Earlier work by the same team of scientists showed that the century-old remedy for aches and pains, long a staple of family pharmacies, can help ward off colon cancer.The new study, published in the British medical journal The Lancet, reveals for the first time that aspirin also helps protect against prostate, lung, brain, and throat cancers, among others.
"These findings provide the first proof in man that aspirin reduces deaths due to several common cancers," said Peter Rothwell, a professor at the University of Oxford and lead author of the study.
Rothwell and colleagues reviewed eight previous clinical trials involving a total of more than 25,500 patients. In each, some subjects took aspirin and others look-alike placebos.
None of the studies were originally designed to measure the impact of the drug on the incidence of cancer.
During the trials, which lasted four-to-eight years, doses of aspirin as low as 75 milligrams -- a fraction the normal dose for a headache -- cut cancer deaths overall by 21 percent.
Risk was especially reduced after five years of treatment with the drug, by 30 to 40 percent depending on the type of cancer.
Three of the eight trials ran long enough to examine the impact of aspirin over a period of two decades.
The 20-year risk dropped on average by a fifth: 10 percent for prostate cancer, 30 percent for lung cancer, 40 percent for colon cancer, and 60 percent for oesophageal cancer.
For cancer of the lung and throat, the protective effect was confined to adenocarcinomas, the type typically seen in non-smokers.
"Perhaps the most important finding for the longer term is the proof of principle that cancers can be prevented by simple compounds like aspirin, and that 'chemo-prevention' is therefore a realistic goal," Rothwell said.
The length of time before the benefits of taking aspirin kicked in also varied: five years for throat, pancreatic, brain and lung cancer, about 10 years for stomach and colorectal cancer, and 15 years for prostate cancer.
The reductions in stomach and brain cancers, however, were more difficult to quantify because of the smaller number of deaths recorded.
"These promising results build on a large body of evidence suggesting that aspirin could reduce the risk of developing or dying from many different types of cancer," said Ed Yong of Cancer Research UK in commenting on the study.
"This tells us that even small doses reduce the risk of dying from cancer provided it is taken for at least five years."
Many doctors recommend regular use of aspirin to lower the risk of heart attack, clot-related strokes and other blood flow problems.
But daily use of the drug, available without prescription, may cause stomach problems, including stomach bleeding. Alcohol use can aggravate these symptoms.
"We encourage anyone interested in taking aspirin on a regular basis to talk to their doctor first," Yong said.
Aspirin is believed to have a preventive effect because it inhibits an enzyme called COX-2, which promotes cell proliferation in cancer tumours.
In rich nations, the lifetime risk of developing cancer is about 40 percent, with rates in the developing world increasing.