Deaths of American men who took the wildly popular impotence pill Viagra have climbed to 16, including seven men who reportedly died during or after sex, a reminder that sexual exertion in older men can be risky with or without a pill. The Food and Drug Administration said on Tuesday there is no evidence that Viagra itself is dangerous, although the agency did repeat warnings that heart patients who take nitroglycerin should never take the impotence pill because the drug mix can be deadly. Two deaths suggest that happened.