The inhibitory effect of these medications is critical in ensuring that an erection occurs. The enzyme PDE5 prevents cGMP (cyclic guanosine monophosphate) from being formed, which is bad because cGMP is responsible for commanding the smooth muscles of the penis to relax and allow more blood to flow into the spongy corpus cavernosum.
PDE5 inhibitors, such as Sildenafil Viagra, help keep cGMP in the penile tissues for up to 4 hours by stopping PDE5 from destroying it.
Since the erectile response is mediated by increased blood flow in the penile tissues, the presence of cGMP must be preserved in order to achieve and sustain an erection.