Prescribed for erectile dysfunction or, more likely in recreational use, ordered from offshore pharmacies with websites, Viagra® (and its relatives like Cialis® and Levitra®) are vasodilators. These pharmaceutical products are designed to lower blood pressure and increase the flow of blood to the extremities like, duh, our dicks.
Side effects of Viagra® are pretty simple: headache, dizziness and, for some guys, a bluish-purple tint to the vision. Serious risks include severe drops in blood pressure and priapism, and hard-ons lasting more than 3 hours. Get thee to the emergency room if you have the latter going on as tissue damage can occur, damage that might make your dick not quite your best friend in the future.