Viagra (sildenafil): how it works, safety and getting it in the UK

Viagra is the brand name of sildenafil, a medicine the NHS says is mainly used to treat erection problems (erectile dysfunction). Viagra's product information gives its date of first authorisation as 14 September 1998. In 2017 the MHRA, the UK's medicines regulator, made one version, Viagra Connect, a pharmacy medicine for men over 18 with erectile dysfunction, sold after a discussion with the pharmacist.

Doses aren't covered here: the NHS says that if a doctor prescribes sildenafil, they'll tell you how much to take and how often.

What is Viagra (sildenafil)?

Viagra is a brand of sildenafil, which belongs to a group of medicines called phosphodiesterase type 5 (PDE5) inhibitors. Sildenafil is the active ingredient whether it's sold as Viagra or as a generic tablet, and the BNF lists many generic sildenafil tablets alongside the Viagra brand. Besides erection problems, the NHS says sildenafil is used to treat pulmonary arterial hypertension.

Pulmonary arterial hypertension is a type of pulmonary hypertension, which the NHS describes as high blood pressure in the blood vessels that supply the lungs. For that use, the BNF says sildenafil is started under specialist supervision.

How does Viagra work?

Sildenafil works by relaxing the blood vessels in your penis, allowing blood to flow in when you're sexually excited and causing an erection. It doesn't cause an erection on its own: the BNF says these medicines don't start an erection, and sexual stimulation is required.

Viagra's product information explains the chemistry: sexual stimulation releases nitric oxide in the penis, which raises the level of a substance called cGMP that relaxes the smooth muscle and lets blood flow in. Sildenafil blocks PDE5, the enzyme that breaks cGMP down.

How long does Viagra take to work, and how long does it last?

In a study of men who hadn't eaten, the typical time to an erection firm enough for sex, among those who got one, was 25 minutes after taking sildenafil, ranging from 12 to 37 minutes. In a separate study, sildenafil could still produce an erection in response to sexual stimulation 4 to 5 hours after it was taken (Viagra product information).

NICE's clinical summary for GPs gives its duration of action as 4 to 5 hours, a window in which an erection can happen with stimulation. An erection that lasts longer than 4 hours (priapism) needs medical help straight away: the product information warns that if it isn't treated immediately, it can damage the penis and cause a permanent loss of erections.

Taken with food, sildenafil is absorbed more slowly, reaching its peak level about an hour later on average (product information), and the NHS says it may take longer to get an erection after a large meal. The NHS also says it's best to avoid drinking too much alcohol, which can make getting an erection more difficult.

Does Viagra work?

In clinical studies it helped many men with erection problems. In fixed-dose studies reported in Viagra's product information, 62% to 82% of men, depending on the dose, said treatment improved their erections, against 25% on a dummy pill. The BNF says a PDE5 inhibitor is the first-line medicine for erectile dysfunction, whatever the cause.

Results vary with the cause. Across the studies, improvement was reported by 84% of men whose erection problems had a psychological cause, 68% with a physical cause, 59% with diabetes and 43% after radical prostatectomy (product information).

If it doesn't help, see your doctor. The Viagra Connect leaflet says to talk to your doctor if you've tried it and still can't get and keep an erection, and NICE's clinical summary suggests a GP follow-up 6 to 8 weeks after starting treatment.

Can you buy Viagra over the counter in the UK?

Yes, in a pharmacy version. The NHS says you can buy sildenafil from a pharmacy for erection problems after speaking with a pharmacist, though it's mostly available with a prescription. The BNF lists the Viagra brand tablets as prescription-only medicines. Viagra Connect is the version the MHRA made a pharmacy medicine in 2017, saying pharmacists would decide whether treatment is appropriate and could advise on erectile dysfunction, side effects and whether to see a GP.

If you buy sildenafil without a prescription, the NHS advises a check-up with your doctor within 6 months of starting it, because erection problems can be a sign of other serious health conditions, such as diabetes or heart problems.

Where to get sildenafil in the UK

	What you get.	Who supplies it.	Things to know.
On prescription	Generic sildenafil; on the NHS, the Viagra brand only for men who meet set criteria	A GP, or a sexual health clinic	Generic sildenafil can be prescribed on the NHS in England without restriction
From a pharmacy, without a prescription	Viagra Connect, for men over 18 with erection problems	A pharmacist, after a discussion to check it suits you	Not sold to men with severe heart or blood vessel disorders, high cardiovascular risk, liver failure, severe kidney failure or some interacting medicines: they need a doctor
Online	Medicines from an online pharmacy	Only UK-registered pharmacies, the MHRA says	Check a pharmacy based in Great Britain on the GPhC register; be suspicious of websites offering prescription medicines with no prescription required

From the NHS, NICE CKS, the BNF and the MHRA. If you buy sildenafil without a prescription, the NHS advises a check-up with your doctor within 6 months of starting it.

Getting sildenafil on prescription

You can get sildenafil on prescription, and the NHS says sexual health clinics can provide the same treatment you would get at your GP surgery. NICE's clinical summary says generic sildenafil can be prescribed without restriction on the NHS in England, while the BNF says the Viagra brand can only be prescribed in NHS primary care for erectile dysfunction in men who meet set criteria.

Buying Viagra online

The MHRA says erectile dysfunction medicines are a popular target for criminals selling unlicensed and counterfeit medicines. Its Criminal Enforcement Unit seized about 19.5 million doses of unlicensed erectile dysfunction medicines between 2021 and 2025, and it warns that such products may contain no active ingredient, the wrong dose, hidden drugs or toxic ingredients.

