"There is a lot of interest in NSAIDs, but it is too early to say whether these drugs will have a role in lowering the risk of breast cancer," says Michael Thun, M.D., vice president of epidemiology and surveillance research for the ACS.

One study, published in the July issue of the British Journal of Cancer (Vol. 83, No. 1) looked at the impact of NSAIDs on breast cancer rates among more than 28,000 women. That study found risk of breast cancer was almost 25 percent lower for women who used NSAIDs the most, compared to women who used them the least.

Women who developed breast cancer tended to have smaller tumors and less chance of having a cancer that had spread to other parts of the body if they had used significant amounts of the drugs in the years before they were diagnosed, according to the study.

But another study, published a month earlier in the British Medical Journal (Vol. 320, Issue 7,250), found no reduction in breast cancer rates associated with use of NSAIDs. However, it did find lower risk of developing cancers of the esophagus, stomach, colon and rectum among NSAID users.

Dr. Thun, who has conducted research on aspirin and colon cancer since 1990, says that NSAIDs continue to show promise for cancer prevention. One such drug, celecoxib, has already been approved to suppress formation of pre-malignant adenomatous polyps in people with familial adenomatous polyposis, a rare inherited condition that causes hundreds of polyps to form and leads to colon cancer if preventive surgery is not done.

Clinical trials of NSAIDs? effectiveness in preventing sporadic precancerous colon polyps will be completed in the next two to three years, according to Dr. Thun. Sporadic polyps are those that are not inherited.

"But the evidence that aspirin-like drugs may be of benefit against breast cancer is much weaker and less consistent," he says. "Neither the American Cancer Society nor other health organizations currently recommend that the general public take aspirin or other NSAIDs for cancer protection because no randomized clinical trials have yet proven that these drugs reduce cancer risk in humans."

People with familial polyposis are an exception. Although studies have not shown that NSAIDs actually prevent cancers in these people, recent research has proven they can reduce the formation and growth of precancerous polyps.

NSAIDs cause some side effects ? such as stomach irritation, ulcers and bleeding ? so solid evidence that the benefits outweigh these risks is needed before doctors should advise people to take them for long periods of time, Dr. Thun says. "Neither the precise mechanism by which these drugs inhibit cancer nor the optimal dose and regimen have yet been defined," he says.

It?s not unusual to see conflicting results from early studies, as with the two case-control studies on NSAIDs and breast cancer, Dr. Thun adds. Such disagreement is often the result of subtle differences in the people included in the studies, and the way the data were collected and analyzed.

"The possibility that NSAIDs may help to treat and prevent recurrence of breast cancer needs to be investigated in randomized clinical trials," Dr. Thun says. "Only then will we know whether the NSAIDs are a safe and effective way to reduce breast cancer risk."

Randomized clinical trials are more difficult to organize and more costly to conduct, but their results are more reliable in assessing the effectiveness of cancer prevention strategies.