You should take your Viagra tablet about one hour before you plan to have sex. It helps you to get and maintain an erection by helping more blood get into your penis when you are sexually aroused. It lasts 4 to 5 hours, or longer if you have mild to moderate ED. Tadalafil Cialis. In pulmonary hypertension it works by relaxing blood vessels in the chest.Viagra is a drug that helps treat erectile dysfunction. It improves blood flow to the penis, and its effects can last for up to 4 hours, although the effect will likely be stronger after 2 . Sildenafil under the name Viagra is used to treat erectile dysfunction (impotence) in men. Another brand of sildenafil is Revatio, which is used to treat pulmonary arterial . Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes .