Viagra (sildenafil citrate) is a potent phosphodiesterase inhibitor that helps men suffering from impotence to regain their sexual ability and perform during the entire period of Viagra efficiency. Viagra works by increasing the blood flow into the tissues of the penis allowing the patient to achieve a fuller erection. The erection is later most likely to go away as usual, except for the rare cases of a prolonged erection called priapism that may be very painful and result in permanent damage to the tissues of the penis. Some of the most common side effects reported by patients taking this medicine include heartburn, headache, nosebleeds, flushing, diarrhea, light sensitivity, changes in color vision, insomnia and muscle aches. Nonarteritic anterior ischemic optic neuropathy is one of the serious side effects of taking Viagra. The risk factors for developing this condition include smoking, being older than 50, having high cholesterol, heart disease, diabetes or high blood pressure.