WASHINGTON - The research could give you whiplash: Aspirin prevents cancer, one study says. Oops, maybe not, says another.
Now comes word that aspirin may fend off cancer only if people take much more than is used to fight heart disease, suggesting some of the earlier back-and-forth may have been due to confusion over the right dose.
Even that evidence is circumstantial, offering no end to the competing headlines.
"A general perspective that people have is, â€˜Why is it so difficult to get a clear answer on a pill that costs a few pennies and is available over-the-counter and taken by millions of people?'" says the American Cancer Society's Dr. Michael Thun, a coauthor of the newest study.
For decades, scientists have chased the hope that aspirin could be an easy way to prevent certain cancers. The idea: Aspirin fights inflammation, and thus pain, by inhibiting substances known as cyclooxygenase, or COX, enzymes. COX enzymes also are involved in the formation of certain kinds of tumors, such as colorectal, prostate and breast cancers.
Aspirin does something else, as well: It makes blood less likely to form clots, giving it an important role in fighting heart disease. A daily baby aspirin - 81 milligrams - is recommended for people with cardiovascular disease or who are at high risk for it.
In contrast, connecting the cancer dots - showing that reducing COX would in turn reduce tumors - is vexing. And because aspirin can cause stomach ulcers and bleeding, firm proof of an anticancer benefit is a must before any health group will recommend using it for that reason.
The hints of that benefit are tantalizing.
"Aspirin and cancer's not going to go away, and there's great value in figuring out how to use it," says Dr. Phillip Febbo, an oncologist at Duke University Medical Center who is closely following the research.
Aspirin and similar anti-inflammatory drugs reduce the risk of many cancers when tested in animals bred to develop human-like tumors. Evidence in people isn't nearly so clear-cut.
In so-called observational studies, people who say they regularly use aspirin seem at lower risk of colorectal cancer in particular, and also prostate, breast and a few other cancers. However, people who take aspirin on their own may be healthier than the general population and thus at lower risk of cancer anyway.
Some more rigorous trials randomly assigned people at high risk of colon cancer to use aspirin, and found those who did developed fewer precancerous growths called polyps - but stopped short of showing the drug really prevented cancer, Thun explains. More disappointing news came in 2005, when a major study that assigned women to use either a baby aspirin or dummy pills found no effect on a whole list of cancers.
Thun and colleagues at the cancer society wondered if many of those earlier studies used too low a dose of aspirin to have an anticancer effect. So they examined adult-strength aspirin - 325 mg or more - in a study tracking more than 140,000 people.
Taking an adult-strength aspirin daily for at least five years was associated with a 30 percent lower risk of colorectal cancer, a 20 percent lower risk of prostate cancer, and 15 percent less cancer overall, they reported last week in the Journal of the National Cancer Institute.
That's not proof of aspirin's benefit, either, but it does suggest that clinical trials start looking at higher doses to tease out an answer.
Duke's Febbo says answers also may come from improved testing of the COX enzyme's role in individual tumors. As with other tumor stimulators, there may be subsets of people for whom COX is a big cancer factor, and others where COX, and thus aspirin, won't matter.
Until then, what's the average person to do? Neither the cancer society nor a government committee that sets health guidelines recommends aspirin to prevent cancer, even for those at high risk of colorectal cancer. Consult a doctor first if you're considering it anyway, Thun stresses - aspirin can be dangerous if someone bleeds easily or has certain other conditions.
For now, cancer specialists quote their cardiac colleagues' advice.
"Stick with what's heart-healthy," says Febbo. And stay tuned: "There's a lot of potential for aspirin" and cancer.