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Drug Reference

aspirin

NSAID, antiplatelet· Brands: Bufferin, Ecotrin, Bayer, Bayer Aspirin, Disprin, Aspro, SolprinPOPR

Mechanism of action
Aspirin irreversibly acetylates serine 530 of cyclooxygenase (COX) isoenzymes, thereby blocking the conversion of arachidonic acid to prostaglandins and thromboxane A2. At low doses, it preferentially inhibits COX-1 in platelets, reducing thromboxane A2 and thereby inhibiting platelet aggregation for the lifespan of the platelet. At higher doses, it also inhibits COX-2, providing anti-inflammatory, analgesic, and antipyretic effects.
Pharmacokinetics
Aspirin is rapidly absorbed from the gastrointestinal tract, with peak plasma concentrations occurring within 30 to 60 minutes for plain tablets. It undergoes rapid hydrolysis to salicylic acid, the active metabolite, with a plasma half-life of 15 to 20 minutes for aspirin and 2 to 3 hours for salicylic acid, though this extends at high doses. Salicylic acid is metabolised in the liver and excreted renally, with dose-dependent elimination kinetics. Enteric-coated formulations delay absorption and thereby onset of action.
Indications & dosing

Mild to moderate pain

Take with food to reduce gastric irritation. Use lowest effective dose.

  • adult · PO: 300–900 mg every 4-6 hours · Take with food to reduce gastric irritation. Use lowest effective dose.

Fever

Avoid in children under 16 with fever due to risk of Reye's syndrome.

  • adult · PO: 300–900 mg every 4-6 hours · Avoid in children under 16 with fever due to risk of Reye's syndrome.

Secondary prevention of cardiovascular events

For prevention of MI and stroke. Consider gastroprotection if high GI risk.

  • adult · PO: 75–100 mg once daily · For prevention of MI and stroke. Consider gastroprotection if high GI risk.

Acute coronary syndrome

Loading dose; follow with 75-100 mg daily.

  • adult · PO: 300 mg single dose · Loading dose; follow with 75-100 mg daily.

Rheumatoid arthritis

High doses for anti-inflammatory effect. Monitor for toxicity.

  • adult · PO: 600–1200 mg every 4-6 hours · High doses for anti-inflammatory effect. Monitor for toxicity.

Kawasaki disease (acute phase)

Start within 10 days of onset. After defervescence, reduce to antiplatelet dose.

  • pediatric · PO: 80–100 mg/kg/day divided into 4 doses · Start within 10 days of onset. After defervescence, reduce to antiplatelet dose.

Pre-eclampsia prophylaxis

For high-risk women from 12 weeks until delivery.

