Painkillers have been linked to a Regular use of common painkillers such as aspirin and ibuprofen reduces the risk of breast cancer, according to an international study.
The research, which looked at information from 2.7 million women, was published in the Journal of the National Cancer Institute.
Aspirin cut the risk by 13%, while ibuprofen lowered it by a fifth.
However, experts warned long-term use of painkillers can have serious side-effects.
Anti-inflammatory drugs can have potentially very serious side-effects when taken over a long period.
There have been many studies looking at the role of painkillers in breast cancer, and the latest is a review of 38 of these, combining their results to give a more reliable picture.
Both aspirin and ibuprofen are Non-Steroidal Anti-Inflammatory Drugs (NSAIDs), and it is their ability to interfere with inflammation in the human body which appears to be key.
Two body chemicals which help produce inflammation, COX1 and 2, are thought to play roles in the development of cancer by influencing how cells divide and die, the production of new blood vessels that can "feed" tumours, and influence the body's immune responses.
It appears NSAIDS inhibit these chemicals.
Women taking either aspirin or ibuprofen regularly had a 12% lower chance of developing breast cancer compared to those who did not use them at all, while regular ibuprofen use appeared to have the biggest effect.
The regular use of painkillers is problematic because, in some people, they can cause serious side-effects, including stomach ulcers, increased risk of stroke, asthma and heart, liver and kidney problems.
The potential benefits of reducing breast cancer risk would have to be balanced against these.
This advice was echoed by Breakthrough Breast Cancer, which urged women worried about breast cancer risk to talk to their GP rather than simply take painkillers.