Effective and well understood. The more important question is what the erectile dysfunction is telling you about your arteries.
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Sildenafil is a PDE5 inhibitor. It relaxes the blood vessels supplying the penis, allowing more blood in when there is sexual stimulation.
It does not create desire and it does not work without arousal — a persistent misunderstanding. It enables a normal response rather than producing one.
A lower-dose version is available from pharmacies without a prescription after a consultation with the pharmacist.
Take around one hour before sex. The effect lasts roughly four hours.
A heavy or fatty meal delays and reduces absorption considerably — this is the commonest reason people conclude it has not worked. Take it on a relatively empty stomach.
Alcohol makes it less effective and is worth limiting.
No more than one dose in 24 hours.
It often needs several attempts before you judge it. Guidance suggests trying it up to eight times at an adequate dose before concluding it does not suit you. A great many people abandon it after one disappointing occasion.
Common and usually mild: headache, facial flushing, indigestion, a blocked nose, and dizziness.
Some people notice a blue tinge to their vision or increased light sensitivity. This is a recognised effect of sildenafil specifically, is temporary, and is not dangerous — but it is disconcerting if unexpected.
Seek emergency care for an erection lasting more than four hours. Priapism is rare but can cause permanent damage if untreated.
Also urgent: sudden loss of vision or hearing in one or both ears. Stop the drug and seek immediate assessment.
Care is needed with alpha blockers such as tamsulosin, where doses may need separating.
Nitrates and nitrites — absolute contraindication. This is the one that matters most, and it is why buying PDE5 inhibitors from unregulated websites is genuinely dangerous rather than merely inadvisable.
Other interactions: alpha blockers, some antifungals, certain HIV medicines, clarithromycin, and grapefruit juice.
What should be checked is not the drug but the cause. Blood pressure, HbA1c, cholesterol and testosterone are all worth measuring, because erectile dysfunction is frequently the first presentation of something else.
Yes, where clinically appropriate — but the more valuable part of the consultation is usually not the prescription.
Erectile dysfunction is often an early cardiovascular warning. The arteries supplying the penis are narrower than the coronary arteries, so they tend to show a problem first. ED can precede a cardiovascular diagnosis by several years, which makes it a genuine opportunity rather than simply an inconvenience to be treated.
What that involves: checking blood pressure, glucose, cholesterol and testosterone; reviewing medications, since several common ones cause ED; and asking about the psychological and relationship context, which matters as much as the vascular picture.
What we will not do is issue a prescription without asking why. Someone treated for ED without anyone checking their cardiovascular risk has been sold a tablet rather than seen by a doctor.
This page is information, not an offer to supply.
This is one of the biggest price gaps in UK pharmacy, and it is worth understanding before you spend anything.
Branded Viagra and generic sildenafil contain the same drug. The generic can cost a small fraction of the branded price for the same dose. There is no clinical difference whatsoever.
A 50mg product is also available directly from pharmacies without a prescription, after a consultation with the pharmacist. For a straightforward case that is often the quickest and cheapest route, and we would rather point you to it than take a fee.
Counterfeit erectile dysfunction tablets are among the most commonly seized fake medicines in the UK, and have been found to contain the wrong dose, the wrong drug, or contaminants.
A UK pharmacy that asks you questions is the safe route, and it is not the expensive one.
Sildenafil is taken as needed and can be stopped at any time. There is no withdrawal and no tolerance in the usual sense.
Most apparent failures are technique rather than the drug, and the fix costs nothing:
Tadalafil lasts far longer — up to 36 hours — which removes the need to plan around a tablet and suits many people better. It can also be taken as a low daily dose.
If sildenafil causes headache or flushing, tadalafil or vardenafil may suit you better.
Never take sildenafil with nitrates — GTN spray, isosorbide, or amyl nitrite poppers. The combination can cause a catastrophic drop in blood pressure. This one has no exceptions.
Identical. Same active drug, same dose, a fraction of the price.
Paying for the brand buys packaging.
Usually food, timing, dose or expectation. A heavy meal delays absorption considerably; it needs 30 to 60 minutes; and it requires arousal to do anything at all.
Try it properly eight times at an adequate dose before concluding it failed.
Not with nitrates — GTN spray, isosorbide mononitrate, or poppers. That combination can be fatal.
Most other heart medicines are compatible, but this needs checking rather than assuming.
It can, and that is worth taking seriously rather than being alarmed by. Erectile dysfunction often precedes cardiovascular disease by several years.
It is an opportunity to find and treat risk factors early, which is why the blood tests matter more than the tablet.
Not physically. There is no tolerance and no withdrawal.
Some men do become psychologically reliant on having it available, which is worth discussing, particularly where anxiety is part of the picture.
No. It improves blood flow in response to arousal and does nothing for libido.
If desire is the problem, testosterone, mood, medication and relationship factors are the things to look at.
A recognised, temporary and harmless effect caused by a related enzyme in the retina.
Sudden loss of vision in one eye is completely different and needs emergency assessment.
Sildenafil is designed for use as needed. Tadalafil is the one licensed for low-dose daily use.
Go to A&E. Priapism is a genuine emergency and delay risks permanent damage.
It is rare, and it is not something to wait out.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 24, 2026
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