Where did you hear this? A quick Google search doesn't reveal studies from any reputable sources that back this up. There's a study from some funky internet site that says 28 out of 32,500 Viagra users they looked at got gallstones, but the study is too badly reported to seem legitimate. It doesn't even state the incidence of gallstones in the general population of people fitting the same profile who didn't use Viagra, and on top of that, the men in the study group were all older, were taking many other medications, and also had other co-morbidities.