The MHRA's advice is to use only UK-registered pharmacies, including online pharmacies showing the official green cross logo, and to check an online pharmacy based in Great Britain on the General Pharmaceutical Council (GPhC) website. Its #FakeMeds campaign lists warning signs of a dodgy website, such as prescription medicines with no prescription required, or claims like "100% safe" and "no side effects".

Viagra side effects

The NHS lists the common side effects of sildenafil as headache, dizziness, a blocked or stuffy nose, stomach ache, diarrhoea or feeling sick, anxiety, and temporary changes in vision, such as blurred vision. The Viagra Connect leaflet says side effects are usually mild to moderate and short-lived.

In clinical studies, the most commonly reported were headache, flushing, indigestion, a blocked nose, dizziness, nausea, hot flushes and changes in vision, including a colour tinge and blurring (product information and leaflet). If you feel dizzy or have blurred vision, the NHS says not to drive, ride a bike or use machinery until you no longer have these symptoms.

Serious side effects

The NHS says sildenafil can sometimes cause serious side effects, although most of them are rare. They include:

• an allergic reaction (anaphylaxis), with symptoms such as a swollen throat or tongue, a raised itchy rash and difficulty breathing
• blistering skin around the mouth, eyes, nose or genitals, with flu-like symptoms
• a heart attack, with symptoms such as chest pain that feels tight or like squeezing on your chest, feeling short of breath, and sweating
• a fast or irregular heartbeat, seizures (fits) or fainting
• painful erections that last longer than 4 hours (priapism)

The NHS says to call 999 if you think you might be having anaphylaxis, a heart attack or a seizure, and to call NHS 111 if you think you're having any other serious side effect. If you get chest pain before, during or after sex, the Viagra Connect leaflet says to seek medical attention immediately, get into a semi-sitting position, try to relax, and not to use nitrates to treat the pain. If your sight suddenly gets worse, the product information says to stop taking sildenafil and contact a doctor immediately.

You can report side effects on the MHRA's Yellow Card website.

Is Viagra safe, and who shouldn't take it?

Most adults can take sildenafil, the NHS says, but it isn't suitable for everyone. It may not be suitable if you:

• have ever had an allergic reaction or serious side effect from sildenafil
• have recently had a heart attack or stroke
• have low blood pressure or uncontrolled high blood pressure
• have liver or kidney problems
• have blood cancer (leukaemia), cancer of the bone marrow (multiple myeloma) or sickle cell anaemia
• have ever had vision loss caused by reduced blood flow to the optic nerve
• have a curved or bent penis, which makes getting an erection painful or difficult

Never combine it with nitrate medicines for chest pain or heart failure (such as glyceryl trinitrate or isosorbide mononitrate), with "poppers" or with riociguat, because together they can cause a dangerous fall in blood pressure (Viagra Connect leaflet).

Sex can put a strain on the heart. The product information says that men for whom sexual activity is inadvisable, such as those with unstable angina or severe heart failure, shouldn't take it, and the Viagra Connect leaflet says to talk to your doctor first if you get very breathless or have chest pain with light or moderate activity, such as walking briskly for 20 minutes or climbing two flights of stairs.

Can you take Viagra with high blood pressure?

Sometimes, with a doctor's advice. The NHS lists uncontrolled high blood pressure as a reason sildenafil may not be suitable, and the Viagra Connect leaflet says to talk to your doctor first if your high blood pressure isn't controlled.

Sildenafil causes mild, temporary drops in blood pressure that usually have no clinical effect, but alpha-blockers such as doxazosin or tamsulosin, used for an enlarged prostate or high blood pressure, can add to this and cause dizziness or fainting (product information and leaflet).

Viagra isn't licensed for women or for anyone under 18 (product information).

Medicines, food and drink that don't mix

The NHS says some medicines and supplements may not mix well with sildenafil, including some types of:

• heart medicines, such as nitrates for chest pain (angina)
• epilepsy medicines
• medicines for pulmonary hypertension or high blood pressure
• antibiotics, including rifampicin, erythromycin and clarithromycin
• medicines for an enlarged prostate or erectile dysfunction
• HIV medicines and antifungal medicines
• supplements containing St John's wort

Taking sildenafil with poppers or illegal drugs such as amphetamines, ecstasy and cocaine can increase the risk of serious side effects, and the NHS says not to eat grapefruit or drink grapefruit juice, because it can make side effects more likely. Before taking it, tell your doctor about any other medicines, herbal remedies, vitamins or supplements you take.

Viagra, Cialis and other erection medicines

Sildenafil is one of four PDE5 inhibitors the NHS lists for erection problems, alongside tadalafil (Cialis), vardenafil (Levitra) and avanafil (Spedra). The BNF says sildenafil, vardenafil and avanafil are short-acting and suitable for occasional use, while tadalafil is longer-acting.

Does Viagra make you last longer?

Sildenafil is licensed for erection problems, not premature ejaculation, and BAUS (the British Association of Urological Surgeons) says the exact role of PDE5 inhibitors in premature ejaculation is uncertain. Our page on whether Viagra makes you last longer goes through the trials, and dapoxetine (Priligy) covers the tablet licensed for premature ejaculation.

When should you see a GP?

See a GP or go to a sexual health clinic if erection problems keep happening: the NHS says it might be a sign of a health condition that can be treated. The NHS also says erectile dysfunction is very common, particularly in men over 40, and usually nothing to worry about.

Erectile dysfunction may be an early sign of cardiovascular (heart and blood vessel) disease, diabetes, high blood pressure or high cholesterol (Viagra Connect leaflet), and the BNF says it increases the risk of cardiovascular disease. Our guides to erectile dysfunction, low sex drive in men and morning wood cover related questions.

While taking sildenafil, call 999 for a suspected anaphylaxis, heart attack or seizure, and NHS 111 for any other serious side effect (NHS).