSRIs such as sertraline reduce platelet serotonin and add to aspirin's bleeding risk, particularly in the stomach.
Why it can occur
additive bleeding risk
What official information says
UK information for sertraline lists aspirin and NSAIDs among medicines that increase bleeding risk and advises telling a doctor; a stomach-protecting medicine may be prescribed.
What to discuss with a clinician
Documented; gastroprotection considered in those at risk; report black stools or bruising.
What may be monitored
Signs of gastrointestinal bleeding
Basis of the status
NHS: Taking sertraline with other medicines (tell your doctor; bleeding risk)
Sources
NHS: Sertraline(Taking it with other medicines and herbal supplements · Medicines that interact with sertraline)UK BNF: Sertraline interactions(Interactions)UK NHS: Aspirin(Low-dose aspirin · Taking it with other medicines and herbal supplements)UK
Review
Facts checked against the cited sources; not yet clinically reviewed · last reviewed 12 September 2026
Therapeutic duplication. Aspirin with another antiplatelet (dual antiplatelet therapy) is a deliberate, time-limited prescription after stents and acute coronary syndromes; taking two antiplatelets without such an indication adds bleeding risk. UK
Food and drink
With
Effect
Official advice
Food
Taking aspirin with or after food reduces stomach irritation; dispersible tablets are dissolved in water. (other)
UK information advises taking low-dose aspirin with food. UK
Alcohol
Alcohol increases the risk of stomach irritation and gastrointestinal bleeding with aspirin. (Additive mucosal injury and bleeding risk)
UK information states alcohol can be drunk in moderation while taking low-dose aspirin, but heavy drinking increases the chance of stomach bleeding. UK
Herbal remedies and supplements
With
Effect
Official advice
Herbal remedies and supplements with antiplatelet effects (for example high-dose fish oil, ginkgo)
A possible additive increase in bleeding tendency
UK information states there is little research on herbal remedies with aspirin and advises telling a pharmacist or doctor about any supplement taken alongside it. UK
Monitoring
What
Why
Tests and markers
Source
Symptoms of bleeding
No routine blood tests are needed for low-dose aspirin; people are advised to report black stools, vomiting blood, prolonged nosebleeds or unusual bruising
Low-dose aspirin is prescribed from 12 weeks for women at risk of pre-eclampsia and UK information states it is safe in pregnancy when recommended; pain-relief doses are not recommended, particularly in the third trimester, because they can affect the baby's circulation and delay labour. UK
Breastfeeding
UK information states low-dose aspirin may be taken while breastfeeding when the baby is healthy and a doctor advises it; higher pain-relief doses are not recommended because of the theoretical risk of Reye syndrome in the baby. UK
Children
Not for children under 16 except on specialist advice (Kawasaki disease, some heart conditions) because of Reye syndrome. UK
Older adults
Bleeding risk, particularly gastrointestinal, rises steeply with age; a proton pump inhibitor is often co-prescribed and the indication is reviewed regularly. UK
Kidney impairment
Avoid analgesic doses in severe renal impairment (sodium and water retention, deterioration of kidney function); low-dose antiplatelet treatment is generally continued with monitoring. UK
Liver impairment
Avoid analgesic doses in severe hepatic impairment because of the increased risk of gastrointestinal bleeding; low-dose use is decided individually. UK
Educational summary of drug–condition cautions in the cited sources; not a personal screening.
Effects on tests and results
Platelets: Platelet function is impaired for 7-10 days after each dose, so bleeding time is prolonged although the platelet count is unchanged. UK
Haemoglobin: Occult gastrointestinal blood loss can lower haemoglobin and ferritin, causing iron-deficiency anaemia. UK
Low-dose aspirin raises serum uric acid by reducing its renal excretion. UK
Salicylate at high doses can interfere with some urine glucose and thyroid-function assays; not relevant at antiplatelet doses. US
Pharmacokinetics
Absorption
Rapidly absorbed from the stomach and small intestine (dispersible faster than enteric-coated, which is absorbed in the intestine over hours)
Peak
Aspirin 30-40 minutes (plain tablets); antiplatelet effect within an hour of a chewed dose
Half-life
Aspirin about 15-20 minutes (hydrolysed to salicylate); salicylate 2-3 hours at low dose, longer at high dose (saturable metabolism); antiplatelet effect lasts the platelet lifespan
Metabolism
Hydrolysed by esterases in the gut wall, blood and liver to salicylic acid, then conjugated in the liver
Elimination
Renal, as salicyluric acid and glucuronides; excretion of salicylate increases in alkaline urine
Dosing is intentionally not described: doses depend on the indication, the country's licence, kidney and liver function, age, weight and other medicines, and are set by a prescriber.
Medicines that share the class of Aspirin. They are separate medicines with their own licences, doses and interactions, not alternative names for Aspirin.
Medicines often discussed alongside Aspirin: used together, compared with it, or acting on the same problem by a different route.
Educational content. Anatomy Nexus provides medical education and does not replace professional medical advice, diagnosis, treatment, prescribing or pharmacist review. Doses, choices and monitoring are decided by a prescriber for an individual; never start, stop or change a medicine on the basis of this page.
