Sildenafil, Vardenafil, Avanafil and Tadalafil are all variations of a type of drug called a PDE5 inhibitor. PDE5 is an enzyme which can cause blood vessels to tighten and become restrictive, letting less blood through them.
It’s when blood vessels at the base of the penis become tight that erectile problems develop, as blood needs to be able to reach and fill the small muscles in the corpus cavernosum in the penis for an erection to occur.
By blocking the action of PDE5, these drugs enable blood to flow more freely into the penis, making it easier to achieve and keep an erection.