There was a recent cohort study that showed an association, not a causal relationship, between Viagra use and melanoma. The same authors used the same cohort study - the Physician's Health Study - to show an association between men treated for prostate cancer and melanoma risk. However, men with melanoma are not at increased risk for prostate cancer. One hypothesis is that men treated for prostate cancer are more likely to use ED meds, which may raise the risk of melanoma, but this has yet to be proven. Keep in mind that the risk of melanoma is very low in the population, even among men taking Viagra, so this is not all that worrisome. It does seem prudent to check for signs of melanoma. I do this anyway because I am fair skinned, have had many sunburns as a child, and have had several basal cell carcinomas - all risk factors for melanoma.
Here are abstracts of the relevant analyses for any who are interested:Sildenafil Use and Increased Risk of Incident Melanoma in US MenPersonal History of Prostate Cancer and Increased Risk of Incident Melanoma in the United States