“Taking a low dose of aspirin every day can prevent and possibly even treat cancer,” BBC News today reported. The painkiller is already taken by thousands to cut their risk of heart attacks and strokes, but new, widely reported research has suggested it may have a role in fighting cancer that this study used.

In three research papers published simultaneously, doctors and researchers looked at cancer data recorded during dozens of trials testing aspirin for heart and circulatory health. They found that daily use of aspirin was linked to a drop in the short-term risk of developing cancer and could reduce both the risk of cancers spreading around the body and the risk of death due to cancer.

However, given that the trials were not originally designed to see whether aspirin would reduce people’s risks of cancer we cannot be sure that the drug’s potential cancer-fighting benefits are not outweighed by its known side-effects. For example, aspirin can cause painful stomach irritation and there is a small but important risk of serious bleeds associated with its use. Therefore, until there have been dedicated studies, it is too soon to recommend that people should start taking daily aspirin unless it has been recommended to them by a doctor.

Overall, aspirin is a highly effective medical treatment when used appropriately, but it is not yet a drug that should be taken unsupervised on a daily basis, even at low doses.

What is the basis for these current reports?

Although first created to act simply as a painkiller, aspirin is now routinely used to treat and prevent a range of health problems. For example, in the UK it can be prescribed as a blood-thinner to reduce the risk of clots, but only as a small daily dose, often around 75mg. This is much lower than the 300mg found in a typical aspirin painkilling tablet.

Some researchers have also suggested that the humble household drug can also reduce the risk of cancer, and various studies have looked at the issue over the years. However, The Lancet journal group has today published three related research papers examining the effects of aspirin on cancer incidence, cancer metastasis (spread) and cancer deaths. To examine whether or not aspirin has cancer-fighting properties, researchers conducted three reviews of the body of evidence provided from previous trials that had indirectly examined how aspirin-use related to cancer. In other words, the trials had been looking at issues other than cancer, but had recorded additional data on cancer that the researchers examined.

The reviews were conducted by Peter Rothwell of Oxford University, along with other researchers. Two reviews were published in The Lancet and the third was published in the group’s specialist cancer publication, Lancet Oncology. The researchers declared they received no external funding for any of the reviews.

What did the research find?

To conduct their review, researchers performed systematic searches of research databases to gather all relevant studies on cancer incidence, cancer metastasis (spread) and cancer deaths. They then used recognised methods to produce three different review papers; the findings of which are summarised below.

Paper 1: Aspirin reduces the short-term risk of cancer incidence and mortality

The first study analysed individual patient data from 51 randomised controlled trials (RCTs) that had compared daily aspirin use with control treatments as a means of preventing vascular events such as heart attacks and strokes. In total, the trials featured 77,549 patients: 40,269 were prescribed aspirin and 37,280 were in control groups and were prescribed either an alternative treatment or no treatment at all.

The study found that aspirin reduces the risk of non-vascular death by around 12% compared with a control treatment. This effect was mainly due to fewer cancer deaths after five years, with the risk of cancer being 37% lower among aspirin users than the control group.

The researchers then considered only those trials examining aspirin use for the prevention of vascular events in people who had not yet had an event such as a heart attack or stroke, a practice known as “primary prevention”. Doctors prescribe aspirin in this way when people are considered to be at risk due to factors such as their age, weight and lifestyle.

When examining aspirin as a primary prevention, the researchers found that after three years of use it reduced cancer incidence in both men and women, and in smokers and non-smokers. Aspirin also reduced the risk of heart attack and strokes, but increased the risk of bleeding – a potentially serious side effect that some people experience when taking aspirin. The effect aspirin had on both cardiovascular risks and bleeds decreased with time, although the effect aspirin had on cancer remained.

Paper 2: Aspirin prevents distant metastasis

One particularly serious complication of cancer can be “metastasis”, where cancerous cells spread to other tissues and organs. It can make the cancer harder to treat, and with many cancers metastasis can reduce the chances of a patient surviving.

Some theories have suggested that regular aspirin use could reduce the risk of metastasis, perhaps by the actions it has upon the blood. The second review examined this issue by analysing individual patient data to determine how common metastasis is in people who developed a solid cancer, such as stomach cancer (as opposed to a blood cancer, such as leukaemia) and whether their aspirin use had any impact on their risk of it. The patients were participants in RCTs comparing groups taking aspirin daily with control groups, for the prevention of vascular events. This study included patient data from five trials performed in the UK. These trials would have excluded patients with a recent diagnosis of cancer from entry into the study, and therefore only participants who were diagnosed with cancer during the course of the trial were looked at.

