A study published in the peer-reviewed medical journal JAMA Internal Medicine showed a possible link between Viagra use and an increased risk of developing melanoma skin cancer.

The study looked at 25,000 men who participated in an earlier Health Professionals Follow-up Study, a Harvard University project that began in 1986. After adjusting for risk factors, men who had used Viagra (sildenafil) had nearly twice the risk of developing melanoma, though there was no risk increase noted for other skin cancers such as basal cell and and squamous cell carcinoma.

There was also no significant link between erectile function by itself and melanoma, Forbes reported.

The study did not include Viagra competitors Cialis (tadalafil) and Levitra (vardenafil) since those medications were not available at the beginning of the study.

“But in their discussion they point out that because these drugs are longer-acting they may potentially result in an even greater increase in risk for melanoma,” according to Forbes.

According to a Harvard health publication, every two years the men who participated in the study filled out health and lifestyle questionnaires. In 2000, the questionnaire began asking about the use of erectile dysfunction drugs. That year, 1,618 men reported having used Viagra.

“Over the next decade, among the 29,929 men who said they had never used Viagra, 128 developed melanoma. Among the 1,618 Viagra users, 14 developed melanoma. In other words, 4.3 of every 1,000 who didn’t take Viagra developed melanoma compared to 8.6 of every 1,000 men who took Viagra.”

Melanoma expert Dr. Hensin Tsao of Massachusetts General Hospital told the Harvard health publication that older men are already at a greater risk for developing melanoma and dying from it.

“An estimated 76,000 Americans (more than half of them men) will be diagnosed with melanoma this year, and almost 10,000 will die from it,” according to the Harvard health publication.

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

Other melanoma risk factors such as a family history of skin cancer, ultraviolet light exposure, hair color, and mole count were taken into account and did not affect the Viagra skin cancer findings, the authors said.

Melanoma is usually caused by ultraviolet radiation, typically from sunshine, resulting in a genetic defect that leads skin cells to rapidly multiply and form malignant tumors.

Melanoma is also the deadliest form of skin cancer. The American Cancer Society estimates that 76,100 new cases of invasive melanoma will be diagnosed in 2014, 43,890 in men and 32,210 in women.

If melanoma is recognized and treated early, it is almost always curable, but if it is not, the cancer can advance and spread to other parts of the body, where it becomes hard to treat and can be fatal.