The authors of this research looked at the evidence for the impact of aspirin on both cancer incidence and death and cardiovascular events. The evidence relating to cancer was collated from the most recent systematic reviews and some individual studies reporting on specific sites or long-term aspirin use, and the evidence for CV events was based on the ATT Collaboration meta-analysis. The overall benefits and harms for taking aspirin for 10 years starting from the age of 50, 55, 60 and 65 years were then considered separately for men and women.
The authors estimate that there is an overall net relative benefit of 4-6% in both women and men, with the number needed to treat (NNT) for ten years ranging from 33 to 127 to prevent one major event (taking into account prevention of cancer, MI or stroke, but major bleeding events caused by treatment). Reductions in cancer account for the largest proportion of the overall benefit. Of note, other side-effects (aside from major bleeding) were not considered in this analysis. For deaths, the estimated NNTs to save one life over 20 years range from 46-213 (almost entirely due to a reduction in deaths from cancer). The authors comment that the benefits of aspirin are at least equivalent in magnitude to those from statins, and are complimentary to statins as they mostly relate to cancer.
The reductions in cancer incidence and mortality were mainly for gastrointestinal cancers. The cancer prevention benefits are associated with longer-term treatment, and the authors suggest that treatment should be continued for a minimum of 5 years (larger benefits for 10 years of use), starting between the age of 50 and 65 years. The go on to discuss some of the uncertainties, including whether there is an upper age at which the harms outweigh the benefits, the optimum dose (currently no evidence of a greater reduction with increasing dose) and duration, and the extent of any carry-over effect after stopping aspirin.
NHS Choices has published an assessment of this study, as it has been featured widely in the press (available via the link below). This notes that it is unclear whether this was a systematic review, and that the researchers did not carry out a meta-analysis, but compiled their own estimates. Studies of varying design and quality were included. The authors of the assessment conclude that “While the findings on aspirin and cancer show promise, it is not clear that the results are reliable from the methods reportedly used to compile this review.”
A spokesperson for Cancer Research UK has been quoted by the BMJ as saying: “Before aspirin can be recommended for cancer prevention some important questions need to be answered, including what is the best dose and how long people should take it for. And tests need to be developed to predict who is likely to have side effects. Given the continued uncertainty over who should take aspirin, Cancer Research UK is funding a number of trials and research projects to make the picture clearer.”