What pill can stop your period the main component of Viagra is Sildenafil Citrate. Sildenafil Citrate 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. Sildenafil Citrate is applied for the treatment of erectile dysfunction (impotence) in men and pulmonary arterial hypertension. Sildenafil Citrate may also be used for other purposes not listed above. The recommended dose of Cialis is 20 mg it should be taken at least 16 minutes before the sexual activity. The effect persists for 36 hours. Take Cialis once daily and do not exceed the recommended dose.