If you read the headlines you would think that Viagra causes Sexually Transmitted Diseases (STD). "Sex Diseases Tripled in Men 40 or Older Taking Viagra, Cialis, Study Says". The Sun Times in Chicago headline read, "Older Viagra Users More Likely to Get STDs".
For teachers of statistics, this is one more case of correlation caused not by causation, but a common cause.
I found the article at a George Mason web site, which points out that the media headlines missed the point.
"Media coverage reveals a classic confusion between causation and correlation, as they implied that Viagra results in greater risk for older men. But if anything, the study suggested just the opposite: Men who are interested in Viagra have riskier sex lives.."