What are the indications for starting aspirin (acetylsalicylic acid) therapy?

Aspirin Indications for Initiation

Aspirin should be started in patients with established atherosclerotic cardiovascular disease (secondary prevention) at 75-162 mg daily, but is generally not recommended for primary prevention in most patients, particularly those over 70 years old, due to bleeding risks that outweigh benefits. 1

Secondary Prevention (Established Cardiovascular Disease) - ALWAYS INDICATED

Aspirin 75-162 mg daily is strongly recommended for patients with: 1

• Coronary artery disease (stable angina, prior myocardial infarction, post-PCI) 1
• Prior ischemic stroke or TIA - use 75-325 mg daily 1
• Symptomatic peripheral artery disease - use 75-325 mg daily 1
• Post-carotid endarterectomy - reduces stroke risk 1
• Acute coronary syndrome - loading dose 162-325 mg, then 75-162 mg daily 1
• Acute myocardial infarction - start within 24 hours at 162.5-325 mg 1
• Acute ischemic stroke - start 160-325 mg within 48 hours 1

For these secondary prevention indications, the benefits clearly outweigh bleeding risks, with aspirin reducing vascular events by 20-28%. 1

Primary Prevention - GENERALLY NOT RECOMMENDED

Do NOT Start Aspirin in:

• Adults over 70 years without established cardiovascular disease - bleeding risk exceeds benefit 1
• Low-risk individuals under 50 years - insufficient benefit to justify bleeding risk 1
• Asymptomatic patients without diabetes or high cardiovascular risk 1

Consider Aspirin (Shared Decision-Making Required) in:

Diabetes with additional high-risk features (but NOT routine): 1

• Age >40 years with additional cardiovascular risk factors
• 10-year cardiovascular risk >10%
• Dose: 75-162 mg daily
• Critical caveat: The ASCEND trial showed only 12% reduction in vascular events but 29% increase in major bleeding 1

High cardiovascular risk (10-year risk >10%) in patients under 70 years: 1

• Must have low bleeding risk
• Patient preference essential
• Benefits are marginal compared to bleeding risks

Acute Treatment Indications

Acute coronary syndrome/suspected MI: 1

• Loading dose: 162-325 mg immediately (non-enteric coated for faster absorption)
• Can be started in prehospital setting
• Continue 75-162 mg daily indefinitely

Acute ischemic stroke: 1

• Start 160-300 mg within 48 hours of symptom onset
• Reduces death/recurrent stroke by 9 per 1000 patients treated

Special Populations

Post-coronary artery bypass grafting: 1

• Start within 6 hours post-surgery
• Dose: 100-325 mg daily for 1 year minimum
• Reduces saphenous vein graft closure

Polycythemia vera: 1

• 100 mg daily prevents thrombotic complications
• Effective even with elevated platelet counts

Pregnancy (preeclampsia prevention): 2

• Low-dose aspirin for women at high risk
• Not a cardiovascular indication but worth noting

Contraindications to Consider

• Active gastrointestinal bleeding 1
• Hemophilia or other bleeding disorders 1
• Age <21 years (Reye syndrome risk) 1
• Aspirin allergy (consider clopidogrel 75 mg daily as alternative) 1

Optimal Dosing

Maintenance therapy: 75-162 mg daily is preferred over higher doses 1

• No additional benefit with doses >162 mg
• Bleeding risk increases dose-dependently (2.0% with <100 mg vs 4.0% with >200 mg) 1
• Most common U.S. formulation: 81 mg daily

Loading doses for acute events: 162-325 mg 1

Key Clinical Pitfalls

Avoid enteric-coated formulations for acute treatment - slower absorption and potentially inadequate platelet inhibition, especially in heavier patients 1

Do not combine aspirin with warfarin unless absolutely necessary (mechanical valve, atrial fibrillation with ACS) - significantly increases bleeding risk 1

Primary prevention in diabetes is no longer routine - the ASCEND trial changed practice by showing comparable bleeding to cardiovascular benefit 1