The researchers, from Stanford University School of Medicine, also found that the longer the participants took aspirin, the lower their risk.
A report on the study is due to appear online this week in the journal Cancer.
Senior author Jean Tang, an assistant professor of dermatology at Stanford, says:
"There's a lot of excitement about this because aspirin has already been shown to have protective effects on cardiovascular disease and colorectal cancer in women."
"This is one more piece of the prevention puzzle," she adds.
Study Based on 60,000 Women In the WHIThe data for the study came from 60,000 Caucasian postmenopausal women aged 50 to 79 across the US taking part in the Women's Health Initiative (WHI).
From this pool, the researchers had access to data about many aspects of participants' lives, including diet, medication, physical activity and history of exposure to the sun.
The researchers chose to study Caucasian women because lighter skin contains less skin pigment, a known risk factor for melanoma.
In the WHI the participants gave information at the start of study and then at periodic follow-up clinical visits.
Also, the investigators actually checked whether the drug was aspirin and not a non-aspirin nonsteroidal anti-inflammatory drug (NSAID) by asking participants to show their medication bottles.
That information helped put the women into one of three groups: those who did not use NSAIDs at all, those who used non-aspirin NSAID, and aspirin users.
The women were followed for about 12 years and note was taken of any cancers they developed.
Dermatologists either evaluated of confirmed any melanomas that developed during the follow-up.
When they analyzed the results, the researchers found the women who took aspirin had a 21% lower risk for melanoma compared to women who did not take it.
They also found that the longer the women took aspirin, the lower their risk for melanoma, with 11% reduced risk if they took it for one year, 22% reduced risk when taking it for between one and four years, and 30% risk reduction if taken for five years or longer.
Implications and LimitationsTang says the findings are important because we know aspirin also has other protective effects in women. If it also reduces melanoma risk, then perhaps it also reduces risk of other cancers, she adds.
The findings also highlight the need to invest more in long-range studies like the WHI, says Tang, because they yield so much information.
Speculating on why aspirin and not other NSAIDs appears to have this protective effect, Tang suggests it is because aspirin uses different molecular pathways than other NSAIDs to reduce inflammation, and it could be something in these pathways that is the key to aspirin's effect on cancer.
Although the results look promising Tang doesn't believe they are strong enough to recommend women should start taking aspirin to avoid developing melanoma.
"We don't know how much should be taken, or for how long, to be most effective," she cautions.
Plus aspirin can produce its own side effects such as stomach upsets, and more serious conditions such as ulcers and bleeding.
The results disagree with other studies that showed taking aspirin every other day with vitamin E made no difference to rate of developing melanoma or any other cancer.
But Tang says perhaps the dose in those studies was too low to show an effect. (She notes that the women who took aspirin in this study were mostly taking regular or extra-strength aspirin, not baby aspirin.)
A criticism that is often levelled at studies that use WHI data is that much of it is self-reported. So a potential weakness of this study is that the researchers are relying on participants' own responses on aspirin intake, sun exposure and other lifestyle factors that could influence the results.
Tang says the only way to prove the link is there is to carry out a "gold standard" clinical trial that links a specific drug or substance directly to melanoma risk.
For instance, the first clinical trial to explore the influence of fish oils on the skin immunity of humans recently reported that consuming omega-3 fish oils can help to prevent skin cancer.
Rates of Melanoma On the Rise GloballyRates of both non-melanoma and melanoma skin cancers have been increasing over the past decades.
Every year around 132,000 melanoma skin cancers occur globally, with younger women and older men carrying the highest risk of developing the disease.
As ozone layers get thinner, the Earth's atmosphere loses more and more of its filter against the sun's UV rays.
The World Health Organization (WHO) says estimates suggest a 10% decrease in ozone levels results in an extra 4,500 melanoma skin cancer cases.
However, the UN agency says although global rates of melanoma continue to increase, much of the cause can be attributed to "recreational exposure to the sun and a history of sunburn," and suggests these "factors lie within each individual's own responsibility".
Funds from the WHI, which is funded by the National Heart, Lung and Blood Institute and the National Institutes of Health, the National Institute of Arthritis and Musculoskeletal and Skin Diseases, a Damon Runyon Clinical Investigator Award and a Selected Professions Fellowship from the American Association of University Women, helped finance the study.
Written by Catharine Paddock PhD