Clinical Inquiries

What are the benefits and risks of daily low-dose aspirin for primary prevention of CV events?

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One nonfatal myocardial infarction (MI) will be avoided for every 126 to 138 adults who take daily aspirin for 10 years (strength of recommendation [SOR]: A, systematic reviews and meta-analyses of multiple randomized controlled trials [RCTs]).

Taking low-dose aspirin for primary prevention shows no clear mortality benefit. A benefit for primary prevention of stroke is less certain. Although no evidence establishes increased risk of hemorrhagic stroke from daily low-dose aspirin, one gastrointestinal hemorrhage will occur for every 72 to 357 adults who take aspirin for longer than 10 years (SOR: A, systematic reviews and meta-analyses of multiple RCTs and cohort studies).




A 2013 systematic review of RCTs, systematic reviews, and meta-analyses examined the prophylactic use of low-dose aspirin for the primary prevention of cardiovascular disease (CVD) among adults 18 years and older.1 Twenty-seven papers met inclusion criteria; the total number of patients wasn’t reported.

A composite finding of nonfatal MI, nonfatal stroke, and CVD death indicated a number needed to treat (NNT) of 138 over 10 years of therapy (relative risk [RR]=0.90; 95% confidence interval [CI], 0.85-0.96). CVD death wasn’t disaggregated from this composite, but an analysis of all-cause mortality didn’t reach statistical significance (RR=0.94; 95% CI, 0.88-1.00). RR for nonfatal stroke alone also wasn’t disaggregated.

Risk of gastrointestinal (GI) bleeding was found to be a number needed to harm (NNH) of 108 over 10 years (RR=1.37; 95% CI, 1.15-1.62) whereas risk of hemorrhagic stroke didn’t reach statistical significance (RR=1.32; 95% CI, 1.00-1.74). This population-level review didn’t report disaggregated findings by age or baseline atherosclerotic cardiovascular disease (ASCVD) risk.

Another review finds benefit only for prevention of nonfatal MI

A 2016 systematic review included 2 good-quality and 9 fair-quality RCTs evaluating the benefits of low-dose aspirin compared with placebo or no treatment for primary prevention of CVD events in 118,445 patients ages 40 years and older.2 The review found benefit only for nonfatal MI, with an NNT of 126 over 10 years (RR=0.78; 95% CI, 0.71-0.87). There was no change in RR for nonfatal stroke (RR=0.95; 95% CI, 0.85-1.06); negligible impact on all-cause mortality (RR=0.95; 95% CI, 0.89-0.99); and no statistically significant benefit for CVD-specific mortality (RR=0.94; 95% CI, 0.86-1.03).

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