Monday August 13 2012
Previous studies have linked aspirin with reduced cancer risk
"Daily dose of aspirin cuts cancer risk," The Daily Telegraph says. The newspaper reports that a daily dose for the over-60s can "cut cancer risk by 40%".
The story is based on a recent long-term cancer prevention study that included more than 100,000 adults.
Looking at the data as a whole, the researchers found that those who were taking daily aspirin at the start of the study had a very slightly lower risk of dying from cancer over the 11 years of the study. This result was not statistically significant (it could have been a chance result). However, when the researchers restricted the analysis to those who had updated information on their aspirin use throughout the course of the study, they did find a significant 16% reduced risk of death from cancer.
This is not the first time aspirin has been associated with reduced cancer risk. A review of clinical trials published earlier this year also observed a reduced risk of developing cancer with daily aspirin. However, the risk reduction found from the pooled results of those trials was greater (37% reduction) than that found in this study (16%). This may be because this was an observational study and not a trial, so people taking daily aspirin were doing so for cardiovascular disease. Health factors associated with cardiovascular disease may also increase the risk of cancer.
It is important to stress that you should never start taking aspirin on a daily basis without first checking with your GP or pharmacist that it is safe or suitable to do so. Aspirin is not without risks and is known to increase risk of gastrointestinal irritation and bleeding, particularly in the elderly or those with a history of stomach ulcers or bleeding problems. It also should not be taken by people with asthma.
Overall, the current evidence is not strong enough to recommend that everyone take daily aspirin purely for cancer prevention.
Where did the story come from?
This study was conducted by researchers of the Epidemiology Research Program of the American Cancer Society, Atlanta, USA, and was funded by the American Cancer Society.
The study was published in the peer-reviewed Journal of the National Cancer Institute.
The Telegraph’s headline claim that aspirin can “cut cancer risk by 40%” is misleading as this result was found by a different study published in March of this year and not by the study discussed in the news article. The Telegraph does appropriately include quotes from one of the lead authors of the study, Dr Eric Jacobs. The paper says that Jacobs “emphasised people should not take aspirin every day before discussing the potential side effects, such as stomach bleeds, with their doctors”.
What kind of research was this?
This was a prospective cohort study looking at the relationship between daily aspirin use and risk of death from cancer.
A previous systematic review published earlier this year, which pooled data from randomised controlled trials (RCTs), found that using aspirin was associated with a reduction in risk of death from cancer. This cohort further investigated the association between daily aspirin use and cancer mortality.
Although this study included a large number of people who were reliably followed up, it is still not the best way of examining the effects of an intervention (in this case aspirin) for reducing the risks of an outcome (in this case cancer mortality). The best way would be using a randomised controlled trial. The difficulty is that, although many randomised controlled trials of aspirin use have been conducted and were included in the 2012 systematic review, most of these trials were designed to assess the effectiveness of aspirin in preventing cardiovascular events such as heart attack and stroke. That is, the participants were taking aspirin for prevention of cardiovascular disease, not to see whether it would reduce their risk of cancer. Therefore, these trials may not give such reliable risk estimates for the outcome of cancer.
Nevertheless, this large cohort study is valuable in adding to the evidence on the association between aspirin use and cancer outcomes.
What did the research involve?
This was part of the Cancer Prevention Study II (CPS-II) Nutrition Cohort. In 1992, people in the CPS-II trial answered questions about themselves, including medical (such as aspirin use) and behavioural factors. They answered follow-up questionnaires to update information and find out about new cancer diagnoses in 1997 and every two years after.
However, the questions about aspirin use were slightly different in 1992 from those asked in 1997 and after. In 1992 people were asked the average number of days a month they used aspirin during the past year, and the average number of pills taken on those days. In 1997 and onwards people were also asked specifically about use of low-dose (75mg) or higher-dose aspirin. Participants reporting use of aspirin (any dose) on 30 or 31 days a month were considered “daily users”.
Excluding people with a cancer diagnosis in 1997 or before, and those without information on aspirin use, left 100,139 participants (44,360 men and 55,779 women) in this study.
