TUESDAY, March 20 (HealthDay News) -- Aspirin, a popular weapon in the war against heart attacks, may also play a role in cancer prevention and treatment, three new British studies suggest.

"We have now found that after taking aspirin for three or four years there starts to be a reduction in the number of people with the spread of cancers, so it seems as well as preventing the long-term development of cancers, there is good evidence now that it is preventing the spread of cancers," said lead researcher Dr. Peter M. Rothwell, a professor of neurology at the University of Oxford and John Radcliffe Hospital in Oxford.

"Because aspirin prevents the spread of cancers, it could potentially be used as a treatment," he added.

But the research is not conclusive, and did not prove that aspirin combats cancer. So, people should not start popping aspirin in the hopes of thwarting cancer, experts said.

Previously, these investigators showed that a daily dose of aspirin taken over 10 years appeared to prevent some cancers, but the short-term benefits and the benefits for women weren't clear.

Currently, a daily low-dose aspirin is recommended for people who have had a heart attack or stroke to prevent another. "It may well be that taking aspirin to prevent cancer becomes the main reason for taking it," Rothwell said.

Aspirin may work against cancer by inhibiting platelets, which promote clotting and also help cancer cells spread, he said.

The papers were published March 21 in The Lancet and The Lancet Oncology.

In one study, Rothwell's team analyzed data from 51 clinical trials comparing aspirin with no aspirin in preventing heart attacks.

Overall, daily low-dose aspirin reduced the risk of dying from cancer 15 percent. Taking aspirin five years or more reduced the risk 37 percent, and over three years, the risk reduction was about 25 percent for both men and women, the researchers noted.

In addition, aspirin was associated with a 12 percent reduction in deaths from non-cardiovascular causes, they found.

In another study, Rothwell's team looked at the effect of aspirin on slowing the spread of cancer, or metastasis.

Their data came from five clinical trials that also looked at daily low-dose aspirin (75 milligrams or more) and heart attack and stroke prevention. The researchers zeroed in on patients who developed cancer.

Over more than six years of follow-up, low-dose aspirin reduced the risk of distant metastasis by 36 percent, compared with cancer patients receiving a placebo, they found.

Moreover, aspirin reduced the risk of metastasis in solid tumors, such as colon, lung and prostate cancer, by 46 percent and by 18 percent for cancers of the bladder and kidney.

It also reduced the risk of diagnosing a cancer that had already spread by 31 percent. For those who continued to take aspirin after a cancer diagnosis, the risk of metastasis was cut by 69 percent, the researchers calculated.

Aspirin also reduced the risk of dying from cancer by about half. These risk reductions remained after taking into account age and sex, the researchers said.

In a third study, Rothwell's group looked at the effect of aspirin on metastases by analyzing observational studies rather than clinical trials.

These studies revealed a 38 percent reduction in colon cancer, which matched well with the risk reduction seen in clinical trials, they said. There were similar findings for esophageal, gastric, biliary and breast cancer, they added.

While the study is attention-getting, not everyone agrees with the overall conclusions.

Among them is Nancy R. Cook, an associate biostatistician at Brigham and Women's Hospital and Harvard Medical School in Boston and co-author of an accompanying journal editorial. She pointed out that these studies only dealt with trials where aspirin was given daily, whereas two large trials in which aspirin was given every other day found no connection with cancer prevention.

"Aspirin seems to work for people who have had cardiovascular disease. Perhaps in the long-term it will turn out to be protective for cancer, but we need to verify that and get more information," Cook said.

And, aspirin is not benign, Cook said, pointing out risks for bleeding and other gastrointestinal problems.

People should not start taking aspirin hoping to preventing cancer, Cook said. "Most of the studies show that the effect doesn't accrue until after 10 years," she noted.

Eric Jacobs, strategic director of pharmacoepidemiology for the American Cancer Society, said that "this study provides important new evidence that long-term daily aspirin, even at low doses, may lower risk of developing cancer."

However, any decision about treatment should be made on an individual basis in consultation with a doctor, he said.

"Because these results are new," Jacobs added, "it will take time for the broader scientific community to evaluate the data in the context of existing knowledge and to consider whether the clinical guidelines should be changed."

More information

For more on cancer, visit the American Cancer Society.