  • adult · PO: 75–100 mg once daily · For high-risk women from 12 weeks until delivery.
Contraindications & precautions
  • Concurrent use of methotrexate (>=20 mg/week) · Increased methotrexate toxicity; avoid or use with extreme caution.
  • Severe hepatic impairment · Aspirin may precipitate hepatic encephalopathy and impair coagulation.
  • Hypersensitivity to aspirin or other NSAIDs, including history of asthma or angioedema · Cross-sensitivity may occur with other NSAIDs.
  • Active peptic ulcer disease or gastrointestinal bleeding · Aspirin increases risk of further bleeding.
  • Severe renal failure · Aspirin may worsen renal function; avoid if eGFR < 30 ml/min.
  • Third trimester of pregnancy · Risk of premature closure of ductus arteriosus and prolonged labour.
  • Children under 16 with viral infection · Risk of Reye's syndrome; use only if specifically indicated (e.g. Kawasaki disease).
  • Use with caution in patients with asthma, as aspirin may precipitate bronchospasm.
  • Avoid in children under 16 with fever or viral illness due to risk of Reye's syndrome.
  • Use with caution in patients with a history of peptic ulcer or bleeding disorders; consider gastroprotection.
  • May increase risk of bleeding when used with anticoagulants or antiplatelet agents; monitor for signs of bleeding.
  • Stop aspirin before planned surgery to reduce bleeding risk, as advised by the clinician.
Adverse effects
  • Bleeding tendency (bruising, epistaxis, prolonged bleeding)serious
  • Renal impairment (especially in dehydrated or elderly patients)serious
  • Hypersensitivity reactions (rash, urticaria, angioedema, anaphylaxis)serious
  • Dyspepsia
  • Gastrointestinal bleedingserious
  • Tinnitus
  • Bronchospasm in patients with asthmaserious
  • Reye's syndrome in childrenserious
Interactions
  • clopidogrelmajorWhen prescribed for a valid indication, use the lowest effective doses (e.g., aspirin 75-100 mg daily and standard clopidogrel maintenance). Monitor for signs of bleeding, and consider gastroprotective strategies such as a proton pump inhibitor, especially in patients at high risk. Avoid concurrent use of other anticoagulants or antiplatelets without careful assessment. Contraindicated in active pathological bleeding or when the patient has a severe bleeding disorder.
  • ticagrelormajorUse the combination only when indicated, such as in acute coronary syndromes or after stent placement. Monitor patients closely for signs of bleeding, including occult blood loss. Consider gastroprotective strategies such as proton pump inhibitors in patients at higher risk. Review the need for continued dual therapy at regular intervals.
  • prasugrelmajorUse dual antiplatelet therapy only when clearly indicated, such as acute coronary syndrome or drug-eluting stent placement. Monitor for signs of bleeding, including occult blood loss. Consider gastroprotection with a proton pump inhibitor where appropriate. The duration of dual therapy should be guided by current cardiology guidelines and individual thrombotic versus bleeding risk.
  • sertralinemajorUse with caution. Monitor patients for signs of bleeding, such as anaemia, black stools, or epistaxis. Consider gastroprotective therapy, such as a proton pump inhibitor, especially in patients with risk factors. Weigh the benefits of antiplatelet therapy against the risk of bleeding. If excessive bleeding occurs, review the need for sertraline and consider alternatives.
  • citaloprammajorAssess the need for aspirin and citalopram co-prescription. If aspirin is required for cardiovascular protection, consider gastroprotective measures such as a proton pump inhibitor. Use the lowest effective doses of each drug. Monitor for signs of gastrointestinal bleeding (e.g., black stools, anaemia) and evaluate haemoglobin periodically. Consider alternative analgesia or anticoagulation strategies where appropriate, and avoid concurrent use in high-risk patients unless benefits clearly outweigh risks.
  • escitaloprammajorBefore co-prescribing, assess the patient's overall bleeding risk and the necessity of each drug. For patients with existing risk factors, consider gastroprotective therapy (e.g., a proton pump inhibitor) and monitor closely for evidence of bleeding. Advise patients to report any unusual bleeding, easy bruising, or black stools. If pain relief is required, alternatives to aspirin (e.g., paracetamol) may be considered where appropriate, but the antiplatelet benefit of aspirin should not be discontinued without specialist advice.
  • diclofenacmajorAvoid concurrent use if possible. If combination is necessary, use the lowest effective dose of diclofenac for the shortest duration, consider gastroprotective therapy (e.g., PPI), and administer aspirin at least 30 minutes before diclofenac to minimise interference. Monitor for signs of GI bleeding and cardiovascular events.
  • ibuprofenmoderateAvoid concomitant use where possible. If an NSAID is required, consider alternatives such as paracetamol or a non-NSAID analgesic. If combined use is necessary, administer aspirin at least 30 minutes before ibuprofen or at least 8 hours after the last ibuprofen dose, to allow aspirin to irreversibly inhibit platelet COX-1 first. Monitor for signs of gastrointestinal bleeding and renal impairment. The evidence for the specific timing recommendation is moderate, and local formulary or specialist guidance should be consulted.
  • warfarinmajorAvoid combined use where possible, especially in patients with a high baseline bleeding risk or those requiring prolonged therapy. If the combination is necessary, use the lowest effective aspirin dose, consider gastroprotection with a proton pump inhibitor, and monitor INR more frequently. Educate the patient on signs of bleeding and advise prompt medical review if they occur.
  • methotrexatemajorAvoid concurrent use if possible. If combination is necessary, use a lower methotrexate dose, monitor plasma methotrexate concentrations, renal function, and full blood count closely, and adjust therapy accordingly.
