When a man is sexually aroused, nitric oxide is launched into the erectile tissues of the penis. Alvitra 60 mg nitric oxide stimulates the enzyme known as guanylate cycle which leads to a surge in the system of a chemical compound referred to as guanosine monophosphate (cGMP), which in flip reasons the penile blood vessels to loosen up and fill it with blood. As quickly as the tender erectile tissues fill with blood, an erection is caused.