Researchers followed up deaths and cause of death through the US National Death Index to the end of 2008. For 99.3% of deaths that occurred they obtained either death certificates or disease classification codes for the cause of death from the database.
The researchers analysed these deaths according to whether people reported that they took aspirin and, if they did, how long they had been taking it for. They also did a separate analysis looking at current (short-term and long-term), past or occasional users.
What were the basic results?
Most participants were over 60 in 1997 and, at this time, 24% reported daily aspirin use. Almost half of these people taking aspirin (46%) were taking the low dose and most of them were taking one tablet daily, suggesting that it was being used for the prevention of cardiovascular disease.
Daily users were also slightly more likely than non-users to be:
- highly educated
- former smokers rather than never having smoked
- using anti-inflammatory drugs regularly (such as ibuprofen)
Over the 11 years of study follow-up (1997-2008), a total 5,138 (5%) participants died from cancer. In the first analysis, compared with no use, daily aspirin use at the start of the study was associated with slightly reduced risk of dying from cancer, though these risk reductions did not reach statistical significance:
- use for less than five years (use in 1997 but not 1992) – non-significant 8% reduction in risk (0.92, 95% confidence interval (CI) 0.85 to 1.01)
- use for five or more years (use in 1997 and 1992) – also a non-significant 8% reduction in risk (0.92, 95% confidence interval, 0.83 to 1.02)
There were, however, significant reductions in risk in subsequent analyses that included aspirin information from later questionnaires (including 3,373 cancer deaths). These were thought to provide a more reliable source of data:
- use for less than five years (daily use in 2003 but not in both the years 1999 and 2001) – 16% reduction in risk (0.84, 95% confidence interval, 0.76 to 0.94)
- use for five or more years (use in 1999, 2001 and 2003) – also a 16% reduction in risk (0.84, 95% confidence interval 0.75 to 0.95)
How did the researchers interpret the results?
The authors say that their results show an association between daily aspirin use and modestly lower cancer mortality. However, the reduction in cancer mortality they record is smaller than that observed with long-term aspirin use in the pooled results of the recent systematic review published in the Lancet (37% risk reduction with more than five years of use).
This study had a large number of participants and the follow-up was reliable. It provides further information that daily aspirin may give a small reduction in risk of dying from cancer.
A cohort study is not the best way of examining the effects of an intervention upon an outcome, as there may be other health or lifestyle factors that differ between those who take aspirin and those who don’t, that might influence their risk of cancer. Also, the study used self-reported questionnaires to assess aspirin use and there may be some inaccuracies in estimates of dose or frequency of use.
The study follows on from a systematic review published earlier this year that had included all randomised trials examining the effect of aspirin in reducing the risk of dying from cancer. The difficulty is that the participants in the clinical trials that were included in this review were taking aspirin for prevention of cardiovascular disease (for example, heart attack or stroke), not to see whether it would reduce their risk of cancer. Therefore, these trials may not give such reliable risk estimates for the outcome of cancer. Similarly, most of the people in the current cohort who were taking daily aspirin also seemed to be doing so for the reason of prevention of cardiovascular disease – not for cancer prevention. Therefore, neither this cohort nor the clinical trials have examined the use of aspirin for cancer prevention, and we don’t know whether the benefits of aspirin outweigh the risks in people without risk factors for cardiovascular disease.
Although aspirin is widely established as an effective treatment for cardiovascular disease, aspirin is not yet recommended as a treatment to prevent cancer as the risks of aspirin might outweigh the benefits. Aspirin can rarely cause serious adverse effects, and is known to increase the risk of stomach irritation and bleeding. Those who may be at higher risk of these complications are the elderly, people who have past history of stomach ulcers, or people who are taking drugs that increase their risk of bleeding or have other medical conditions that increase their risk of bleeding. Aspirin can also cause breathing problems in people with asthma and some people can have allergic reactions to aspirin.
There are many other lifestyle changes that people can make that may reduce their risk of cancer, including giving up smoking, eating a healthy balanced diet and taking regular exercise.
Analysis by NHS Choices. Follow Behind the Headlines on twitter.