It's not the first study that finds the lowly aspirin may protect against the deadliest kind of skin cancer, but it is one of the largest.
And it adds to a mounting pile of studies suggesting that cheap, common aspirin lowers the risk of many cancers — of the colon, breast, esophagus, stomach, prostate, bladder and ovary.
The new study, in the journal Cancer, looked at melanoma in 60,000 post-menopausal Caucasian women. (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, tells Shots. "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 NSAID painkillers, such as ibuprofen and naproxen, did not have a lower risk of melanoma — a point whose possible significance we'll come back to in a moment.
Tang says the study accounted for the women's sun exposure and other lifestyle factors, such as smoking, that increase the melanoma risk.
The data came from the 22-year-old, federally financed Women's Health Initiative. But this study merely observed whether women chose to take aspirin or not, and then correlated that with whether they got melanoma. This kind of "observational" study doesn't prove anything.
"We would have to do a large clinical trial, randomizing women to receive aspirin versus placebo, following them for 10-plus years," Tang says.
She acknowledges that type of gold-standard study isn't likely to happen because "it's just too expensive to do."
So this study (like so many others) leaves open the question: What should anybody do?
"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."
She says "high risk" means anybody who's already had skin cancer — either melanoma or less dangerous kinds; either is an indicator of how much sun damage the skin has had.
People who sunburn easily and don't tan so much are also at higher risk of melanoma.
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.
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 Cox-2 that controls inflammation.
But Jacobs thinks something else is going on. He thinks aspirin may reduce cancer by inhibiting platelets, 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.
But skeptics like Jacobs agree with enthusiasts like Tang and Harris on one point: "The worst thing," Tang says, "would be [for people to think], 'I can take aspirin, and that justifies me doing indoor tanning.' That is not the right message."