Aspirin could cut a cancer patient's risk of dying with the disease by 20%, research suggests.
The over-the-counter painkiller has long been linked to improved heart health due to its blood-thinning properties.
Previous studies have suggested aspirin may also have potential when it comes to treating cancer. It remained unclear, however, whether the drug is an "an acceptable and effective" add-on to conventional treatments.
To learn more, scientists from Cardiff University analysed 118 studies, made up of patients with 18 different types of cancer.
Results – published in the journal eCancerMedicalScience – suggest aspirin is "associated with a reduction of about 20% in cancer deaths" and the benefit "appears not to be restricted to one or a few" forms of the disease.
Aspirin therefore "appears to deserve serious consideration" as an add-on treatment for cancer, according to the scientists.
They have stressed, however, the painkiller cannot be used in place of conventional cancer treatments.
In 1974, scientists from Wales' Medical Research Council's Unit found a daily aspirin tablet reduces the risk of death from heart disease or a stroke by nearly a quarter (24%).
The BMJ later judged this to be one of the top 50 most important studies published since 1945.
Aspirin's anti-cancer potential has also been discussed for more than 50 years. "The bulk of published evidence" was of dubious quality, however.
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"In recent years, my research team and I have been struck by the actions of aspirin on the biological mechanisms relevant to cancer and these seem to be the same in many different cancers," said lead author Professor Peter Elwood.
"We therefore wanted to review the scientific evidence available on the use of aspirin as an additional treatment for a wide range of cancers."
Within the 118 studies, around a quarter of a million cancer patients reported taking aspirin. How often the painkiller was taken, or its dose, is unclear.
"Overall, we found that at any time after a diagnosis of cancer, about 20% more of the patients who took aspirin were alive, compared with patients not taking aspirin," said Professor Elwood.
The results also suggest aspirin may prevent cancer from spreading.
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"There is now a considerable body of evidence to suggest a significant reduction in mortality in patients with cancer who take aspirin and that benefit appears not to be restricted to one or a few cancers," said Professor Elwood.
"Aspirin therefore appears to deserve serious consideration as an adjuvant treatment of cancer, and patients with cancer and their carers should be informed of the available evidence."
An adjuvant strengthens a patient's immune system after initial cancer therapy.
"We must also stress aspirin is not a possible alternative to any other treatment," said Professor Elwood.
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Aspirin's blood-thinning properties means it has been linked to dangerous bleeding, making it unsuitable as a daily drug for some people.
To better understand the extent of the side effect, the Cardiff scientists wrote to the authors of the 118 studies.
"As expected", bleeding "increased" among the patients who took aspirin.
Fatal bleeds were "rare", however. None of the studies' authors reported an increased risk of deadly bleeds due to aspirin itself.
Further research is pending, which should provide more information on aspirin's potential for bowel, breast and prostate cancer, specifically.
Nevertheless, "the biological effects of aspirin appear likely to be of relevance to cancer generally", according to the scientists.
More than 100 studies were analysed in the review, however, over 1,000 aspirin-cancer trials are said to be carried out every year.
"Further research into aspirin and cancer would clearly be of great value, and new studies should be encouraged, especially if focused on some of the less common cancers," said Professor Elwood.
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