Taking a low-dose aspirin every day will reduce your chances of dying from cancer, scientists say today, confirming the over-the-counter pill as the most extraordinary drug yet discovered.

Daily aspirin has already been shown to cut the chances of heart attacks and stroke in people who are at risk. The study published today in the Lancet medical journal reveals that it also has a powerful preventive action against a range of cancers – and possibly more of them than there is currently enough evidence to prove.

While the doctors who carried out the study say it is not for them to make recommendations, the lead author, 46-year-old Prof Peter Rothwell from Oxford University, says he has been taking aspirin for the last two years. The beneficial dose is 75mg – a quarter of a standard tablet, which is 300mg. Some pharmacies sell low-dose tablets, but at a higher price.

Rothwell and his colleagues have already shown that daily aspirin cuts death rates from colorectal cancer by more than a third. For the study published today, they examined all the data they could find from well-conducted trials that had assessed the use of aspirin against a control drug and had recorded deaths from cancer.

In eight trials involving more than 25,000 patients, they found there were 21% fewer deaths after five years among those who took a daily aspirin tablet, compared with those who did not. The effect was most noticeable in gastrointestinal cancers, where deaths dropped by 54%.

Patients in aspirin trials 20 years ago were still 20% less likely to die of a solid tumour and 35% less likely to die of gastrointestinal cancer.

They found that the effects of the aspirin seemed to kick in for pancreatic, brain, oesophageal and lung cancer (generally not the sort triggered by smoking) after patients had been taking it for five years or more. For stomach and colorectal cancer, the effects were seen after 10 years and for prostate cancer after 15 years. After 20 years, the risk of death from prostate cancer was reduced by 10%, for lung cancer by 30%, for colorectal cancer by 40% and for oesophageal cancer by 60%.

The scientists believe the benefits may be even greater than they could establish. The impact on pancreas, stomach and brain cancers was difficult to quantify exactly because of smaller numbers of deaths, they say. There could be an effect in breast and ovarian cancers, but there is not enough trial data to prove it.

Those in the trials took aspirin for only four to eight years. Rothwell believes that if people took aspirin for 20-30 years, from the age of around 45 or 50, they might substantially reduce the risk of cancer, which steadily increases as people age.

Rothwell and colleagues believe their findings should tip the balance of risk/benefit in favour of daily aspirin dosing. While millions of people started taking the tablets after evidence that it cut their chances of heart attacks and stroke, there has been something of a medical back-pedalling, because aspirin can cause stomach bleeding.

"At the moment, GPs all over the country have been taking patients off aspirin because the fashion has changed. I certainly say we should stop doing that," Rothwell said.

Prof Peter Elwood, a medical epidemiologist who carried out the first randomised trial of aspirin in heart attack patients, said the increased risks of a stomach bleed were relatively small – about one patient per 1,000 per year. "But there is no increase in deaths from bleeding," he said. "Aspirin appears to be increasing the less serious bleeds."

Against that, he said, a substantial survey in south Wales showed that 80% of men over 50 already had a high enough risk of heart attack or stroke to justify taking aspirin under all existing guidelines.

Prof Alastair Watson, professor of translational medicine at the University of East Anglia, highlighted the risk of bleeding. "People wishing to take aspirin should first discuss it with their GP," he said. "But this study remains a very important new development. It is further proof that aspirin is, by a long way, the most amazing drug in the world."