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.

• 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).

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

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)

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.

These are not all the side effects of sildenafil.

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

Sildenafil is not recommended in pregnancy and while breastfeeding.

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.