Viagra is the brand name for the drug sildenafil, Cialis is the brand name for tadalafil. Erectile Dysfunction (ED) drugs all act in a similar way. They are all PDE5 inhibitors. This means they help to relax the muscles and increase blood flow to the body.This makes it easier for blood to get into the penis, and it allows people with ED to experience a lasting erection during sexual activity.The drugs act relatively quickly. Manufacturers of PDE5 inhibitors recommend taking the pill at least 30 minutes to 1 hour before sexual stimulation.Viagra is used only when you need it. A person takes the drug 30 minutes to 1 hour before sexual activity, and the effects typically last for 3-5 hoursThe effect of a similar dose of Cialis may last up to 36 hours. For this reason, people looking for a long-lasting drug may choose Cialis.