Though safe and effective for most users, Viagra may produce a number of undesirable side effects. The most common of these effects include acid indigestion, headache, blurred vision, sensitivity to light, nasal congestion, diarrhea, and urinary tract infection. A large number of less common side effects have also been reported by some users, but men with previously existing cardiac problems seemingly incur the greatest danger of all Viagra users. In fact, the manufacturer of the drug cautions doctors against prescribing Viagra to anyone who has recently experienced a heart attack or stroke, or those with serious arrhythmias, severe high or low blood pressure, or history of heart failure. The reason for such an advisory is that heart attack, stroke, and other dangerous cardiac conditions have all been reported after use of Viagra, chiefly in persons previously exhibiting cardiac risk factors and sometimes resulting in death.