The Food and Drug Administration said today that it had asked Pfizer Inc., the world's largest drug maker, to amend its warnings on Viagra in response to scattered reports of vision loss by people taking the drug. Some blindness was reported by 38 men taking Viagra - a tiny fraction of the 23 million people who have used the drug - and among four men taking Cialis, a newer competitor. None of the 100 or so clinical trials on Viagra have suggested that the drug causes vision loss, but the F.D.A. said it had suggested updating the drug's warning label...