Men who take Viagra for erectile dysfunction may be at almost twice the risk for melanoma, according to a 2014 study.

The study was published in the journal JAMA Internal Medicine. It followed a cohort of over 25,000 men from 2000 to 2010, excluding men who reported cancers at baseline. The study collected information on incidence of melanoma, squamous cell carcinoma, and basal cell carcinoma over ten years.

About Viagra

Viagra was initially developed in the 1980s as a treatment for angina and high blood pressure. It had already been patented for those uses in Britain for seven years when the FDA initially approved it as a treatment for erectile dysfunction in March 1998.

Since then, Viagra has been a phenomenally successful moneymaker for its manufacturer Pfizer Inc. In 2012, Viagra was prescribed 8 million times, resulting in sales of around $2 billion. Pfizer will also continue to enjoy extended patent protection for Viagra, thanks to a 2011 decision by a federal judge. The extension will keep generics versions of Viagra from entering the U.S. market until 2019.

Viagra Study Suggests Increase In Melanoma Risk

Researchers found that men who had used sildenafil, the active ingredient in Viagra, had almost twice the risk of subsequently developing melanoma. They found no similar increase in the risk of the two other types of skin cancer. They also found that those who had used sildenafil at all, not just recently, also had a similarly elevated risk of melanoma. The researchers were able to rule out any possible relationship between the Viagra melanoma and the erectile dysfunction itself.

The researchers concluded that although the results of the study did not warrant any change in clinical recommendations, they did call for further investigation into a possible relationship between sildenafil and melanoma.

Doctors theorize the link may have something to do with the way Viagra inhibits the action of an enzyme called phosphodiesterase. Most melanomas have a particular mutation that also blocks the action of that enzyme. The JAMA study states that low expression of phosphodiesterase by use of sildenafil may increase the invasiveness of melanoma cells.

It’s also unknown whether these study results have any implications for other erectile dysfunction drugs besides Viagra. The study considered the histories only of those who used Viagra, not other erectile dysfunction drugs such as Levitra, Cialis or Stendra. Like Viagra, these newer drugs also block phosphodiesterase. So it’s conceivable they could have a similar relationship with melanoma.

Other Theory Suggests Lifestyle Factor In Viagra Melanoma

It’s also been suggested that these findings might be more related to lifestyle than to Viagra. Some say that since Viagra isn’t cheap, men who take it tend to be wealthier and can therefore afford more leisure time spent outside. That outdoor time increases their sun exposure and with it their risk of developing melanoma.

However, the JAMA study results aren’t fully consistent with that theory. Doctors say increased sun exposure would result in an increase of all types of skin cancer, not just melanoma. But the results of the sildenafil study showed an increase only in incidence of melanoma.