In an analysis of eight separate studies, people who took a low daily dose of aspirin were much less likely to die of cancer. In the three studies with the longest follow-up (20 years), people who took low-dose aspirin for five years were 20% less likely to die of any type of cancer than people who took no aspirin.

'These results do not mean that all adults should immediately start taking aspirin. But they do demonstrate major new benefits that have not previously been factored into guideline recommendations.'

The protective effect appeared to take at least five years to develop and was stronger in older individuals. Increasing aspirin dose above 75 mg per day did not increase the protective effect (most low-dose aspirin sold in the U.S. is 81 mg).

These results are new enough that not all doctors have figured out yet how to incorporate them into their recommendations. But they will.

Peter M. Rothwell, MD, PhD, FRCP, professor of clinical neurology at Oxford University and lead researcher of the analysis, finds several different take home points: "These results do not mean that all adults should immediately start taking aspirin. But they do demonstrate major new benefits that have not previously been factored into guideline recommendations."

While cautioning that more research is necessary to better understand the meaning of the results, Rothwell believes that those who started taking aspirin their late 40s or 50s — before a person's risk of cancer begins to climb — and then continued for 20 or 30 years might see the most benefit.

The eight studies chosen for the analysis all compared the effect of taking aspirin to not taking any and had a follow-up of at least four years. Overall, there were 25,530 study participants and 674 total deaths from cancer.

Looking at all patients in the eight studies, after five years of taking aspirin, death rates were 34% lower for all cancers and up to 54% lower from gastrointestinal cancers.

Looking at the three studies with the longest follow-up, it took about 5 years to see a benefit from taking aspirin for esophageal, brain, liver and pancreatic cancer; about 10 years for stomach and bowel cancer, and about 15 years for prostate cancer. The 20-year risk of death was reduced by about 10% for prostate cancer, 30% for lung cancer, 40% for bowel cancer and 60% for esophageal cancer.

There are some limitations to these studies. They were originally designed to measure the effect of taking aspirin on vascular events such as stroke, not on cancer. That's one reason Rothwell's team was only able to look at deaths from cancer, not the effect of aspirin on the number of cases of cancer. And while the number of subjects was large, the number of deaths from some types of cancer was small. The studies also included too few women to look at aspirin's effect on breast and gynecological cancers. Future studies will likely be designed to target all of these factors.

Rothwell adds that: "We can't say with absolute certainty that there won't be some unknown harm in taking aspirin for 30 years, but it looks as if there would be pretty large benefits in reducing cancer deaths. People have to accept there's some uncertainty here."

Currently, taking aspirin to prevent a first heart attack is not generally recommended. Aspirin does lower the risk of a first heart attack but also increases the risk of gastrointestinal bleeding and other events. The benefits and risks are roughly equal, so on average, there is no gain from taking aspirin. If aspirin also has an anti-cancer effect, these recommendations may need to be re-evaluated.

Medical recommendations change from year to year. It should certainly be interesting to see how these studies affects doctors' consensus on taking daily low-dose aspirin.

An early online version of an article detailing the analysis was published by The Lancet on December 7, 2010.