How does Viagra work?

Written with
Amanda Barge

‘Phosphodiesterase 5’ (PDE5) inhibitors are ED pills taken to help men get an erection if they have erectile dysfunction. 

PDE5 inhibitor drugs include: 

  • Sildenafil (brand name: Viagra)
  • Tadalafil (brand name: Cialis)
  • Avanafil (brand name: Stendra)
  • Vardenafil (brand names: Levitra and Staxyn)

Although these famous pills are used by lots of men all over the world, not many of us actually know how they work. 

That's because things get a little complex when understanding how these PDE5 inhibitors help with ED, so Urologist Ramesh Krishnan helps us break it down below. 

Getting an erection

To get our heads around how ED pills work (and why they may or may not work for you), we need to start by understanding how men get erections. 

The key parts to getting erect:

  1. Your brain senses sexual arousal (this could be from physical touch or other mental stimulation like sight or thoughts)
  2. Nerve signals are sent to the penis
  3. These signals release chemicals (cGMP), which tell the muscles in the penis to relax
  4. This allows the erectile bodies in the penis to fill with blood, and an erection is formed 
  5. Afterward, the PDE enzyme kicks in, to tell your erection to end

If you’d like to get more stuck into the science, read the sections below. If not, you can skip to the section about PDE5 inhibitors

cGMP & smooth muscle relaxation: 

cGMP gets the erection party started.  

When someone is stimulated and becomes aroused, signals are sent from the brain to the penis, which releases a chemical called ‘cGMP’.

cGMP causes ‘smooth muscle relaxation’. In the penis, when smooth muscle cells relax, it allows more blood in, becomes hard, and an erection can form. 

Erectile bodies in the penis & blood flow:

Your penis is a pretty amazing organ that can take on very different forms. One minute it can be floppy and relaxed, and the next minute it can be hard and excited.

It’s able to do this because of the parts of the penis called the ‘erectile bodies’: 

  • ‘Erectile bodies’ in the penis are made up of spongy tissue called ‘the corpora cavernosum’ and a stretchy covering called ‘the tunica’
  • When you get an erection, blood fills the spongy tissue in the penis
  • This allows it to get hard, and increases the length and girth of the penis
  • The stretchy covering then stretches and traps the blood in the spongy tissue
  • This stops the flow of blood out of the spongy tissue in the penis – helping you to maintain your erection 

PDEs & your erection going down:

Once you’ve climaxed (or you’re just not feeling aroused anymore), ‘Phosphodiesterases’ (PDEs) close the erection party down. 

PDEs are enzymes that break down cGMP and signal for your erection to switch off, and for normal life to resume. Once they’ve done their job, blood will flow out of the penis and the smooth muscle cells will go back to normal. 

Without PDEs, men would be walking around with erections all the time. Awkward.

PDE5 inhibitors 

PDE5 inhibitors work in 2 ways: 

1. Getting an erection

PDE5 inhibitors block signals from the PDE enzyme, which allows more cGMP to hang around for longer (remember, cGMP is what causes smooth muscle relaxation, which leads to increased blood flow).

Therefore, in short, PDE5 inhibitors help smooth muscle relaxation occur.

This leads to ‘vasodilation’ (the smooth muscle relaxation allows for the blood vessels to widen, which increases blood flow around the body, including to the erectile bodies in the penis).

So, they can help men get an erection.

2. Keeping an erection

The increased blood flow from taking PDE5 inhibitors leads to a firmer and harder erection.

A firmer, harder erection stretches the tunica more, helping to trap in the blood.

So, this allows the erection to last longer, as less blood escapes out.

Erectile dysfunction (ED) & PDE5 inhibitors 

If you have ED, your body or brain might block the natural part of the erection process, and won’t send the blood to the penis (which we now know is needed to get erect).

PDE5 inhibitors were designed for physical ED, but not for psychological ED. We’ll explain why below. 

Mojo aims to provide useful wellbeing resources to its users; however, you should not solely rely on opinions or advice available on the Website or given by the Community. Always seek advice from a qualified medical doctor or other healthcare professional before acting.

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