The findings of a long-term study that concluded Viagra (sildenafil) users may be at an increased risk of developing malignant melanoma presents a “biologically plausible mechanism to explain the findings,” according to the American Council on Science and Health, which routinely lambastes “junk studies.”

The study results, published in the peer-reviewed medical journal JAMA Internal Medicine, found that over a 10-year period, Viagra users developed malignant melanoma at twice the rate of men who did not use the wildly popular erectile dysfunction drug. The wide-ranging study followed nearly 26,000 men.

The American Council on Science and Health noted that men with erectile dysfunction (ED) who did not take Viagra showed no increase, ruling out the possibility that the condition itself was a risk factor for higher rates of melanoma.

Melanoma skin cancer expert Dr. Hensin Tsao told Harvard Health Publications that the study is “provocative” and “warrants more careful replication,” but that it’s too soon to definitively say that men should be worried that Viagra causes melanoma, though they should “definitely be worried about melanoma.”

The study came out of lab research looking at how Viagra acts on cell-to-cell signaling pathways. It found that Viagra “mimics key parts of a process that lets melanoma cells spread to other parts of the body.”

To explore whether Viagra might have the same effect on melanoma in humans, a team of researchers looked at data collected from men taking part in the Health Professionals’ Follow-up Study.

Beginning in 1986, the men — medical professionals, including doctors in the United States and China — began completing health and lifestyle questionnaires every two years. The questionnaires started asking men about use of erectile dysfunction drugs in 2000. A total of 1,618 men reported having used Viagra.

Over the course of a decade, 128 men of the nearly 30,000 who said they never used Viagra developed melanoma skin cancer. Of the 1,618 Viagra users, 14 developed the deadly skin cancer.

Viagra users, according to the research data, were 84 percent more likely to develop melanoma than those who did not use the erectile dysfunction drug.

The figure remained steady even when the researchers adjusted for a family history of skin cancer, ultraviolet light exposure in the states where the men lived, other kinds of cancer and major illnesses and other factors.

Melanoma is caused mainly by intense, occasional UV exposure (frequently leading to sunburn), especially in those who are genetically predisposed to the disease. Melanoma skin cancer kills an estimated 9,710 people in the United States annually.

With early detection, there is a high cure rate for melanoma tumors that are surgically removed. The prognosis is grim if the melanoma tumor has deeply penetrated the layers of the skin. Chemotherapy, immunotherapy and radiation therapy are treatment options. In the United States there is a five-year survival rate of 91 percent.

Manufactured by drug giant Pfizer, the U.S. Food and Drug Administration (FDA) approved Viagra to treat impotence — now clinically known as erectile dysfunction — in 1998. In the first year, the drug yielded about $1 billion in sales. Some 30 million men in the United States suffer from erectile dysfunction and 20 million men in the United States have tried Viagra.

While Viagra does not cause a man to become sexually aroused, it increases blood flow to the penis so a man can get and keep an erection needed for sex. The pill must be ingested 30 minutes to four hours hour before sexual activity.