Metastudy says aspirin lowers risk of some cancers, but questions remain

A new study is presenting the most current meta-analysis of published observational research investigating whether long-term aspirin use reduces the risk of developing digestive tract cancers. While the results suggest aspirin may help prevent some cancers, some experts question how useful this kind of associational study is, particularly when a recent randomized clinical trial came to a very different conclusion. Aspirin’s use as a preventative medicine arose in the late 20th century. First thought to reduce risk of heart attack and stroke, more recently it has been floated as a possible cancer prevention compound. However, evidence of the benefits from daily aspirin use is inconsistent, and scientists are still debating whether the risks outweigh the benefits.

A new study, led by a team of Italian researchers, is offering the largest meta-analysis conducted to date, focusing on a very specific relationship between long-term aspirin use and a reduction in incidences of digestive tract cancers. The analysis pooled data from 113 observational studies covering cancers such as bowel, pancreatic and liver.

The study defined regular use of aspirin as at least one or two tablets per week, and concluded there is a dose-dependent link between aspirin use and a reduced risk of several cancers. In general, aspirin use was associated with a 27 percent reduction in risk of colorectal cancer, a 36 percent reduction for stomach cancer, and a 22 percent reduction for pancreatic cancer. Higher doses, taken for longer periods of time, were associated with greater reductions in cancer risk according to the study.

“We found that the risk of cancer was reduced with increased dose; an aspirin dose between 75 and 100mg a day was associated with a 10% reduction in a person’s risk of developing cancer compared to people not taking aspirin; a dose of 325mg a day was associated with a 35% reduction, and a dose of 500mg a day was associated with a 50% reduction in risk,” says Cristina Bosetti, corresponding author on the new study. “However, the estimate for high dose aspirin was based on just a few studies and should be interpreted cautiously.”

The analysis suggests long-term use of aspirin confers the most benefit in reducing cancer risk, particularly in relation to colorectal cancer. The biggest benefits were seen when aspirin was used for between five and ten years, however, the researchers note few studies examine very long term (over ten years) aspirin use.

“Compared to people who did not take aspirin regularly, the risk of bowel cancer declined in regular aspirin users up to ten years,” adds Bosetti. “The risk was reduced by 4% after one year, 11% after three years, 19% after five years and 29% after ten years.”

All these results may sound like great news, and offer a green light for daily aspirin use, but it is important to note there is plenty of other research to suggest regular aspirin use is not particularly beneficial if you are otherwise healthy.

Another large meta-analysis of observational research published early last year looked at the relationship between daily aspirin use and cardiovascular events, but this analysis compared the potential benefits of aspirin against the known harms.
Pooling data from 13 studies, that analysis concluded there was a very small association between reduced cardiovascular events and aspirin use, but, aspirin use also raised the incidence of major bleeding events. The general conclusion of that analysis was daily aspirin use may do more harm than good, especially if a person is otherwise healthy and not at a high risk of cardiovascular disease.