What are the indications, recommended dosing, contraindications, and precautions for acetylsalicylic acid (aspirin)?

Acetylsalicylic Acid (Aspirin): Clinical Overview

Primary Indications

Aspirin is definitively indicated for secondary prevention in patients with established atherosclerotic cardiovascular disease (ASCVD), where benefits far outweigh bleeding risks 1, 2. For primary prevention, the evidence has shifted dramatically against routine use, with aspirin now recommended only in highly selected patients aged 40-59 years at elevated cardiovascular risk with low bleeding risk 1, 3.

Secondary Prevention (Strong Indication)

• Documented ASCVD (prior MI, stroke, or known coronary disease): 75-162 mg daily indefinitely 2
• Acute coronary syndrome/unstable angina: Initial dose 162-325 mg, then 81 mg daily maintenance 4, 5
• Post-PCI/stenting: 81 mg daily (higher initial doses no longer recommended) 5
• Heart failure with atherosclerotic disease: 81-325 mg daily 6
• Atrial fibrillation with vascular disease (if OAC contraindicated): 81 mg daily 7

Primary Prevention (Highly Restricted)

The 2023 ADA guidelines and 2022 USPSTF recommendations have substantially narrowed indications 1, 3:

May consider (shared decision-making required):

• Age 40-59 years with ≥10% 10-year ASCVD risk AND low bleeding risk 1, 3
• Diabetes with ≥1 additional major risk factor (family history of premature ASCVD, hypertension, dyslipidemia, smoking, chronic kidney disease) AND age ≥50 years 1

Do NOT use:

• Age ≥60 years for primary prevention (harms exceed benefits) 1, 3
• Age <40 years or low ASCVD risk (<10% 10-year risk) 1
• Age <21 years (Reye syndrome risk) 1, 8

---

Recommended Dosing

The optimal dose is 75-81 mg daily for most indications 1, 2, 9. Higher doses increase bleeding without improving efficacy 9.

Specific Dosing by Indication:

• Chronic secondary prevention: 75-81 mg daily 2, 9
• Acute coronary syndrome (initial): 162-325 mg loading dose, then 81 mg daily 4, 5
• Primary prevention (if indicated): 75-81 mg daily 1, 8
• Post-stroke/TIA: 50-81 mg daily effective 10

The ASCEND trial in diabetes used 100 mg daily with demonstrated efficacy, but U.S. practice favors 81 mg tablets 1. Doses >325 mg daily offer no additional cardiovascular benefit and substantially increase gastrointestinal bleeding risk 9.

---

Contraindications

Absolute Contraindications:

• Active bleeding (any site) 4, 11
• Hemophilia or bleeding disorders 4, 11
• Aspirin allergy/hypersensitivity (asthma with nasal polyps) 4, 12
• Age <21 years (Reye syndrome risk) 1, 8
• Severe uncontrolled hypertension 4, 11

Relative Contraindications:

• Active peptic ulcer disease 4, 11
• History of gastrointestinal bleeding (consider only with PPI co-therapy) 13
• Thrombocytopenia or platelet dysfunction 12
• Concurrent anticoagulation (significantly increases bleeding risk 3-6 fold) 12
• Renal insufficiency (increases bleeding risk) 12

---

Critical Precautions and Monitoring

Bleeding Risk Assessment

The ASCEND trial demonstrated that aspirin increases major bleeding from 3.2% to 4.1% over 7.4 years, primarily gastrointestinal 1. This 29% relative increase in bleeding must be weighed against cardiovascular benefits in every patient 1.

High bleeding risk factors:

• Age >70 years 1, 8
• History of GI bleeding or peptic ulcer 13
• Concurrent anticoagulation or antiplatelet therapy 12
• Chronic kidney disease 12
• Anemia 8
• Uncontrolled hypertension 4

Drug Interactions

• Ibuprofen blocks aspirin's antiplatelet effect: Take ibuprofen ≥30 minutes after immediate-release aspirin or ≥8 hours before 4
• Warfarin/anticoagulants: Increases INR by up to 15% and bleeding risk 3-6 fold; requires close INR monitoring and GI prophylaxis 12
• Other NSAIDs: Additive bleeding risk; avoid combination 6, 12
• ACE inhibitors: Theoretical interaction reducing vasodilatory effects, but clinically insignificant—continue both 4

Gastroprotection Strategy

For patients requiring aspirin with high GI bleeding risk 13:

• Add PPI therapy (reduces endoscopic ulcers by 90%) 13
• Test and treat H. pylori if present (especially with prior ulcer history) 13
• Use lowest effective dose (75-81 mg) 1, 9
• H2-receptor antagonists are inadequate for protection 13

Perioperative Management

• High-risk cardiac patients (recent MI, recent stent): Continue aspirin perioperatively 12
• Other patients: Stop 7-10 days before surgery 12
• Other NSAIDs: Stop for 5 half-lives (ibuprofen 2 days, naproxen 2-3 days) 12

---

Common Clinical Pitfalls

1. Overuse in primary prevention: 27.6% of patients in the PINNACLE registry were receiving aspirin inappropriately, particularly those >70 years 14. Aspirin should be discontinued in most patients ≥60 years without established ASCVD 3.

2. Abrupt discontinuation in secondary prevention: Withdrawal increases risk of acute coronary events and stent thrombosis 4. Emphasize indefinite continuation unless contraindication develops.

3. Inadequate dose in acute settings: Use 162-325 mg loading dose for ACS, not maintenance 81 mg 4, 5.

4. Combining aspirin with anticoagulation without gastroprotection: This combination dramatically increases bleeding risk and requires PPI co-therapy 12, 13.

5. Using aspirin in diabetes without risk stratification: The 2023 ADA guidelines emphasize that primary prevention in diabetes requires careful consideration—not automatic prescription 1. Benefits are modest (12% relative risk reduction) and offset by bleeding in many patients.

6. Prescribing enteric-coated formulations for acute use: Non-enteric-coated aspirin provides faster buccal absorption in acute settings 5.

---

Pregnancy and Breastfeeding

Avoid aspirin during the last 3 months of pregnancy unless specifically directed by a physician, as it may cause problems in the unborn child or complications during delivery 15. Consult a healthcare professional before use if pregnant or breastfeeding 15.