Sildenafil (including Viagra)

1. About sildenafil

Sildenafil is a medicine used to treat erectile dysfunction. It increases blood flow to the penis to help men get an erection. At least two-thirds of men have improved erections after taking it.

Sildenafil is also sometimes used to treat pulmonary hypertension (high blood pressure in the blood vessels that supply the lungs).

This medicine is available on prescription and can also be bought from most pharmacies. It comes as tablets that you swallow, chewable tablets, and as a liquid that you drink.

2. Key facts

It usually takes 30 to 60 minutes for sildenafil to work for erectile dysfunction. You can take it up to 4 hours before you want to have sex.
Taking sildenafil alone will not cause an erection. You need to be aroused for it to work.
The most common side effects are headaches, feeling sick, hot flushes and dizziness. Many men have no side effects or only mild ones.
It can be dangerous to take sildenafil if you also take medicines called nitrates (often given for chest pain). The combination can cause a dangerous fall in your blood pressure. If you’re not sure if you’re taking a nitrate, ask your doctor or pharmacist.
Sildenafil is called by the brand names Viagra (for erectile dysfunction) and Revatio (for pulmonary hypertension).

3. Who can and can’t take sildenafil

Sildenafil for erectile dysfunction (Viagra) can be taken by men aged 18 and over. It’s not for women or children.

Sildenafil for pulmonary hypertension (Revatio) can be taken by adults and children aged 1 and over.

Sildenafil is not suitable for some people.

Do not take sildenafil if you:

have had an allergic reaction to sildenafil (Viagra) or any other medicines in the past
are taking medicines called nitrates for chest pain
have a serious heart or liver problem
have recently had a stroke or a heart attack
have low blood pressure
have a rare inherited eye disease, such as retinitis pigmentosa

Check with your doctor before taking sildenafil if you:

have sickle cell anaemia (an abnormality of red blood cells), leukaemia (cancer of blood cells) or multiple myeloma (cancer of bone marrow)
have a deformity of your penis or Peyronie’s disease (curved penis)
have a heart problem. Your doctor should carefully check whether your heart can take the additional strain of having sex.
have a stomach ulcer or a bleeding problem like haemophilia

4. Where to get sildenafil

On the NHS

You can get sildenafil on the NHS if you have erectile dysfunction or pulmonary hypertension.

Branded versions of sildenafil such as Viagra are not available on the NHS, except in special circumstances.

Private prescription

You can also get sildenafil on a private prescription from your doctor. This means you’ll need to pay the full cost of the medicine.

Prices vary, but on average 4 tablets costs around £20.

From a pharmacy

You can buy sildenafil from a pharmacy following a discussion with the pharmacist.

They’ll ask you some questions about your general health to make sure sildenafil is safe for you to take. You can also talk to them about side effects.

If the pharmacist has any concerns about whether sildenafil is safe for you, they may advise you to see your doctor.

Online

You can buy sildenafil/Viagra over the internet. Be very careful if you do this as many websites sell fake medicines.

Online medicines are not always regulated and the ingredients in them can vary from one pack to another. They can cause unpleasant side effects or may not be suitable for you.

It’s best to see your doctor before buying medicines online. They know your medical history and can discuss whether you might benefit from treatment.

If you choose to buy sildenafil/Viagra over the internet, make sure:

any online pharmacy is registered with the General Pharmaceutical Council (GPhC)
any online doctor service is registered with the Care Quality Commission (CQC)
all doctors are registered with the General Medical Council (GMC)

5. How and when to take it

It’s important to take it as advised by your doctor or pharmacist.

How much will I take for erectile dysfunction?

Sildenafil tablets for erectile dysfunction come in different strengths ranging from 25mg to 100mg.

The usual dose is 50mg, when you need it (no more than once a day).

The dose can be increased to 100mg or decreased to 25mg depending on its effect.

Take sildenafil up to 4 hours before you want to have sex. For sildenafil to work properly, you’ll need to be sexually excited.

How much will I take for pulmonary hypertension?

Sildenafil tablets for pulmonary hypertension come in 10mg and 20mg tablets.

The usual adult dose is 20mg taken 3 times a day.

Doses for children can range from 10mg to 20mg taken 3 times a day, depending on their weight.

How to take it

Swallow tablets whole with a glass of water or juice (but not grapefruit juice).

For erectile dysfunction, it’s best to take sildenafil on an empty stomach. It may take longer to work if you take it with food.

Liquid sildenafil

If you or your child are taking sildenafil as a liquid, 1ml is usually the same as taking a 10mg tablet, but it’s important to always check the medicine label.

Liquid sildenafil will usually be made up for you by your pharmacist.

The medicine will come with a plastic syringe or spoon to help you take the right amount.

If you do not have a plastic syringe or spoon, ask your pharmacist for one. Do not use a kitchen teaspoon as it will not give the right amount.

What if I forget to take it?

If you have pulmonary hypertension and you forget to take a dose of sildenafil, take it as soon as you remember and then continue as usual.

Never take 2 doses at the same time. Never take an extra dose to make up for a forgotten one.

If you often forget doses, it may help to set an alarm to remind you.

You could also ask your pharmacist for advice on other ways to help you remember to take your medicine.

What if I take too much?

Taking too much sildenafil by accident can cause unpleasant side effects, such as:

headaches
dizziness
indigestion
blocked nose
altered vision

Talk to your doctor if you have taken too much sildenafil and you’re worried about these side effects.

Like all medicines, sildenafil can cause side effects in some people. Many people have no side effects or only minor ones.

Common side effects

Common side effects happen in more than 1 in 100 people.

