Sildenafil Uses: What It Treats and How It Works

What sildenafil is prescribed for, how it works in the body, how long it lasts and which dose is right — reviewed by UK clinicians.

Sildenafil is mainly used to treat erectile dysfunction (ED) by increasing blood flow to the penis during arousal. It is also prescribed, under the brand Revatio, for pulmonary arterial hypertension. Sildenafil is the active ingredient in Viagra and works within 30–60 minutes, lasting up to 4 hours.

Part of the Erectile Dysfunction Treatment Guide.

What is sildenafil used for?

Sildenafil belongs to a group of medicines called PDE5 inhibitors (phosphodiesterase type 5 inhibitors). In the UK it is licensed for two conditions:

• Erectile dysfunction (ED) — the main use. Taken as a 25mg, 50mg or 100mg tablet before sex, sildenafil helps men get and keep an erection firm enough for intercourse. This is the use most people know as Viagra.
• Pulmonary arterial hypertension (PAH) — under the brand name Revatio (usually 20mg three times daily), sildenafil relaxes blood vessels in the lungs to lower pulmonary blood pressure and improve exercise capacity.
• Raynaud's phenomenon (off-label) — specialists occasionally prescribe sildenafil off-label to improve circulation in severe Raynaud's, although this is not a licensed use.

This guide focuses on sildenafil for erectile dysfunction. ED affects roughly half of men aged 40–70 to some degree, and sildenafil is usually the first tablet treatment offered because it is effective, well studied and inexpensive as a generic.

How does sildenafil work?

An erection is a blood-flow event. During arousal, nerves in the penis release nitric oxide, which triggers production of a messenger chemical called cGMP. cGMP relaxes the smooth muscle in the walls of the penile arteries, letting blood flow in and become trapped in the erectile tissue. An enzyme called PDE5 constantly breaks cGMP down — in many men with ED, it breaks it down faster than it can build up.

Sildenafil works by blocking PDE5. With the enzyme inhibited, cGMP levels stay higher for longer, blood vessels stay relaxed, and the natural erection response becomes stronger and easier to maintain.

How long does sildenafil take to work — and how long does it last?

Most men find sildenafil starts working within 30–60 minutes of taking a tablet. Blood levels peak at around the one-hour mark, and the effect then gradually tails off, giving a typical window of up to 4 hours (sometimes 5) in which getting an erection is easier.

Tips for getting the best from each dose

• Take it on an empty stomach where possible. A large or fatty meal can delay absorption by an hour or more and blunt the peak effect.
• Allow time. Take the tablet about an hour before you plan to have sex rather than at the last minute.
• Arousal is still needed. The tablet will not produce an erection without sexual stimulation.
• Give it a fair trial. NICE guidance recommends trying a treatment on at least 8 separate occasions at the maximum tolerated dose before concluding it does not work.

Sildenafil doses: 25mg, 50mg, 100mg — what's the difference?

Sildenafil for ED comes in three tablet strengths. The usual starting dose is 50mg, adjusted up or down depending on how well it works and how well you tolerate it. Whatever the dose, the rule is the same: no more than one tablet in 24 hours.

Side effects are generally mild and dose-related — the most common are headache, facial flushing, indigestion, a blocked nose and temporary changes to colour vision. For a full breakdown, see our sildenafil dosage and side effects guide.

Who shouldn't take sildenafil?

Sildenafil is safe for most men, but there are some situations where it must not be taken and others where it needs extra care. This is why a clinical assessment is required before every prescription.

Do not take sildenafil if you:

• Take nitrates in any form, or use poppers
• Have severe heart or liver problems, or have been told sexual activity is unsafe for your heart
• Have had a recent stroke or heart attack (generally within the last 6 months)
• Have very low blood pressure (below 90/50 mmHg)
• Have ever lost vision because of non-arteritic anterior ischaemic optic neuropathy (NAION)
• Have certain rare inherited eye conditions such as retinitis pigmentosa

Take extra care (tell your prescriber) if you have:

• An alpha-blocker prescription (doxazosin, tamsulosin, alfuzosin) — doses may need spacing or lowering
• Kidney or liver impairment, or you take strong CYP3A4 inhibitors such as ritonavir, ketoconazole or clarithromycin
• A deformity of the penis (such as Peyronie's disease), sickle cell disease, leukaemia or multiple myeloma — these raise the risk of priapism
• A stomach ulcer or a bleeding disorder

Sildenafil vs tadalafil: which lasts longer?

Tadalafil lasts much longer. Both medicines work the same way, but tadalafil (the active ingredient in Cialis) stays active for up to 36 hours, compared with around 4 hours for sildenafil. Sildenafil's shorter window suits planned, occasional use; tadalafil suits men who want more spontaneity, and it is also available as a low daily dose.

Neither is "stronger" — head-to-head studies show similar effectiveness, and the best choice comes down to how you prefer to plan sex. You can order tadalafil online through the same consultation process, and our tadalafil vs sildenafil comparison covers the differences in depth. If climaxing too quickly is also a concern, sildenafil does not treat this — see our premature ejaculation treatment page instead.

Getting sildenafil through Access Doctor is straightforward: complete a confidential online consultation, and a GPhC-registered pharmacist independent prescriber will check the medicine is safe for you before it is dispensed and posted in plain packaging. If sildenafil isn't the right fit, longer-acting tadalafil is available through the same process.