Viagra can promote blood clots and may be dangerous for people already at risk of heart disease and stroke, researchers said yesterday. The findings could explain incidents of fatal cardiovascular disease in a small number of men taking the drug, they said. However, because the study was carried out on blood cells in a test-tube rather than in people, others urged caution. Pfizer, the company that makes Viagra, said there was no evidence that the drug, taken by 20 million men worldwide, increased the risk of heart disease or stroke. Viagra was originally developed as a treatment for angina. It...