How PDE5 Inhibitors Actually Work, In Plain English
August 9, 2026 · 5 min read
Understanding the mechanism clears up most of the myths, including the biggest one: that these pills work whether or not you're turned on.
Step one: arousal releases nitric oxide
Sexual stimulation — physical, mental, or both — causes nerve endings and the lining of blood vessels in the penis to release nitric oxide. No arousal, no nitric oxide, no chain reaction.
Step two: cGMP relaxes the smooth muscle
Nitric oxide triggers production of cyclic GMP (cGMP). cGMP tells the smooth muscle inside the arteries and the erectile chambers to relax. Relaxed muscle means wider vessels, which means blood rushes in and the chambers expand. The expansion compresses the outflow veins, which traps the blood in place.
Step three: PDE5 shuts it all down
An enzyme called phosphodiesterase type 5 (PDE5) breaks cGMP down. That's normal and necessary — it's how an erection ends. But if PDE5 is too aggressive relative to your cGMP production, the erection never gets firm enough or doesn't last.
Where the medication comes in
Sildenafil, tadalafil, vardenafil and avanafil block PDE5. cGMP hangs around longer, the muscle stays relaxed longer, and blood flow is maintained. Because the whole cascade starts with arousal, the medication does nothing on its own — which is exactly why 'I took it and nothing happened' is often a stimulation or timing issue rather than a dosing failure.
Why side effects happen where they do
PDE5 exists in other tissues too, and related enzymes are found in blood vessels, the retina and the nasal lining. That's the source of flushing, congestion, headaches and the occasional visual color shift — the drug is doing its job in places you didn't ask it to.
The short version
PDE5 inhibitors remove a brake. They don't press the gas — arousal still has to do that part.
This article is general health information, not medical advice, and does not create a patient–provider relationship. Prescription treatment is only available after an evaluation by a licensed provider who determines it is appropriate for you. Do not take a PDE5 inhibitor if you use nitrates or riociguat. Seek emergency care for chest pain, sudden vision or hearing loss, or an erection lasting more than 4 hours. See our Important Safety Information.