Function cialis and viagra Cialis and Viagra are PDE5 inhibitors that help to relax the muscles and increase blood flow. Viagra is the brand name for the drug sildenafil, Cialis is the brand name for tadalafil, and Levitra is the brand name for vardenafil. Each of these drugs works in much the same way. The drugs are considered PDE5 inhibitors. This class of drugs helps to relax the muscles and increases blood flow to the body. This makes it easier for blood to get into the penis, and allows people with erectile dysfunction (ED) to experience a lasting erection during sexual activity. The action of the drugs is relatively fast. Manufacturers of PDE5 inhibitors recommend that users take the pill at least 30 minutes to 1 hour before sexual stimulation. How long these drugs last is one of the key differences between them: Viagra is a drug that is used only when needed. The drug is taken 30 minutes to 1 hour prior to sexual activity, and the effects typically last for 3-5 hours. Levitra may start working faster than Viagra, but lasts for about the same amount of time. A similar dose of Cialis may be felt for up to 36 hours. For this reason, people looking for a long-lasting drug may choose Cialis.