If you get these side effects, keep taking the medicine, but tell your doctor or pharmacist if these side effects bother you or do not go away:

headaches
feeling sick
hot flushes, including facial flushing
indigestion
a colour tinge to your vision or blurred vision
stuffy nose
dizziness

Serious side effects

Serious side effects are rare and happen in less than 1 in 1,000 people.

Stop taking sildenafil and call a doctor straight away if you get:

chest pains – if this happens during or after sex, get into a semi-sitting position and try to relax; do not use nitrates to treat your chest pain
prolonged and sometimes painful erections – if you have an erection that lasts for more than 4 hours, contact a doctor immediately
a sudden decrease or loss of vision
a serious skin reaction – symptoms may include fever, severe peeling and swelling of the skin, blistering of the mouth, genitals and around the eyes
seizures

Serious allergic reaction

In rare cases, it’s possible to have a serious allergic reaction (anaphylaxis) to sildenafil.

Immediate action required: Call 999 or go to A&E if:

you get a skin rash that may include itchy, red, swollen, blistered or peeling skin
you’re wheezing
you get tightness in the chest or throat
you have trouble breathing or talking
your mouth, face, lips, tongue or throat start swelling

You could be having a serious allergic reaction and may need immediate treatment in hospital.

These are not all the side effects of sildenafil.

For a full list, see the leaflet inside your medicines packet.

You can report any suspected side effect to the UK safety scheme.

7. Pregnancy and breastfeeding

Sildenafil is not recommended in pregnancy and while breastfeeding.

Non-urgent advice: Tell your doctor if you’re:

trying to get pregnant
pregnant
breastfeeding

8. Cautions with other medicines

Some medicines do not mix well with sildenafil.

Tell your doctor if you’re taking these medicines before you start sildenafil:

nitrates for chest pain
riociguat for pulmonary hypertension
recreational drugs called poppers, such as amyl nitrite

Mixing sildenafil with herbal remedies and supplements

Some complementary therapies may have ingredients that could interfere with sildenafil and cause side effects.

Important

Tell your doctor or pharmacist if you’re taking any other medicines, including herbal remedies, vitamins or supplements.

9. Common questions

Sildenafil belongs to a group of medicines called phosphodiesterase 5 (PDE5) inhibitors.

It works if you have erection problems by temporarily increasing blood flow to the penis when you get sexually excited.

It works in pulmonary hypertension by relaxing blood vessels in the chest.

This increases the blood supply to the lungs and reduces the workload of the heart.

Sildenafil works well for erectile dysfunction.

In general, at least two-thirds of men say they have harder erections after taking it.

If sildenafil does not work for you, it may be because:

you have not waited long enough after taking it before trying to have sex
you have waited too long after taking it to have sex
the dose is not high enough
you’re not sexually aroused enough

Sildenafil usually starts to work within 30 to 60 minutes.

It’s triggered by sexual stimulation, so you need to be aroused for it to work.

You should be able to get an erection at any time between 1 and 4 hours after taking sildenafil.

Your erection should go away after you have had sex.

If you’re taking sildenafil for erectile dysfunction, do not take it more than once a day.

If you’re taking sildenafil (Revatio) for pulmonary hypertension, do not take it more than 3 times a day.

Sildenafil is safe to take for a long time.

There do not seem to be any lasting harmful effects from taking it for many months and years.

If sildenafil does not work, there are other tablets you could try that work in a similar way, such as:

tadalafil (Cialis) – can be taken up to 36 hours before sex
avanafil (Spedra) – can be taken up to around 4 hours before sex
vardenafil (Levitra) – can be taken up to around 4 hours before sex

You could also try a medicine called alprostadil. This is a man-made hormone that helps to stimulate blood flow to the penis.

Alprostadil comes as:

an injection directly into your penis
a small pellet placed inside your urethra (the tube that carries urine from your bladder to the tip of your penis)

If you cannot get erections because of a hormonal problem, your GP may refer you to an endocrinologist (who specialises in the treatment of hormonal problems).

You may benefit from treatment using injections of man-made hormones to restore your hormone levels to normal.

Other treatments for erectile dysfunction include:

a vacuum pump
psychological treatments
penile implants
pelvic floor muscle exercises

Some complementary therapies, such as acupuncture, have claimed to treat erectile dysfunction. But there’s little evidence they work.

There are lots of treatments that claim to be a “herbal Viagra”. In fact, there’s no such thing.

A herbal Viagra has never been approved as a treatment for erectile dysfunction.

Plus, these treatments may be fake or unsafe.

Women can use sildenafil for pulmonary hypertension.

There’s no good evidence that sildenafil works for women with sexual problems.

There’s no firm evidence to suggest that taking sildenafil will reduce fertility in either women or men.

But speak to a pharmacist or your doctor before taking it if you’re trying to get pregnant.

Sildenafil does not affect any type of contraception, including the contraceptive pill and emergency contraception.

A beer or a couple of glasses of wine are not likely to stop or delay sildenafil working.

But drinking lots of alcohol can make it more difficult to get an erection.

To get the most benefit from sildenafil, do not drink heavily before taking it.

Taking poppers (amyl nitrite) with sildenafil can have a dangerous effect on your heart.

It can also be potentially dangerous to take sildenafil with:

stimulants like MDMA (ecstasy) or cocaine
hallucinogens like LSD
novel psychoactive substances (which used to be known as legal highs) like mephedrone

Sildenafil has not been properly tested with recreational drugs.

Talk to your doctor if you think you might use any recreational drugs with sildenafil.

Do not drink grapefruit juice if you’re taking sildenafil (Revatio) for pulmonary hypertension. It interferes with the way the medicine works.

You can eat and drink normally while taking sildenafil (Viagra) to get an erection.

Erectile dysfunction can often be improved if you make changes to your lifestyle, such as:

Related conditions

Useful resources

Page last reviewed: 22 January 2019
Next review due: 22 January 2022

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