Is Aspirin (ASA) 81mg ordered once or twice a day?

Aspirin (ASA) 81mg Dosing Frequency

Aspirin 81mg is administered once daily, not twice daily, for cardiovascular prevention based on current guidelines. 1

Dosing Rationale

The American College of Cardiology Foundation/American Heart Association (ACCF/AHA) guidelines clearly recommend a daily aspirin dose of 81mg as an accepted maintenance regimen for cardiovascular protection, particularly after percutaneous coronary intervention (PCI). This recommendation is based on:

• Consistent platelet inhibition achieved with once-daily dosing
• Reduced bleeding risk compared to higher doses or more frequent administration
• Equivalent efficacy to higher maintenance doses for cardiovascular protection

Evidence for Once-Daily Dosing

The ACCF/AHA guidelines specifically state that "a daily aspirin dose of 81 mg per day is an accepted regimen in preference to higher maintenance doses" 1. This recommendation is based on extensive clinical trial data showing that:

• Once-daily low-dose aspirin (75-100mg) provides sufficient platelet inhibition for 24 hours
• The antiplatelet effect of aspirin is primarily due to irreversible inhibition of cyclooxygenase-1 (COX-1), which lasts for the lifetime of the platelet (8-10 days)

Specific Clinical Applications

For Patients with Coronary Artery Disease:

• For patients on ticagrelor: "The recommended maintenance dose of ASA to be used with ticagrelor is 81 mg daily" 1
• After stenting: 81mg daily is the preferred maintenance dose after the initial higher-dose period 1

For Primary Prevention:

• When indicated, the standard dose is 81mg once daily

Potential Pitfalls and Considerations

1. Morning vs. Evening Dosing: Some research suggests that evening dosing may provide better platelet inhibition during the critical early morning hours when cardiovascular events are more common 2. However, this timing consideration has not been incorporated into formal guidelines.

2. Body Weight Considerations: While some studies have examined the relationship between BMI and aspirin efficacy 3, current guidelines do not recommend dose adjustment based on body weight for the 81mg dose.

3. Alternate-Day Dosing: Research has shown that alternate-day dosing (100mg every other day) provides significantly less platelet inhibition than daily 81mg dosing 4, 5. The Women's Health Study used alternate-day dosing, but this is not the standard of care.

Conclusion

For patients requiring low-dose aspirin therapy, the standard regimen is 81mg once daily. Twice-daily dosing is not recommended by current guidelines and may unnecessarily increase bleeding risk without providing additional cardiovascular benefit.