Taking an aspirin every other day may help healthy women ward off colon cancer according to a new long-term study published in the Annals of Internal Medicine on Tuesday.

However, the researchers, led by Nancy Cook, Associate Biostatistician at Brigham and Women’s Hospital and Professor of Medicine at Harvard Medical School in Boston, found it took years for this benefit to emerge, and this should be weighed against the raised risk of gastrointestinal bleeding from long-term use of aspirin and the fact the treatment made no difference to cancer deaths.

For over 20 years, since an Australian study first suggested aspirin may have anti-cancer properties, researchers have been finding the headache drug may prevent and also treat all sorts of cancer, including colorectal.

For example, research suggests colon cancer survival improves with aspirin use, and that aspirin and other common painkillers may reduce risk of skin cancer.

However, this latest study is the first to look at the long term effect on cancer risk of taking a low dose of aspirin every other day.

For their study, Cook and colleagues examined data on 39,876 women aged 45 and over who took part in the Women’s Health Study (WHS), run by Brigham and Women’s Hospital and Harvard Medical School.

The WHS was set up to test the benefits and risks of low-dose aspirin and vitamin E in the prevention of heart disease and cancer and the participants are all female health professionals. The WHS has been going for 19 years and so far 400 published research reports have used its data.

The WHS participants were enrolled between 1994 and 1996 and randomly assigned to one of two groups. One group took 100 mg of aspirin every other day and the other group took placebo.

Then every year after enrollment, until the end of the trial in March 2004, the women received 12 months’ supply of aspirin or placebo, and every 6 and 12 months they also filled in questionnaires that asked them about how well they were adhering to the dosing schedule, were there any adverse effects, were they taking aspirin over and above the dose for the study, and other information that might affect the results such as additional risk factors.

At the end of the trial, the women were invited to have further post-trial annual follow ups with the option to opt out. Those who did not opt out were followed until March 2012. 33,682 of the women stayed for the extra follow up.

During the trial period there were 5,071 cancer cases among the women, these included 2,070 breast, 451 colorectal, and 431 lung cancer cases. Also, 1,391 women died from cancer.

The results showed that over the entire follow-up, aspirin use had no effect on risk for developing breast or lung cancer.

However, aspirin use was linked to a reduced risk of colorectal cancer, but this did not emerge until well into the post-trial follow up, that is after 10 years of use.

Long term use of low-dose aspirin was also linked to more gastrointestinal bleeding and peptic ulcers, (as reported by the women themselves in their questionnaires).

There was no effect on risk of death from cancer.

In an accompanying editorial, Professor Peter Rothwell of Oxford University in the UK, a world expert on aspirin, notes that the study gives further evidence that we need to consider the benefits and risks of regular aspirin use in men and women separately.

He points out that fewer than 8% of cancer cases in this study of women were due to colon, esophagus, and stomach cancers combined, which compares with around 23% of cases in trials involving men.

Rothwell also argues that the raised risk of gastrointestinal bleeding, together with no evidence that long term aspirin use reduced deaths in the women, or overall risk of cancer, “should temper any recommendations for widespread use of aspirin in healthy middle-aged women”.

The study will no doubt add more fuel to the debate around whether regular aspirin use poses more benefit than risk.

The main source of funding for the study was the National Institutes of Healt (NIH).

Written by Catharine Paddock PhD