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 about the right dose.

Even that evidence is circumstantial, offering no end to the competing headlines.

“A general perspective that people have is, ‘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 co-author 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.

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 anti-cancer 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 following the research.