In a study where they followed patients for several years they found drastic reduced risk for several cancers indicating that maybe an aspirin a day could be used as a prevention drug. Most likely the aspirin is active at early stage of cancer as the best improvement is only found after 5 years of low daily aspirin intake.
"In The Lancet, Peter Rothwell and colleagues1 report additional data supporting the hope that daily aspirin can ward off a panoply of cancers. Aspirin's role in reducing risk of vascular events is well established, although which risk factor profiles derive net benefit remain controversial.2 The strongest proof of aspirin's antineoplastic effect has been in colorectal tumours. Several epidemiological studies,3 randomised controlled trials of colon polyp recurrence,4 and randomised trials in patients with hereditary colorectal cancer syndromes [5] and [6] have shown that aspirin reduces incidence of colorectal neoplasia. In long-term follow-up of participants in five randomised trials of cardiovascular prevention, Rothwell and colleagues previously reported that daily aspirin at any dose reduced risk of colorectal cancer by 24% and of associated mortality by 35% after a delay of 8–10 years.7"
