Is aspirin effective for preventing blood clots?

Aspirin for Prevention of Blood Clots

Aspirin is effective for preventing blood clots, but anticoagulants are substantially superior and should be used instead whenever possible. 1

Primary Role: Secondary Prevention Only

Aspirin should NOT be used as first-line therapy for venous thromboembolism (VTE) prevention or treatment. 2 The evidence clearly demonstrates that:

• Reduced-dose direct oral anticoagulants (DOACs) prevent 46 additional recurrent VTE events per 1,000 patients compared to aspirin, with only 4 additional bleeding events per 1,000 patients 1
• Rivaroxaban reduces recurrent VTE by 39 events per 1,000 patients over 2-4 years compared to aspirin, with minimal bleeding risk increase (4 events per 1,000) 1
• Aspirin increases the risk of recurrent pulmonary embolism 3-fold (RR 3.10) and deep vein thrombosis 3-fold (RR 3.15) compared to continued anticoagulation 2

When Aspirin May Be Considered for VTE Prevention

Limited Acceptable Scenarios:

Aspirin (75-100 mg daily) should only be used when patients refuse or cannot tolerate anticoagulation after completing initial VTE treatment. 1, 2 This represents a weak recommendation based on low-certainty evidence. 1

Specifically:

• For patients with unprovoked proximal DVT or PE who are stopping anticoagulant therapy and have no contraindication to aspirin, use aspirin over no treatment 1
• Aspirin reduces recurrent VTE by approximately 53 events per 1,000 patients over 2-4 years compared to placebo, with only 3 additional major bleeding events per 1,000 1, 2
• This benefit is substantially inferior to anticoagulation—aspirin provides only 30-35% reduction in VTE recurrence compared to placebo 2

Critical Pitfall to Avoid:

Never substitute aspirin for anticoagulants in the primary treatment of acute VTE, as this leads to treatment failure and recurrent thromboembolism. 2 Aspirin is NOT recommended for acute PE management. 2

Aspirin for Arterial Thrombosis Prevention

Secondary Prevention (Established Cardiovascular Disease):

Aspirin 75-100 mg daily is strongly recommended for all patients with previous myocardial infarction or stroke. 1, 3 The evidence demonstrates:

• 34% reduction in myocardial infarction or sudden death in patients with chronic stable angina 1
• 50% reduction in myocardial infarction and death in patients with unstable angina 1
• 23% reduction in vascular death and 50% reduction in non-fatal vascular events after acute MI 1

Primary Prevention (No Prior Cardiovascular Events):

Aspirin for primary prevention has a marginal benefit-to-risk ratio and should only be considered in highly select patients after comprehensive shared decision-making. 1, 3

The 2025 American College of Cardiology/American Heart Association guidelines downgraded aspirin to Class IIb (may be considered) because:

• The number of cardiovascular events prevented equals the number of bleeding episodes induced in patients with ASCVD risk >1% per year 3
• Major bleeding increases from 3.2% to 4.1% (29% relative increase) even in carefully selected diabetic patients 1, 3

Absolute Contraindications for Primary Prevention:

Do not use aspirin if any of the following are present: 3

• Age >70 years (harm exceeds benefit) 3
• History of gastrointestinal bleeding or peptic ulcer disease 3
• Concurrent anticoagulation therapy 3
• Thrombocytopenia or coagulopathy 3
• Chronic kidney disease 3
• Uncontrolled hypertension 3
• Concurrent NSAID or steroid use 3

Special Population: Diabetes

For diabetic patients aged ≥50 years with additional cardiovascular risk factors and low bleeding risk, aspirin may be considered only after comprehensive discussion of risks versus benefits. 1, 3 The ASCEND trial showed a 12% reduction in serious vascular events but major bleeding increased significantly. 1

Optimal Dosing

Use 75-100 mg daily (81 mg in US) for long-term prevention—the lowest effective dose to minimize bleeding risk. 1, 3

For acute situations requiring immediate antiplatelet effect:

• Loading dose of 160-300 mg should be given at diagnosis to ensure rapid and complete platelet inhibition 1

Bleeding Risk Quantification

Major bleeding occurs in 2-4 per 1,000 middle-aged adults over 5 years (4-12 per 1,000 in older adults). 3 The relative risk of major gastrointestinal bleeding is 1.6 even with low doses. 3

Clinical Decision Algorithm for VTE Prevention

1. First, determine if patient has active VTE or history of VTE requiring extended therapy 1

   • If yes → Use reduced-dose DOAC (apixaban 2.5 mg twice daily or rivaroxaban 10 mg once daily) 1

2. If patient refuses or cannot tolerate anticoagulation after completing initial treatment 1

   • Screen for aspirin contraindications 3
   • If no contraindications → Use aspirin 75-100 mg daily 1
   • If contraindications present → No pharmacologic prophylaxis 1

3. For patients already on aspirin when diagnosed with VTE 2

   • Suspend aspirin during anticoagulation period 2
   • Continuing both increases major bleeding risk (RR 1.26) 2

4. Reassess annually 1

   • Evaluate continued need for therapy 1
   • Assess bleeding risk changes 1
   • Review patient preferences 1