The research team, from the Veterans Affairs Medical Center in Kansas City and the University of Kansas Medical Center, say tests on cancer cell lines and in mice show that aspirin not only significantly slows growth of cancer cells and shrinks tumors, but also stops tumor cells spreading to new sites.

Their study investigated the effect of aspirin on two types of cancer, including the so-called aggressive "triple-negative" breast cancer, which is immune to most treatments.

Triple-negative breast cancers are so-called because they lack receptors for estrogen, progesterone and Her2/neu.

The researchers presented their findings at the annual meeting of the American Society for Biochemistry and Molecular Biology.

A meeting abstract of the study appears in the FASEB journal.

Aspirin's Effect on Cancer First Suggested 20 Years Ago

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

For example, there are reports that colon cancer survival improves with aspirin use, and that aspirin and other commonly used painkillers may also help guard against skin cancer.

It has also has been shown to reduce the risk of squamous cell esophageal cancer and prostate cancer.

At first it was thought the effect only kicked in after ten years or so, but in 2012, three Lancet studies of people in middle age taking low-dose aspirin suggested that the anti-cancer benefits may start after only three years.

Aspirin May Halt Cancer By Acting on Cancer Stem Cells

However, despite all this evidence, the underlying mechanism through which aspirin confers its anti-cancer benefits have been somewhat difficult to establish.

Now this latest study suggests that for breast cancer, it may be that aspirin interferes with the stem cells that are believed to fuel the growth and spread of tumors.

In a press statement, senior author Sushanta Banerjee, director of the cancer research unit and a professor at the University of Kansas Medical Center, says first-line chemotherapy does not destroy stem cells: eventually the tumor will start to grow again.

"If you don't target the stemness, it is known you will not get any effect," he adds, "It will relapse."

Banerjee and colleagues found that in the mouse model they used, cancer cells treated with aspirin formed no or only partial stem cells.

And in lab tests, aspirin blocked the growth of two different breast cancer lines.

Aspirin Halted Triple-Negative Breast Cancer In Cell Lines

One of the cell lines the researchers used was of what is often called triple-negative breast cancer, which is a less common but much more difficult form of breast cancer to treat.

Banerjee, who is also a professor of medicine in the university's division of hematology and oncology, says he and his team are mainly interested in triple-negative breast cancer because the prognosis is very poor for patients who find themselves with this form of the disease.

The reaction from cancer campaigners has been welcoming but cautious.

According to The Independent, Eluned Hughes, of the UK charity Breakthrough Breast Cancer, urges caution over the "incredibly early stage research" which is yet to be replicated in human patients. But she says they will be watching its progress closely, "it could be promising for the future", and they hope to continue to see new options for patients.

Aspirin May Also Boost Treatments for Hormone-Positive Breast Cancers

The researchers say aspirin may also improve the effectiveness of current treatments for hormone-receptor-positive breast cancers. In their study, they found aspirin boosted the effect of tamoxifen, which is used to treat hormone-positive breast cancers.

Aspirin treats many different conditions. Banerjee says its ability to target several metabolic pathways could be why it is effective against cancer:

"Cancer is not a single-gene disease," explains Banerjee, "Multiple genes are involved."

Many people take a daily low dose of aspirin to lower their risk of a further heart attack or stroke, or if they have a high risk of either.

But taking aspirin is not without risks; for instance daily aspirin use can increase the risk of gastrointestinal bleeding.

Researchers are continuing to investigate whether the advantages outweigh the potential disadvantages.

In 2012 the National Cancer Institute invited scientists to explore how drugs like aspirin, primarily intended for other purposes, might also reduce the risk of developing cancer or extend survival for those who have it.