What is the recommended dosage of aspirin (acetylsalicylic acid) for acute myocardial infarction (AMI)?

Aspirin Dosage for Acute Myocardial Infarction

For acute MI, administer 162 to 325 mg of non-enteric-coated aspirin immediately (chewed or crushed), followed by 81 mg daily indefinitely. 1, 2

Initial Loading Dose

The loading dose should be 162 to 325 mg of non-enteric-coated aspirin, administered as soon as possible upon presentation. 1, 2

• The 162 mg dose has Level of Evidence A support, while the 325 mg dose has Level of Evidence C support from ACC/AHA guidelines 1, 2
• The aspirin must be chewed or crushed for rapid buccal absorption—enteric-coated formulations delay absorption and should not be used for the initial dose 1, 2
• This applies to both STEMI and NSTEMI presentations 2
• Administration should occur within 24 hours of hospital arrival, though ideally immediately upon diagnosis or even in the prehospital setting 1, 2

Evidence Supporting the 162 mg Dose

Recent comparative data suggests 162 mg may be preferable to 325 mg. A large analysis of 48,422 STEMI patients treated with fibrinolytics found no difference in 24-hour mortality (2.8% vs 2.9%), 7-day mortality (4.9% vs 5.2%), or 30-day mortality (6.5% vs 7.1%) between 162 mg and 325 mg doses 3. However, the 325 mg dose was associated with significantly higher moderate/severe bleeding (9.3% vs 12.2%, adjusted OR 1.14, p=0.003) 3.

Maintenance Dosing

After the loading dose, continue with 81 mg daily indefinitely. 1, 2, 4

• The 81 mg daily maintenance dose is preferred over higher doses to minimize bleeding risk while maintaining efficacy 2, 4
• This dose should be continued indefinitely for secondary prevention, reducing serious vascular events by 1.5% per year 2, 4
• Aspirin produces a 23% relative risk reduction in 5-week vascular mortality when administered within 24 hours of STEMI 2, 4

Mechanism and Timing

• Aspirin in doses of 162 mg or more produces rapid and near-total inhibition of thromboxane A2 production, creating an immediate clinical antithrombotic effect 1
• The drug reduces coronary reocclusion and recurrent ischemic events after fibrinolytic therapy 1
• In the ISIS-2 trial, aspirin alone reduced death from AMI, and its effect was additive to that of streptokinase 1

Critical Implementation Points

Do not delay aspirin administration. The only absolute contraindications are:

• Known aspirin allergy 1, 2
• Active gastrointestinal hemorrhage 1, 2

Common pitfalls to avoid:

• Using enteric-coated formulations for the loading dose—these delay absorption and should never be used initially 1, 2
• Withholding aspirin while awaiting definitive diagnosis—aspirin should be given immediately upon suspicion of MI 2
• Using maintenance doses higher than 81 mg long-term—this increases bleeding risk without additional efficacy 2, 4

Special Considerations

For patients who cannot take oral aspirin due to severe nausea or vomiting, aspirin suppositories (300 mg) are safe and effective 1.

If aspirin is contraindicated due to true allergy or intolerance, clopidogrel 75 mg daily is a safe and effective alternative, showing slight superiority in the CAPRIE trial (5.32% vs 5.83% annual vascular event rate) 4.