A new piece of US research backs the idea that aspirin protects against certain types of breast cancer.
Mounting evidence suggests aspirin reduces breast cancer risk
It found women who used aspirin or similar painkillers at least once per week for six months reduced their risk of breast cancer by 20%.
However, the University of Columbia researchers say it is too soon to advise women to start taking aspirin against breast cancer, however.
Their findings appear in the Journal of the American Medical Association.
Previous studies have suggested aspirin may protect against a range of cancers, including breast, colon and bowel.
Dr Mary Beth Terry and colleagues looked at almost 1,500 women with breast cancer and a similar number of healthy women acting as controls.
They then examined what effect the frequency and duration of use of aspirin or other non-steroidal anti-inflammatory drugs (NSAIDs) had on the women's risk of breast cancer.
The researchers found a dose effect. Women who had taken aspirin at least once per week for six months or longer were 20% less likely to develop certain breast tumours.
But those who used seven or more tablets a week reduced their risk by 28%.
Women who had gone through the menopause showed the greatest benefits.
The reduced risk was only seen among those with a type of breast cancer that is thought to be linked to oestrogen, however.
This is the most common type of breast tumour.
Almost two-thirds of tumours from pre-menopausal women and three-quarters of tumours from post-menopausal women contain detectable oestrogen receptors, which means their growth is stimulated by the presence of oestrogen, according to Cancer Research UK.
The results for NSAIDs such as ibuprofen was slightly weaker.
Use of a painkiller which works in a different way to aspirin and NSAIDs did not appear to reduce the risk of breast cancer.
Dr Terry and colleagues said: "These data add to the growing evidence that supports the regular use of aspirin and other NSAIDs as effective chemopreventive agents for breast cancer."
Balancing risks and benefits
Aspirin and NSAIDs are associated with gastric side effects, in particular bleeding, among regular users.
Dr Terry's team said it was important to have more research to look at what dose of aspirin might be protective.
"The potential benefits need to be balanced against potential harmful effects of long-term aspirin use such as peptic ulcer disease and gastrointestinal bleeding," they said.
"It is also important to study whether these findings are supported in more racially and ethnically diverse populations," they added.
Pamela Goldberg, chief executive of Breast Cancer Campaign said: "This is a very interesting study and opens up a whole range of further research, not only in the possible use of aspirin to reduce risk but also in conjunction with existing drugs to improve treatments."
But she added: "Aspirin is not without side effects and we don't know what the negative consequences of long-term use might be nor the dose.
"Many people are already taking low dose aspirin to reduce the risk of heart disease and it would be really interesting if this drug which we take so much for granted could also reduce the risk of breast cancer."
Samia al Qadhi, joint chief executive of Breast Cancer Care said: "We would like to stress caution to all women who are considering taking aspirin as a result of this study because aspirin can be associated with other health issues such as gastrointestinal problems.
She advised all women to consult their local GP to discuss the benefits and risks.
"We feel it is vital that more clinical research is undertaken to examine the effect of aspirin on breast cancer," she said.
Breakthrough Breast Cancer and Cancer Research UK also recommended further research.
Dr John Toy, medical director at Cancer Research UK, said: "The new study suggests that aspirin can prevent hormone positive tumours and not hormone negative tumours.
"This is the first report of this observation and, as ever in science, the findings need to be replicated before we can draw definitive conclusions."