Results from three new studies add support to earlier findings that aspirin reduces cancer risk and, in some cases, metastasis.

The studies, two of which appear in The Lancet and one in The Lancet Oncology, were all conducted by groups led by Peter M. Rothwell, FMedSci, of the University of Oxford and John Radcliffe Hospital in Oxford, United Kingdom. (All three studies were published online on March 21, 2012.) Previous work by Rothwell and colleagues established that daily aspirin reduces the long-term risk of death from cancer, but the short-term effects are less certain, particularly in women, and it is also unclear how the risk/benefit of aspirin changes over time.

One of the Lancet studies showed that in 34 trials of low-dose aspirin for primary prevention of vascular events, involving 69,224 participants, persons allocated to aspirin had a risk of cancer death 15% lower than that of controls (562 deaths vs 664 deaths). This effect was particularly evident for those who had been on aspirin therapy for at least 5 years; members of this group were 27% less likely than controls to die from cancer. In six trials of daily low-dose aspirin for primary prevention, involving 35,535 participants, aspirin use for 3 years or more reduced cancer incidence by 24% in women and by 23% in men, compared with controls.

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In the other Lancet study, the authors collected new data on metastases of cancers that had been diagnosed during all five large randomized trials of daily aspirin (75 mg or more per day) vs control for the prevention of vascular events in the United Kingdom. During the mean follow-up of 6.5 years, new solid cancers were diagnosed in 987 of 17,285 trial participants. Allocation to aspirin reduced risk of cancer with distant metastasis by 36%, risk of adenocarcinoma (common solid cancers including colon, lung, and prostate cancers) by 46%, and such other solid cancers as bladder and kidney cancers by 18%. These decreases were due mainly to a reduction in proportion of adenocarcinomas that had metastatic rather than local disease.

In addition, aspirin use reduced the risk of adenocarcinoma with metastasis at initial diagnosis by 31% and, on follow-up, reduced the risk of metastasis in patients without initial metastasis by 55%—particularly in persons with colorectal cancer (by 74%) and in persons who remained on trial treatment up to or after diagnosis (by 69%).

Aspirin use reduced death due to cancer in patients who developed adenocarcimona, particularly in those without metastasis at diagnosis, by 50%. Consequently, aspirin use reduced the overall risk of fatal adenocarcinoma in the trial populations by 35%, but did not reduce the risk of other fatal cancers.

The Lancet Oncology paper described a systematic review of studies published from 1950 to 2011 that reported associations between aspirin use and risk or outcome of cancer. Observational studies showed a 38% reduced risk of colorectal cancer, which corresponded with the effect of daily aspirin use on 20-year risk of death from colorectal cancer from the randomized trials (42%). Similarly consistent reductions were seen in risks for esophageal, gastric, biliary, and breast cancers.