Viagra (Sildenafil) is a prescription medication used to treat male erectile dysfunction.  Viagra works by helping arteries in the penis relax and widen. This allows for more blood flow to enter the penis. As the arteries in the penis expand and harden, veins that normally carry away blood flow to the penis are compressed resulting in an erection. When nerves or blood vessels aren't working properly, a man may not be able to achieve an erection.  Viagra appears to work for 85% of the men who use it, however it may take longer to act depending on whether you eat a high fat meal with administration. For research purposes, rats are tested with the product to judge the amount of blood pressure and blood flow during the administration of Sildenafil.