  • rivaroxabanmajorUse with caution and only if the expected benefit outweighs the bleeding risk. If combined, prescribe the lowest effective doses, add gastroprotection with a proton pump inhibitor if appropriate, and monitor for signs of bleeding (e.g., haemoglobin, stool colour, mental status). Consider alternative antithrombotic strategies or shorter duration of combination therapy where possible.
  • dabigatranmajorPrescribers should carefully assess the indication for combined therapy, weighing the reduction in thrombotic events against the increased bleeding risk. When the combination is considered necessary, use the lowest effective doses and monitor for signs of bleeding (e.g., haemoglobin, symptoms). Consider gastroprotection with a proton pump inhibitor, particularly in high-risk patients. Avoid concurrent use of additional antithrombotics unless there is a clear indication. Re-evaluate the need for continued combination therapy regularly.
  • apixabanmajorAvoid co-prescription unless there is a clear indication for both. If used together, use the lowest effective doses, consider gastroprotection with a proton pump inhibitor, and monitor for signs of bleeding. Review the need for antiplatelet therapy regularly.
  • enoxaparinmajorUse with caution and only if clearly indicated. Consider alternatives such as clopidogrel or other antiplatelets if antiplatelet therapy is necessary, or consider non-steroidal anti-inflammatory drugs with lower gastrointestinal bleeding potential, but note that all NSAIDs can increase bleeding risk. When co-prescribed, monitor for signs of bleeding, including occult blood loss, and check haemoglobin, haematocrit, and platelet counts periodically. Consider gastroprotective agents such as proton pump inhibitors to reduce gastrointestinal bleeding risk. Adjust doses and durations of both agents to the minimum necessary. If possible, separate administration times, though this does not eliminate the interaction. Do not use in patients with active bleeding, recent major surgery, or high bleeding risk unless benefit outweighs risk.
  • valproic acidmajorAvoid co-prescription if possible. If combined therapy is necessary, monitor the patient closely for signs of valproate toxicity and consider monitoring free valproic acid concentration (if available) in addition to total levels. Reduce the valproic acid dose as clinically indicated. Educate the patient about symptoms of toxicity and bleeding. Alternative analgesics or antiplatelet agents should be considered.
  • edoxabanmajorAvoid concomitant use unless the benefit of antiplatelet therapy outweighs the bleeding risk (e.g., acute coronary syndrome or coronary stenting). If combined treatment is necessary, use the lowest effective aspirin dose, limit the duration of dual therapy, and monitor closely for signs of bleeding (e.g., haemoglobin, stools). Review the need for aspirin regularly and consider gastroprotection with a proton pump inhibitor.
Renal & hepatic adjustment
  • eGFR 60+: No dose adjustment required.
  • eGFR 30–59: Use with caution. No dose adjustment generally required for antiplatelet doses. Monitor renal function.
  • eGFR 15–29: Avoid for analgesic and antipyretic doses. For antiplatelet use, use with caution and monitor closely; consider alternative antiplatelet if bleeding risk high.
  • eGFR 0–14: Contraindicated in severe renal impairment. (avoid)
  • Hepatic impairment (mild): No dose adjustment required. Use with caution, especially in patients with bleeding disorders or on anticoagulants.
  • Hepatic impairment (moderate): Use with caution. Consider reducing dose or frequency for high-dose therapy; monitor liver function and signs of bleeding.
  • Hepatic impairment (severe): Avoid in severe hepatic impairment, particularly if decompensated or with significant bleeding risk. (avoid)
Monitoring, pregnancy & toxicity
  • Signs of bleeding (e.g., GI bleed, bruising, epistaxis) · Periodically during long-term therapy
  • Serum creatinine and eGFR · At baseline and at least annually if renal impairment
  • Liver function tests (ALT, AST) · At baseline and periodically during prolonged use
  • Salicylate levels · In high-dose therapy or if toxicity suspected
  • Platelet count · If high-dose or concurrent with anticoagulants
  • Pregnancy: Low-dose aspirin (≤150 mg/day) may be used for specific indications such as prevention of pre-eclampsia under specialist supervision. High-dose aspirin is avoided, particularly in the third trimester, due to risk of premature closure of the ductus arteriosus, bleeding, and other fetal adverse effects.
  • Toxicity: Salicylism (tinnitus, headache, dizziness) · Reduce dose or discontinue. Provide supportive care.
  • Toxicity: Respiratory alkalosis and metabolic acidosis · Supportive care, IV fluids, sodium bicarbonate for severe acidosis.
  • Toxicity: Hypoglycemia · Monitor blood glucose; administer IV glucose if needed.
  • Toxicity: Severe poisoning (seizures, coma, hyperthermia) · Intensive care, activated charcoal if recent, hemodialysis in severe cases.
Antidotes & reversal
  • Sodium bicarbonate (urinary alkalinisation) · Moderate-severe salicylate toxicity: 1.5 L D5W with 100 mmol bicarbonate at 150-200 mL/h, target pH 7.5-8 and urine 6-8 mL/kg/h; haemodialysis if indicated.
Clinical notes
Aspirin is used for relief of mild to moderate pain, fever, and inflammation, and as an antiplatelet agent in cardiovascular diseases such as acute myocardial infarction and stroke prevention. It is contraindicated in children and adolescents with viral infections due to the risk of Reye's syndrome, in those with active peptic ulcer, bleeding disorders, or aspirin-sensitive asthma. Adverse effects include dyspepsia, gastrointestinal bleeding, tinnitus at high doses, and bronchospasm. It potentiates the effects of anticoagulants and should be used with caution in renal impairment. Stop therapy 7 days before elective surgery unless indicated.

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BNF; local formulary