A new study adds support to the idea that daily aspirin use results in fewer cancer deaths, but the effect may not be as large as previous research might suggest. The researchers say although the collected evidence seems encouraging, it is still too early to recommend routine taking of aspirin just to prevent cancer, because even at low doses, it can increase the risk of serious bleeding in the gut.

The study, by a team of epidemiologists from the American Cancer Society, appeared early online in the Journal of the National Cancer Institute on 10 August.

In March 2012, researchers from Oxford University in the UK, published an analysis of data pooled from 51 randomized trials and calculated that daily aspirin use reduced the risk of cancer death by 15%, and this reduction increased with prolonged use. For instance, taking aspirin every day for 5 years or more, appeared to reduce risk of death from cancer by 37%.

But in the current study, lead author Eric J. Jacobs and colleagues, write:

“However, the magnitude of the effect of daily aspirin use, particularly long-term use, on cancer mortality is uncertain.”

For their study, Jacobs and colleagues analyzed data on over 100,000 mostly elderly people who took part in the Cancer Prevention Study II Nutrition Cohort where they had filled in questionnaires that included questions on use of aspirin. None of the participants had cancer at the start of the study, which followed them for up to 11 years.

When they analyzed the data, the researchers found daily aspirin use was linked to an estimated 16% lower risk of cancer death, both among participants who reported taking aspirin every day for at least five years, and also among those who reported using it for less than that.

The biggest effect (40% lower mortality) on lowering cancer deaths came from lower deaths from cancers of the gastrointestinal tract, including esophageal, stomach, and colorectal cancer, and to a much lesser extent (about 12% lower mortality) from cancers elsewhere in the body.

A 16% reduction in overall cancer mortality is considerably lower than the 37% from the earlier pooled analysis of randomized trials, and the authors do concede that their study was observational and not randomized, so there is a possibility they could have under or over estimated effects from different underlying risk factors among aspirin users and non-users.

However, to counter that potential limitation, they also point out their study has strength because of the very large number of participants.

In conclusion, they write:

“These results are consistent with an association between recent daily aspirin use and modestly lower cancer mortality but suggest that any reduction in cancer mortality may be smaller than that observed with long-term aspirin use in the pooled trial analysis.”

Nevertheless, in a press statement, Jacobs cautions:

“Although recent evidence about aspirin use and cancer is encouraging, it is still premature to recommend people start taking aspirin specifically to prevent cancer.”

“Even low-dose aspirin can substantially increase the risk of serious gastrointestinal bleeding,” he adds.

Expert panels that issue clinical guidelines will look at all the accumulated evidence on the risks and benefits of routine aspirin use, and update the guidelines, says Jacobs.

In the meantime, he suggests decisions about whether or not to take aspirin routinely should be made on an individual basis, taking into account the patient’s medical history, and in consultation with a health care professional.

In an accompanying editorial, John A Baron, professor of medicine at the University of North Carolina at Chapel Hill School of Medicine, agrees: people should not start taking aspiring routinely just to reduce their risk of cancer.

We still don’t know how aspirin might prevent cancer, what the underlying biological mechanisms might be, despite the fact the drug has been around for over 100 years and is a well-known occupant of medicine cabinets around the world.

Baron also points out that the effect on cancer takes time to appear, for instance with colon cancer it could take ten years or more. And in the meantime, people could develop serious gastrointestinal or even brain bleeding from routine aspirin use.

The risks have to be balanced against the benefits, he notes, “particularly when the benefits are delayed whereas the risks are not”.

Written by Catharine Paddock PhD