How to take sildenafil: doses, timing, and safety

Sildenafil is a prescription-only medicine for erectile dysfunction, and taking it correctly matters as much as taking it at all.

This covers the doses licensed in the UK, how and when to take it, and the situations where you shouldn't take it under any circumstances. If you're after the side effects specifically, we've written about the side effects of sildenafil separately.

This page describes sildenafil as licensed in the UK. Tablets sold elsewhere come in strengths that aren't available or approved here, and the guidance below won't apply to them.

Do not take sildenafil if you use nitrates or poppers

Nitrate medicines, usually prescribed for angina or chest pain, and amyl nitrite (poppers) must never be combined with sildenafil. Together they can cause a sudden and dangerous drop in blood pressure.

If you take either, sildenafil isn't an option for you, and your doctor will suggest something else.

Sildenafil doses in the UK

Sildenafil is licensed in the UK in three strengths: 25mg, 50mg, and 100mg.¹

• The usual starting dose is 50mg, taken about an hour before sex.

• The maximum dose is 100mg. Your doctor may adjust up or down depending on how you respond and on other conditions you have.

• Take it no more than once in any 24 hours. Taking more doesn't improve the effect, and it raises the chance of side effects.

• Don't combine film-coated and oro-dispersible tablets. If you've been given both forms at different times, take one or the other.

If you've come across sildenafil sold at 200mg or 300mg, those aren't UK-licensed products. They're above the maximum approved dose, and nothing is known about how they've been made or what's in them. Don't take them.

If 100mg isn't working, talk to your doctor before changing anything.

How and when to take it

Swallow the tablet whole with a glass of water, roughly an hour before sex.¹

Most people notice the effect within about 45 minutes, and it generally lasts a few hours, though this varies between people. Read more about how long sildenafil lasts.

You still need to be sexually aroused for it to work. Sildenafil doesn't produce an erection on its own, which surprises a lot of people the first time.²

Food: You can take it with or without food, but a heavy meal will slow it down.¹

Alcohol: Drinking makes erections harder to get and keep, so it works against what you've taken the tablet for. A drink won't stop it working. A heavy night might.

Grapefruit juice: It can raise sildenafil levels slightly. The effect is modest and for most people it won't be noticeable, but it's worth knowing if you drink a lot of it.²

When sildenafil isn't suitable

Some of these rule it out completely, and some mean your doctor needs to weigh it up. They're not equivalent, so they're split below.

Never take sildenafil if you're taking:¹

• Nitrates, including nitroglycerin, isosorbide mononitrate, or isosorbide dinitrate

• Amyl nitrite, butyl nitrite, or any other poppers

• Riociguat (a medicine for high blood pressure in the lungs)

• Another medicine of the same type (a PDE5 inhibitor), such as tadalafil or another sildenafil product

Tell your doctor before taking it if you:¹

• Take protease inhibitors (some HIV medicines), alpha-blockers (for blood pressure or prostate problems), or medicines for pulmonary arterial hypertension

• Have heart, liver, or kidney problems

• Have low blood pressure, or have had a recent stroke or heart attack

• Have an inherited eye condition such as retinitis pigmentosa

• Have a blood disorder such as sickle cell disease, leukaemia, or myeloma

• Have a curvature or deformity of the penis

• Have a stomach ulcer or a bleeding disorder

• Are intolerant of lactose, since some formulations contain it

Sildenafil for erectile dysfunction is prescribed to men over 18. It's also licensed, under a different brand and at different doses, for pulmonary arterial hypertension in both men and women, which is a separate use and a separate prescription.

When to stop taking it and get help

These are rare, but recognising them quickly is what matters.

• Your erection lasts longer than four hours. A prolonged, painful erection is called priapism, it's a surgical emergency, and waiting it out can cause permanent damage. Go to A&E.¹

• You lose vision, or part of your vision, suddenly. In one or both eyes.¹

• You lose your hearing suddenly, or develop ringing in your ears.¹

• You have chest pain. And if you're prescribed nitrates for chest pain, do not take them to relieve it. Tell the paramedic or clinician you've taken sildenafil.¹

• You have an allergic reaction: wheezing, difficulty breathing, or swelling around the eyes, lips, or throat.¹

• You feel severely dizzy or faint.¹

The chest pain one in particular is worth remembering before you need it.

What to expect the first time

• It won't work on its own. Arousal is still required.

• It may not work the first time, and that's common rather than a sign it won't work at all.

• Headache, flushing, and a blocked nose are the most frequent side effects, and they usually ease.

• Dizziness and changes to vision are possible, so find out how you react before driving or using machinery.

• If nothing happens across several attempts, tell your doctor rather than increasing the dose yourself.

The numan take

Sildenafil is well understood and generally well tolerated when it's taken as prescribed. The two things worth carrying away are the maximum dose of 100mg, and the fact that nitrates and poppers rule it out completely rather than conditionally.

Read the Patient Information Leaflet that comes with your tablets, because it's specific to the product you've actually been given.

References

^{1 Sildenafil 50 mg film-coated tablets. medicines.org.uk. [accessed 7 October 2026] Available from: https://www.medicines.org.uk/emc/product/7141/smpc} 

^{2 Smith BP, Babos M. Sildenafil. StatPearls Publishing, Treasure Island; 2025. [accessed 21 October 2026] Available from: https://www.ncbi.nlm.nih.gov/books/NBK558978/}