Taking over-the-counter painkillers such as aspirin or ibuprofen could reduce the risks of breast cancer by a fifth or more, says a study.
Aspirin can be bought over the counter
However, doctors have urged women not to start taking painkillers regularly for this reason without taking medical advice first - as long-term painkiller use holds health risks of its own.
The study, which involved more than 80,000 post-menopausal US women, is published in the Proceedings of the Annual Meeting of the American Association for Cancer Research.
The findings applied to non-steroidal anti-inflammatory drugs - aspirin and ibuprofen are the best known examples of these, but paracetamol and codeine belong to other classes of painkiller.
Women who took two or more tablets a week were considered "regular users".
Doctors would advise people not to take aspirin or ibuprofen regularly without good reason
Professor Chris Paraskeva, University of Bristol
Regular use of aspirin for between five and nine years was linked to a reduction in risk of breast cancer of 21% over an average of three and a half years.
Those who had been taking the drug even longer had an even bigger reduction - 28%.
Ibuprofen users did even better, recording a 49% reduction in risk.
The results are suggestive that longer-term ibuprofen or aspirin use could produce longer-term benefits against breast cancer.
However, regular use of "low-dose" aspirin had no apparent protective effects.
"These results suggest that even women at high risk for breast cancer may be protected by taking NSAIDs," said Dr Randall Harris, who led the study.
Aspirin is already recommended by many doctors to patients who are at a high risk of heart disease.
Studies also suggest strongly that it could prevent or slow down bowel cancer if taken in slightly higher doses.
Smaller studies had already pointed to a protective effect against breast cancer.
Professor Chris Paraskeva, a researcher from Bristol University who has looked at the effects of NSAIDs on cancer, said that the US findings were "not surprising".
He told BBC News Online: "The evidence in bowel cancer is quite strong, and people are already looking to see whether it has an effect in other cancers.
"However, the doses involved in bowel cancer protection are higher than those recommended for heart disease prevention.
"Doctors would advise people not to take aspirin or ibuprofen regularly without good reason."
Both types of NSAID are associated with gastric side effects, in particular bleeding, among regular users.
A spokesman for Breakthrough Breast Cancer backed this view.
She said: "This is a very interesting study.
"It has been known for some time that aspirin is beneficial in preventing heart disease and recently more evidence to suggest it may also help reduce risk of other cancers including lung, prostate and colon.
"Clearly this study is exciting however we would express caution before recommending that women take aspirin on daily basis until studies have established long-term side effects. Any woman with concerns should consult their GP."