A PDE5 inhibitor for erectile dysfunction
Sildenafil
A short-acting PDE5 inhibitor that helps achieve an erection when a man is sexually aroused — taken before sex, and never with nitrate heart medicines.
What is Sildenafil?
Sildenafil is the original and best-known PDE5 inhibitor, used for erectile dysfunction, which is difficulty getting or keeping an erection firm enough for sex. It works by improving blood flow to the penis when a man is sexually aroused.
Education and reference only. This is a plain-language guide to Sildenafil — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Sildenafil is the original and best-known PDE5 inhibitor, used for erectile dysfunction (difficulty getting or keeping an erection firm enough for sex). It is taken on demand, before sexual activity, rather than every day. It is the active ingredient in Viagra; the same ingredient under the brand Revatio is used for a quite different problem — raising pressure in the lung arteries (pulmonary hypertension) — so the brand reflects the use, but the medicine is the same. Importantly, sildenafil does not create desire: it only helps the physical erection happen when arousal is already present, so it is not a "libido" drug.
How it works
When a man is sexually aroused, nerves in the penis release a signal that relaxes the blood vessels so the penis fills with blood and becomes erect. The body normally winds this signal down using an enzyme called PDE5. Sildenafil blocks PDE5, allowing the natural arousal signal to last longer and work more effectively, so an erection is easier to achieve and maintain. Because it only amplifies the body's own arousal response, sexual stimulation is still needed — it does nothing on its own.
Company & origin
Originated / developed by: Pfizer.
Sildenafil was developed by Pfizer (UK) in the early 1990s, originally investigated for angina before its erectile-function effect was noted. It was approved by the US FDA in 1998 as Viagra (and later as Revatio for pulmonary hypertension).
What it treats
Conditions Sildenafil is used for
Practical use
How to take Sildenafil
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Taken before planned sexual activity, usually around an hour beforehand, and it only works alongside sexual arousal.
- A heavy or fatty meal can delay how quickly it starts to work, so it may act faster on a lighter stomach.
- Never take it with nitrate medicines for chest pain, as the combination can cause a dangerous drop in blood pressure.
- Avoid combining it with recreational 'poppers', which are also nitrates.
- Limit alcohol, which can make erections harder to achieve.
- Seek urgent medical help if an erection is painful or lasts several hours.
Weighing it up
Advantages & disadvantages of Sildenafil
Advantages
- Effective for many men with erectile dysfunction.
- Well-established medicine with extensive experience of use.
- Available as an inexpensive generic, and some versions can be bought from pharmacies after a consultation.
Disadvantages
- Can cause headache, flushing, indigestion and a stuffy nose.
- Must never be combined with nitrates because of the risk of a severe blood-pressure drop.
- Needs to be taken in advance and only works with sexual arousal.
- Not suitable for some men with certain heart conditions, so a check is needed first.
Practical use
Good to know
It is taken a while before sex (not daily) and works best on an empty stomach, as a large or fatty meal can slow it down. A few attempts may be needed to find what works, since timing and arousal both matter. It is short-acting, covering roughly the few hours after a dose. Alcohol in any quantity can make erections less reliable and add to dizziness. Many men buy sildenafil from a pharmacy without a prescription after a consultation, but a proper check first matters because erectile difficulty can be an early warning sign of heart or circulation problems.
Who should not take it / use with caution
- Anyone taking nitrates for angina or chest pain (such as GTN spray or tablets, isosorbide mononitrate or dinitrate) — combining them can cause a dangerous, sometimes fatal, drop in blood pressure. This also includes the recreational drug "poppers" (amyl/alkyl nitrites).
- People who have recently had a heart attack or stroke, have very low blood pressure, or have certain inherited eye diseases of the retina.
- Used with caution in men taking alpha-blockers for blood pressure or prostate symptoms, as both lower blood pressure; the two are usually separated and started carefully.
Monitoring
- Blood pressure and heart health before starting, especially with other heart medicines
- How well it works and whether the timing suits you
- Any vision changes or prolonged erections
Side effects
- Headache, facial flushing, a stuffy or runny nose, and indigestion are the most common.
- Dizziness or light-headedness from the blood-pressure-lowering effect, and a temporary blue tinge or sensitivity to bright light in the vision.
- Rarely, a painful erection that will not go down (priapism): an erection lasting more than 4 hours is a medical emergency and needs urgent hospital care to avoid lasting damage. Sudden loss of vision or hearing also needs urgent review.
Key interactions
- Nitrates and nitrite "poppers" must never be combined with it — this is the single most important interaction (dangerous blood-pressure drop).
- Alpha-blockers and other blood-pressure-lowering medicines add to its effect and can cause dizziness or fainting, so they are introduced carefully.
- Some medicines raise its levels (certain antifungals, some HIV and hepatitis-C treatments, and some antibiotics like clarithromycin), increasing side effects; grapefruit juice can do the same.
Available as: Tablets (the familiar form), and orally disintegrating or chewable tablets for some people. Revatio (the pulmonary-hypertension brand) also comes as a liquid and an injection used in hospital.
Answers
Sildenafil: frequently asked questions
Will sildenafil increase my sex drive?
No. Sildenafil only helps the physical side of an erection by improving blood flow when you are already aroused — it does not create desire or libido. If low desire is the main problem rather than the erection itself, that is worth discussing separately, as the causes and treatments are different.
Why can't I take it with my angina spray?
Nitrate medicines for angina (like a GTN spray or tablets) and sildenafil both lower blood pressure, and together they can cause a sudden, severe and potentially fatal drop. They must never be combined. The same applies to recreational "poppers". Always tell any prescriber or pharmacist about all your heart medicines before using sildenafil.
How long before sex should I take it?
It is usually taken a while before sex and works best on an empty stomach, because a heavy or fatty meal can delay it. It does not work instantly and you still need sexual stimulation. It is short-acting, so it covers the next few hours rather than the whole day — which is one difference from tadalafil.
My erection has lasted a long time — what should I do?
An erection that lasts more than 4 hours (priapism) is a medical emergency. Go to A&E without delay, as a prolonged erection can permanently damage the penis if not treated quickly. This is uncommon but important to act on fast.
What is the difference between sildenafil and Viagra?
They are the same medicine — sildenafil is the generic (active-ingredient) name and Viagra is the original brand. Generic sildenafil contains the identical active ingredient and works the same way; it is usually much cheaper. Revatio is the same ingredient sold for a different condition (lung-artery pressure).
The wider class
About PDE5 inhibitors
Sildenafil belongs to the pde5 inhibitors class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF: Sildenafil.
- electronic Medicines Compendium (SmPC): Sildenafil (Viagra/Revatio).
- NICE CKS: Sildenafil (Viagra).
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