Back in June 2014, a skin cancer study published in the peer-reviewed medical journal JAMA: Internal Medicine caused a stir when researchers found evidence that Viagra could increase the risk of melanoma.

The Viagra melanoma study was a prospective cohort study, a type of study where researchers follow a group of test subjects over a long period of time. A group is identified where most of the test subjects are as similar as possible, ideally varying in only a single factor. In this case, researchers tracked a group of nearly 26,000 men through a program called the Health Professional’s Follow-Up Study. Among the factors tracked by this study were drugs they were taking, including Viagra.

When researchers crunched the numbers, they found that patients who took Viagra had an increased risk of melanoma, a type of skin cancer. In fact, researchers found that men taking Viagra had a more than doubled risk of developing melanoma compared to their peers.

In this type of Viagra melanoma study, researchers cannot “prove” that Viagra caused melanoma in these men. In order to prove it to the standard used in science, researchers would have to perform tightly controlled laboratory tests with human subjects. Not only would that type of Viagra study be very expensive, it would also be highly unethical. However, the results of this Viagra melanoma study do have several other factors that increase the power of the results suggesting the erectile dysfunction drug may cause melanoma skin cancer.

In vitro testing, or testing outside of the human body, has suggested that the active ingredient in Viagra can suppress certain systems the body uses to block cancers like melanoma. Additionally, the JAMA Viagra study itself has several factors that lend credence to the idea of a Viagra melanoma risk. Specifically, researchers controlled for several cofounders. This means that researchers performed the same statistical tests on other possible risk factors for melanoma. For example, it is possible that erectile dysfunction increases a person’s risk of melanoma, and the fact that most men taking Viagra have erectile dysfunction could give the appearance that Viagra was increasing the risk, when erectile dysfunction itself could be the risk factor.

However, researchers had enough data to test the association between melanoma and erectile dysfunction itself. When they did, researchers found that untreated erectile dysfunction was not linked to an increased risk of melanoma. Along similar lines, researchers found that patients taking Viagra had an increased risk of melanoma but not other types of skin cancer. This specificity adds further credence to the idea of a Viagra melanoma link.

As a result of this Viagra skin cancer research, many men may choose to file Viagra melanoma lawsuits against the drug’s manufacturer Pfizer Inc. for allegedly failing to warn consumers of this Viagra side effect.