In total, 987 new solid cancers were diagnosed among 17,285 participants during the trials. Patients who received aspirin were 46% less likely to have cancer with distant metastasis than those using a control treatment.

The researchers then turned their attention to a particular type of cancer called adenocarcinoma, which can affect a range of different organs in the body. They found that:

Aspirin reduced the risk of adenocarcinoma with metastasis at initial diagnosis by 31%.

When patients were diagnosed with cancer but no metastasis, the risk of developing metastases later was 55% lower among aspirin users.

Aspirin also reduced the chances of death in patients with adenocarcinoma.

The effects were independent of age and sex, but smokers benefitted the most from taking aspirin.

Paper 3: The results of observational studies are consistent with the results of RCTs, and show that regular use of aspirin reduces the long-term risk of several cancers and the risk of distant metastases

The third study was a systematic review of observational studies that had reported associations between aspirin, risk of cancer and the outcome of cancer cases. The researchers compared the result with that of RCTs to see if they were consistent across the reviews.

Observational studies provide less evidence than RCTs, and cannot show causation. However, this study found that the results of the observational studies were consistent with those obtained from RCTs, which is that regular use of aspirin reduces the long-term risk of several cancers, including:

colorectal or bowel cancer

oesophageal or foodpipe cancer

gastric or stomach cancer

cancer of the bile duct

breast cancer

Observational studies also supported the finding that aspirin lowers the risk of distant metastasis.

Where there any limitations to the studies?

All of these studies were systematic reviews, which are considered to provide a higher amount of evidence, particularly those examining RCTs, as two of these reviews did. However, it should be noted that the RCTs were designed to assess the effectiveness of aspirin as a means to prevent vascular events such as heart attack and stroke, and not as a means to prevent cancer. Given that the studies were not designed to look at cancer, they may have had limitations in the type of cancer-related information they recorded.

For many people at risk of cardiovascular disease, the benefits of taking prescribed daily aspirin under their doctor’s supervision outweigh the risks of taking the drug. However, this may not be the case in otherwise healthy people who are not at cardiovascular risk. Therefore, caution should be applied when applying these results to the general population. The authors report that, so far, there have only been a few small RCTs of aspirin for the treatment of cancer, but that two new trials have recently begun.

Should I start taking aspirin?

Although these studies provide compelling evidence, taking aspirin is not yet recommended to prevent cancer and people should not start taking it daily as a precautionary measure. This is because aspirin can potentially cause serious harmful effects, such as a small-but-important increase in the risk of stomach irritation and bleeding (haemorrhage).

The risk of complications also tends to be higher in:

the elderly

people with a history of stomach ulcers

people taking drugs that increase their risk of bleeding

people who have other medical conditions that increase their risk of bleeding.

Aspirin can also cause breathing problems in people with asthma. Other common side effects of aspirin include feeling sick and indigestion. Some people have allergic reactions to it.

Given that the potential risks could outweigh any benefits, it is not currently advised that healthy people with no risk factors for cardiovascular disease take aspirin to prevent possible cardiovascular events such as heart attack and stroke. Furthermore, as the evidence for taking aspirin purely as cancer prevention or treatment is even less substantial than for blood thinning, again we cannot be sure that the potential benefits are not outweighed by the known risks.

Before taking aspirin, including for pain relief, talk to your doctor or pharmacist if:

You are pregnant, trying for a baby or breastfeeding.

You have a blood disorder.

You have ever had a stomach ulcer.

You suffer from asthma.

You have liver or kidney problems.

You have high blood pressure.

You have had an unusual or allergic-type reaction to any medicine.

You are taking other medicines.

Aspirin should not be given to children under the age of 16 due to the risk of Reye's syndrome, a rare but dangerous condition that can be triggered by aspirin. Aspirin has been removed as an ingredient from all UK child and baby medicines, so they should not carry a risk of Reye's syndrome.

The news has understandably generated a great deal of interest in aspirin as a potential way to cut cancer risk, but it should be remembered that there are many other lifestyle changes that people can make to reduce their risk of cancer, including giving up smoking, which do not have harmful side effects.