Non prescription chlamydia treatment sildenafil affects the response to sexual stimulation. It acts by enhancing smooth muscle relaxation using nitric oxide, a chemical that is normally released in response to sexual stimulation. This smooth muscle relaxation allows increased blood flow into certain areas of the penis, which leads to an erection. Viagra is contraindicated in patients taking another medicine to treat impotence or using a nitrate drug for chest pain or heart problems. This medicine should not be taken by women and children as well as in patients with a known hypersensitivity to any component of the tablet.