They are cheap pain-killers you can buy over the counter but, if the latest research is right, they can halve your risk of breast cancer. As the front page of the Daily Mail screamed yesterday, "Aspirin to beat breast cancer." The report was based on a study of more than 80,000 women from the US Women's Health Initiative. It found that those who had regularly taken aspirin or similar drugs such as ibuprofen for five to nine years substantially reduced their risk of breast cancer. Just two standard doses a week could reduce the risk by 21%, it said.

No one was more impressed than Dr Randall Harris, the study's lead investigator. "My personal recommendation," he told the world's press, "is that women over 40 might consider taking a standard dose of these compounds on a daily basis, since there is low frequency of side effects."

So does this mean that women of a certain age are now sorted? That an aspirin a day really will beat breast cancer? Or is this yet again a reporting triumph of hype over hope?

Dr Fiona Godlee, editor of Clinical Evidence, a digest of the best available research, believes it's a bit of both. "The problem with enthusiastic media reports of studies are that they raise people's hopes," she says. "If we can discover anything that could generally reduce the risk of breast cancer it would be wonderful. This is a large study and it's well done, but the way it's designed it can never do more than show a possible link."

Harris's study was purely observational. "The only way to confirm that aspirin does reduce the risk of breast cancer is randomly to allocate women either to get aspirin or not," Godlee says. "Then you would need to follow them up for a long enough time to see if they get cancer or not."

It was studies of this quality that showed aspirin reduces the risk of heart attack. One of the biggest trials that randomly allocated people to take aspirin or an inactive tablet was carried out on doctors in the 1980s. That showed low-dose aspirin almost halved the risk of a heart attack in people over 50. Other studies that did not randomly allocate people have also suggested that aspirin may protect against bowel cancer.

What is not clear is whether this latest study has been through the usual process of being critiqued by other breast cancer experts prior to being published. Rather than appearing in one of the top medical journals it has been, so far, only published in the proceedings for the 94th annual meeting of the American Association for Cancer Research. But the same team published a smaller study of 2,000 women showing much the same results - a 40% reduction in breast cancer risk in those taking ibuprofen - to less media interest, in 1996 in the journal Epidemiology.

A big problem with assessing studies of risk reduction is that first you have to know your risk. Otherwise how can you know what reducing it means? If you have a very low risk of breast cancer, reducing it by 50% may not be a big deal.

So should women over 40 be reaching for the aspirin bottle? Not yet. Aspirin can cause bleeding, both internally and externally, and can irritate the stomach. "I would be cautious about starting long-term medication on the basis of this research without seeing your doctor," warns Pamela Goldberg, chief executive of the research charity Breast Cancer Campaign. "Aspirin does have side effects."

More research is clearly needed. Mike Dixon, consultant breast surgeon at the Edinburgh Breast Unit, and Professor Nigel Bundred, of the Withington Hospital in Manchester, are looking at the effects of drugs such as aspirin on women who have a form of pre-breast cancer. They have anti-cancer effects because they interfere with the essential blood supply to growing cancers. They may also stop cancer cells dividing.

"If you want to see if a drug can prevent cancer then you need to look at its effects on the early stages of cancer," says Dixon. "We are starting women with pre-breast cancer on these drugs to see if they have biological effects we can measure - if they can stop the very early stages of cancer from progressing. But aspirin can cause bleeding, bruising and other side effects in some people. Women shouldn't be rushing out to take it yet. "

Meanwhile GPs are bracing themselves for an onslaught of inquiries. "This is another study showing that aspirin is a wonder drug," says Alex Vass, a London GP. "I'll be telling people that more studies are needed, that this is not conclusive and that for an individual the risks of taking aspirin might outweigh the benefits."