Sildenafil and tadalafil are often described as if they “cause” erections. They don't. They protect a chemical signal the body produces during arousal, so that signal lasts longer and works harder. Understanding that one idea explains nearly everything else about the drugs: why they need arousal, why food and timing matter, why the side effects are what they are, and why combining them with nitrates can be fatal. This is the mechanism in plain language.
How an Erection Happens Without Drugs
- Arousal. Sexual stimulation — physical or mental — sends nerve signals to the penis.
- Nitric oxide. Those nerves, and the vessel lining, release nitric oxide (NO) in the erectile tissue.
- cGMP. Nitric oxide activates an enzyme that produces cyclic guanosine monophosphate (cGMP), a messenger molecule.
- Relaxation. cGMP makes the smooth muscle in the erectile chambers and their arteries relax. Arteries widen; the chambers fill with blood.
- Trapping. The swelling chambers compress the veins that drain them, so blood stays in. The erection holds.
- Termination. An enzyme, PDE5, breaks cGMP down. As cGMP falls, muscle contracts, arteries narrow, veins reopen, and the erection subsides.
PDE5 is the off-switch. It's always running, constantly degrading cGMP; an erection happens when production outpaces destruction, and ends when it doesn't.
What the Drug Does
Sildenafil and tadalafil block PDE5. With the off-switch partly disabled, the cGMP that arousal produces isn't broken down as fast. It accumulates to higher levels and stays there longer. The smooth muscle relaxes more completely, more blood enters, and the erection is firmer and more sustained. For a man whose nitric oxide signal is weakened — by vascular disease, diabetes, age — the drug makes the reduced signal go further.
Where Everything Else Comes From
Timing and food
The drug has to be absorbed into the blood and reach the erectile tissue before it can block PDE5 there. Swallowed tablets go through the stomach, intestine and liver, which takes 30–60 minutes and is slowed by food, particularly fat, for sildenafil. Tadalafil's absorption is less food-dependent. Sublingual formulations absorb through the mouth and skip the gut.
Duration
The drug keeps working for as long as enough of it remains in the blood to block PDE5. Sildenafil's half-life is about 4 hours, giving a 4–6 hour window. Tadalafil's is about 17.5 hours, giving up to 36. Same mechanism, different molecule, different pharmacokinetics.
Side effects
PDE5 isn't only in the penis. It's in vascular smooth muscle throughout the body, in the lungs (where sildenafil is also used for pulmonary hypertension), and in the gut. Blocking it relaxes vessels everywhere a little: dilated vessels in the head cause headache, in the face flushing, in the nose congestion; relaxed muscle at the top of the stomach causes reflux. All are dose-related because more drug blocks more enzyme.
Vision
PDE6 is a closely related enzyme in the retina involved in colour signalling. Sildenafil blocks it weakly, causing a temporary blue-green tint in some men at higher doses. Tadalafil is more selective for PDE5 over PDE6 and rarely does this.
Tadalafil and the prostate
PDE5 is also present in prostate and bladder smooth muscle. Daily tadalafil relaxes it, which is why 5mg daily is approved for urinary symptoms of an enlarged prostate.
Get Sildenafil or Tadalafil Through Roman →FDA-approved generics · Free evaluationWhy Nitrates Are Dangerous
Alpha-blockers lower blood pressure by a different route, so the combined effect is additive rather than multiplicative; it's manageable with dose and timing, which is why they're a caution rather than a ban.
Why Some Men Don't Respond
The mechanism also explains failure. If the nerve signal is severely damaged (advanced diabetic neuropathy, nerve injury from prostate surgery), too little nitric oxide is released and there's not enough cGMP to amplify. If arteries are severely narrowed, relaxing them isn't enough to admit sufficient blood. If the venous trapping mechanism has failed structurally, blood leaks out regardless. And if testosterone is very low, the enzymes that make nitric oxide are downregulated. In each case the drug is working on its target; the target isn't sufficient. That's when a urologist's alternatives — injections that act directly on the muscle, devices, implants — come in. See When to See a Urologist.
Sildenafil vs Tadalafil, Mechanistically
| Sildenafil | Tadalafil | |
|---|---|---|
| Target | PDE5 | PDE5 |
| Half-life | ~4 hrs | ~17.5 hrs |
| Window | ~4–6 hrs | Up to ~36 hrs |
| PDE6 (retina) effect | Weak; blue tint possible | Minimal |
| PDE11 (muscle) effect | Minimal | Weak; possible link to back ache |
| Food sensitivity | Significant (fat) | Minimal |
| Daily low-dose use | No (for ED) | Yes |
| BPH indication | No | Yes (5mg daily) |
Our Verdict — September 2026
PDE5 inhibitors protect the body's own erection signal rather than replacing it, which is why they need arousal, why they're safe for most men, and why they're lethal with nitrates. Knowing the mechanism makes you a better patient: you'll time the dose, expect stimulation to matter, understand the headache, and never touch a nitrate. Get the drug from a provider whose evaluation checks for the one interaction that matters; Roman's does.
Frequently Asked Questions
What does PDE5 stand for?
Phosphodiesterase type 5, an enzyme concentrated in penile smooth muscle that breaks down cyclic GMP, the messenger molecule that keeps the tissue relaxed and blood flowing in during an erection.
How does sildenafil cause an erection?
It doesn't, directly. It blocks PDE5 so that the cGMP produced during arousal lasts longer and builds higher, making the smooth-muscle relaxation stronger and more sustained. Without arousal there's no cGMP to protect, so nothing happens.
Why do I need to be aroused for it to work?
Because the drug acts downstream of the nitric oxide signal that arousal triggers. It amplifies the signal; it doesn't generate one.
Why does sildenafil cause headaches and flushing?
PDE5 exists in blood vessels throughout the body, not just the penis. Blocking it relaxes vessels elsewhere, causing headache, flushing, congestion and a small drop in blood pressure.
Why are nitrates dangerous with sildenafil?
Nitrates work by releasing nitric oxide, the start of the same pathway. Combined with a drug that blocks the pathway's off-switch, the result is uncontrolled vessel relaxation and a potentially fatal drop in blood pressure.
Why does sildenafil affect vision?
Sildenafil has a weak effect on PDE6, a closely related enzyme in the retina, which can cause temporary blue-tinted vision. Tadalafil is more selective and rarely does this.
Why does tadalafil last longer?
It's a different molecule with a much longer half-life (about 17.5 hours vs 4). It stays in the body long enough to keep blocking PDE5 for up to 36 hours.