Aspirin (acetylsalicylic acid) is used in both low-dose (81 mg) and full-dose (325 mg) forms for different therapeutic purposes, primarily due to its antiplatelet, anti-inflammatory, and analgesic properties. Here's a breakdown of the key indications for each dose:

Aspirin 81 mg (Low-Dose Aspirin)

Mainly used for its antiplatelet (blood-thinning) effects to reduce the risk of cardiovascular events.

✅ Indications:

1. Secondary prevention of:
   • Myocardial infarction (MI)
   • Ischemic stroke or TIA
   • After coronary artery stent placement
   • Post coronary artery bypass graft (CABG)
   • Peripheral arterial disease (PAD)
2. Primary prevention in select high-risk individuals (based on age, cardiovascular risk factors, and bleeding risk – per recent guidelines):
   • Typically men and women aged 40–70 with increased cardiovascular risk and low bleeding risk
3. Atrial fibrillation:
   • In low-risk patients or those unable to take anticoagulants (less common now due to preference for DOACs)

Aspirin 325 mg (Full-Dose Aspirin)

Used for anti-inflammatory, analgesic, and antipyretic effects, as well as stronger antiplatelet activity when urgently needed.

✅ Indications:

1. Acute coronary syndrome (ACS):
   • Initial management of suspected MI: 325 mg chewed immediately
   • STEMI/NSTEMI management
2. Stroke prevention/treatment:
   • Acute ischemic stroke: After ruling out hemorrhage (usually initial dose 160–325 mg)
3. Post-vascular procedures or events:
   • Higher-dose sometimes used short-term post-stenting or after carotid endarterectomy
4. Pain, inflammation, and fever (historically):
   • Arthritis, rheumatic diseases (now less common due to GI risk and availability of other NSAIDs)
5. Kawasaki disease (pediatric use):
   • Initial high dose for anti-inflammatory effects, followed by low-dose antiplatelet therapy
6. Essential thrombocythemia or polycythemia vera:
   • As an antiplatelet agent to prevent thrombotic complications

⚠️ Key Considerations:

• GI bleeding, peptic ulcers, bleeding disorders, or concurrent use of other anticoagulants are major contraindications.
• Enteric-coated formulations may reduce gastric irritation but do not significantly reduce bleeding risk.