The short answer: sildenafil blocks an enzyme called phosphodiesterase type 5 (PDE5) that normally winds down the blood-flow response behind an erection. With that enzyme quietened, blood vessels in the penis relax more fully and stay open for longer when you are aroused. It amplifies a natural process; it does not replace it.
What is sildenafil?
Sildenafil is a medicine used to treat erectile dysfunction, and it belongs to a group of drugs called PDE5 inhibitors. It first reached the market as a branded sildenafil tablet in the late 1990s, and since the patent expired it has been widely available as a lower-cost generic. The NHS describes sildenafil as a treatment for erection problems and, at a different dose, for pulmonary hypertension. In everyday use for erectile dysfunction it comes as 25mg, 50mg and 100mg tablets, with 50mg the usual starting dose.
An erection is a blood-flow event
When you are sexually aroused, nerve signals trigger the release of nitric oxide in the penis. That sets off a chemical chain reaction ending in a messenger molecule called cGMP, which relaxes the smooth muscle in the walls of the penile arteries. Relaxed muscle means wider vessels, more blood flowing in, and the spongy erectile tissue filling and stiffening. The swelling also compresses the veins that drain blood away, which is what keeps the erection maintained.
In many men with erectile dysfunction, the signalling works but the response is too weak or fades too quickly, often because the vessels themselves are less responsive than they used to be.
Where PDE5 comes in
Your body does not want an erection to last indefinitely, so it also produces an enzyme, PDE5, whose job is to break down cGMP and switch the process off. Sildenafil inhibits PDE5. With less of the enzyme active, cGMP builds up and persists, the smooth muscle stays relaxed, and the erectile response is stronger and easier to sustain.
This is why the whole class of medicines, which also includes tadalafil, is called PDE5 inhibitors. If you are weighing the two options, our sildenafil versus tadalafil comparison sets out the practical differences.
Arousal is still required
This is the most misunderstood part. Sildenafil raises cGMP levels only where the nitric-oxide signal has already started, and that signal comes from arousal. Take a tablet and sit watching the news, and typically nothing happens. The medicine widens the window in which your body can respond; it does not press the button itself. Most men find this reassuring once they understand it: there is no loss of control, and no erection arriving unbidden.
Onset, duration and food
Sildenafil is usually taken 30 to 60 minutes before sex. It reaches peak levels in the blood at around an hour on an empty stomach, and remains active for roughly 4 hours, though the strongest effect is in the first couple of hours. A large or fatty meal slows its absorption noticeably, which is one of the more common reasons a dose seems not to work. We cover the practical timing questions in our guide to how long sildenafil lasts. The manufacturer's product information confirms that food, especially a high-fat meal, can delay the onset of effect.
Prescription-only, and why
For erectile dysfunction in the UK, sildenafil is a prescription-only medicine at the doses prescribers typically use. That is not bureaucracy for its own sake. Erectile dysfunction can be an early sign of cardiovascular disease, some medical conditions and medicines make PDE5 inhibitors unsafe, and the right dose varies between people. A UK prescriber checks your health history, blood pressure and current medicines before deciding whether it is appropriate, and reviews how you get on.
A pharmacy-supplied version of sildenafil can also be bought after a consultation with a pharmacist, but the same safety checks apply, and anyone with heart disease, low blood pressure or relevant medicines is directed to a doctor first.
Common side effects
Because sildenafil relaxes blood vessels throughout the body, not only in the penis, its side effects follow directly from how it works. The most common are headache, facial flushing, a blocked or runny nose and indigestion, and some men notice a mild blue tinge to their vision or sensitivity to light, which reflects a minor effect on a related enzyme in the eye. These effects are usually mild and short-lived, easing as the dose wears off. We set out the full picture in our guide to ED treatment side effects. Seek urgent medical help for an erection lasting more than four hours, chest pain during sex, or a sudden change in vision or hearing.
Does sildenafil work for everyone?
For most men with erectile dysfunction, sildenafil helps, but response varies and a first dose that disappoints does not mean it has failed. Absorption, timing, arousal, alcohol and the meal beforehand all matter, and some men simply need a higher dose or several attempts before it settles. If it still does not work after a fair trial at the right dose, that is worth reviewing rather than abandoning: a prescriber may adjust the dose, suggest a longer-acting option such as tadalafil, or look for an underlying cause. Understanding the mechanism helps here, because it explains why arousal, timing and food each make a difference to whether a given dose works.
What sildenafil does not do
It is worth being clear about the limits. Sildenafil does not increase sexual desire; it acts on blood vessels, not on the brain circuits behind libido. It does not treat the underlying cause of erectile dysfunction, whether that is vascular disease, diabetes, low testosterone or anxiety, so it manages the symptom while the cause is addressed separately. And it does not work for everyone: a meaningful minority of men need a dose adjustment, a different medicine, or investigation of something the tablet cannot fix. None of that makes it a poor treatment; it makes it a treatment with a specific, well-understood job.
