One major limiting factor is that most data come from CVD prevention trials, and only a limited number of trials have focused specifically on cancer prevention. For the USPSTF, these data showed no statistically significant risk reduction in overall cancer mortality (RR = 0.96; 95% CI, 0.87-1.06) or in total cancer incidence (RR = 0.98; 95% CI, 0.93-1.04).4 Other ongoing trials may yield more definitive data.14
The particular interest in CRC was due to it being the first cancer found to be preventable with aspirin therapy. The USPSTF, while acknowledging the homogeneous nature of supporting studies, noted that their significant number and resulting evidence made CRC the only cancer warranting evaluation. Population studies have now shown more benefit than the few randomized control trials. The Women’s Health Study and the Physicians’ Health Study were both limited by their duration. But such studies conducted over a longer period revealed notable benefits in the second decade of use, with a statistically significant lower CRC incidence (RR = 0.60; 95% CI, 0.47-0.76). Additionally, CRC mortality at 20 years was decreased in patients taking aspirin regularly (RR = 0.67; 95% CI, 0.52-0.86).4 Multiple studies are in progress to better establish aspirin’s CRC benefit.
While not directly applicable to the general population, use of aspirin for patients with Lynch syndrome to prevent CRC has strong supporting evidence.15 Beyond CRC, there is nascent evidence from limited observational studies that aspirin may have a preventive effect on melanoma and ovarian and pancreatic cancers.16-18 Further studies or compilations of data would be needed to draw more significant conclusions on other types of cancers. Larger studies would prove more difficult to do, given the smaller incidences of these cancers.
Interestingly, a recent study showed that for individuals 70 years and older, aspirin might increase the risk for all-cause mortality, primarily due to increased cancer mortality across all types.19 Although this result was unexpected, caution should be used when prescribing aspirin particularly for patients 70 or older with active cancer.
A look at the harms associated with aspirin use
Aspirin has long been known to cause clinically significant bleeding. Aspirin inhibits platelet-derived cyclooxygenase-1 (COX-1), a potent vasoconstrictor, and thereby decreases platelet aggregation, reducing thromboembolic potential and prolonging bleeding time. These effects can confer health benefits but also carry the potential for risks. A decision to initiate aspirin therapy for primary prevention relies on an understanding of the benefit-to-harm balance.
Continue to: Initial aspirin studies...