Taking 300 mg of aspirin daily for at least 5 years was shown to prevent colorectal cancer in an analysis of two large randomized trials. The effect was seen beginning 10 years after treatment was initiated.
Although this strategy might be effective in certain high-risk groups, further research is needed to elucidate the risks and benefits of aspirin chemoprevention in various clinical settings, the researchers wrote. The effectiveness of colonoscopy screening and the risk of bleeding complications with long-term aspirin use also should be considered, they noted.
Dr. Andrew Chan of the gastrointestinal unit at Massachusetts General Hospital, Boston, concurred in an accompanying commentary. “These findings are not sufficient to warrant a recommendation for the general population to use aspirin for cancer prevention,” he wrote (Lancet 2007;369:1577–8).
Previous observational studies had reported a decreased incidence of colorectal cancers in regular users of aspirin, but two large trials did not demonstrate a decreased risk over 10 years of follow-up. Longer follow-up is needed, given that the delay is 10–15 years between initiation of development of an adenoma and colorectal cancer, wrote Dr. Peter Rothwell, professor of clinical neurology at the University of Oxford (England), and associates (Lancet 2007;369:1603–13).
Their analysis focused on the British Doctors Aspirin Trial and the UK Transient Ischaemic Attack Aspirin Trial; there was a median follow-up of 23 years in both trials.
During that follow-up period, subjects who took at least 300 mg of aspirin a day for at least 5 years were significantly less likely to develop colorectal cancer than were controls (hazard ratio 0.63), according to a pooled analysis of the two trials. The researchers found no significant effect on any other type of cancer.
The chemopreventive effect was strongest in years 10–19, when the hazard ratio for aspirin users was 0.60, but a significantly reduced hazard ratio of 0.74 was seen in years 20 and later for the subjects who took aspirin. No significant chemopreventive effect was seen at 0–9 years (hazard ration 0.92).
The British Doctors Aspirin Trial randomized doctors in 1978 and 1979 into a group of 3,429 taking a daily dose of 500 mg of aspirin and a control group of 1,710 who took nothing. Treatment continued for 5–6 years.
The UK Transient Ischaemic Attack Aspirin Trial randomized 2,449 patients over age 40 who had already experienced a transient ischemic attack or mild ischemic stroke to receive daily doses of either 1,200 mg of aspirin, 300 mg of aspirin, or placebo.
Recruitment took place between 1979 and 1985, with the trial ending in 1986. The researchers performed a subgroup analysis of only those patients who took aspirin for at least 5 years.
The researchers reported that they identified trial participants who had developed cancer through cancer registries and death certificates.
The findings are not sufficient enough to warrant a recommendation.