Women who take aspirin for five years are 30% less likely to develop melanoma.

Now in its 22 year, the Women’s Health Initiative continues to produce interesting news and associations. The latest study published in the March 2013 issue of Cancer reports on melanoma incidence in 60,000 post-menopausal Caucasian women. Caucasian women were selected because light skinned people have the highest risk of this cancer. Over a 12-year period, women who took aspirin at least a couple of times a week had a 20 percent lower risk of developing melanoma.

“We’re really excited aspirin could be used as a potential preventive agent for melanoma,” Dr. Jean Tang of Stanford University Medical School, the study’s senior author reported to Medscape. “In terms of cancer prevention, a lower melanoma risk by 20 percent is very large and significant.”

Tang says women who regularly took aspirin for five years or more had a 30 percent lower risk.

“There’s nothing else that I know of that has as large an effect as what we’re seeing with aspirin,” Tang says.

Most women in the study took regular doses of aspirin, not baby aspirin. Interestingly, women who took other so-called painkillers, such as and , did not have a lower risk of melanoma — a point whose possible significance we’ll come back to in a moment.

“I know you want to pin me down and say should women go out and take aspirin for melanoma,” the Stanford researcher says. “In somebody who’s at high risk for melanoma, I would say that taking aspirin is a good idea.”

But opinion is split on whether the evidence is strong enough to justify regular use of aspirin to prevent melanoma — or any type of cancer.

“There’ve been about eight studies that have looked at melanoma, and about half of them find slightly lower risk and half find no connection at all,” says Eric Jacobs, an epidemiologist at the American Cancer Society. “You look at the totality of the evidence, and right now it’s rather mixed.”

Jacobs says it’s important to remember that aspirin is a real drug with real side effects. It can cause serious and even fatal stomach bleeding, even at low doses. The official standing of the American Cancer Society is to not endorse aspirin for cancer prevention.

But Dr. Randall Harris of Ohio State University thinks it’s reasonable for people to take some aspirin for its anti-cancer effects. He’s used Women’s Health Initiative data to show that aspirin may reduce the risk of breast cancer.

“You don’t need to take too much,” Harris tells Shots. “You just need to reset the inflammatory mechanism, in my opinion. And so you can get by with just a couple of tablets a week. That’s what I do — been doing it for a long time.”

Harris is pretty convinced that aspirin prevents cancer by damping down a master gene called that controls inflammation.

But Jacobs thinks something else is going on. He thinks aspirin may , blood components that promote clotting.

“We know that activated platelets release substances that can encourage cancer growth and development,” Jacobs says.

The anti-platelet effect could explain why the new study did not find a reduced risk of melanoma from anti-inflammatory drugs such as ibuprofen. These NSAIDs don’t inhibit platelets the way aspirin does.

Scientists’ lack of understanding about how aspirin may exert an anti-tumor effect is a caution against its widespread use as a cancer preventive agent.