On February 12^th, a man from Arkansas filed a lawsuit against Eli Lily and Co., the manufacturers and distributors of the drug tadalafil, also known commercially as Cialis, alleging that the drug was the cause of his melanoma and that he could have avoided the risk of skin cancer if the drug had proper warnings.

In the lawsuit, filed in federal court in California, the plaintiff alleged that Eli Lily and Co. omitted to mention or warn that data showed a link between an increased risk of developing melanoma with taking its popular erectile dysfunction medication.

Cialis was approved by the U.S. Food and Drug Administration (FDA) in 2003, and in 2013, Eli Lily and Co. reported record sales of over $2 billion. Yet, there has been no mention on Cialis’ packaging warning of the increased link of developing melanoma.

According to data in the Cialis lawsuit, the mechanism of the drug’s action is to inhibit an enzyme in the body known as PDE5. The successful inhibition of this enzyme relieves the consumer of his erectile dysfunction. But inhibiting this enzyme has also been shown to cause “cell mutation cultivating melanomagenesis, or the creation of melanocytes which develop into melanoma.”

A 2014 study, published by the Journal of the American Medical Association(JAMA), “Sildenafil Use and Increased Risk of Incident Melanoma in U.S. Men,” found that men who used sildenafil, also known as the drug Viagra, had almost twice the risk of developing melanoma compared to men who had never taken the drug.

Through this lawsuit, the plaintiff joined multidistrict litigation (MDL) No.2691, In re: Viagra and Cialis Products Liability Litigation, which has been set up in the Northern District of California to allow for federal court streamlining.

This litigation may only just be the beginning. Similar lawsuits are being filed against other erectile dysfunction medications and manufacturers, including Viagra, Revatio, and Adcirca. PDE5 inhibitors have also been found in other drugs prescribed for a high blood pressure condition known as pulmonary arterial hypertension. In June 2016, the FDA announced that it would study the possible link between melanoma and drugs like Viagra and Cialis.