The PDE5 inhibitors sildenafil (ViagraR), vardenafil (LevitraR) and tadalafil (CialisR) are drugs which are taken orally. They work by blocking the action of PDE5, which causes cGMP to degrade. cGMP causes the smooth muscle of the arteries in the penis to relax, allowing the corpus cavernosum to fill with blood. That in turn causes erection. PDE5 (cGMP specific phosphodiesterase type 5 is an enzyme (EC 126.96.36.199) from the phosphodiesterase class). PDE5 inhibitors (A phosphodiesterase type 5 inhibitor is a drug used to block the degradative action of phosphodiesterase type 5 on cyclic GMP in the smooth muscle cells lining the blood vessels supplying the corpus cavernosum of the penis). cGMP (Cyclic guanosine monophosphate is a cyclic nucleotide derived from guanosine triphosphate (GTP)).