Sildenafil (Viagra): uses, safety and side effects

Sildenafil is mainly used to treat erectile dysfunction and also to treat pulmonary arterial hypertension, high blood pressure in the arteries of the lungs. Viagra is the brand name mentioned by the NHS. For erection problems it comes as tablets, and for pulmonary hypertension it can also come as a liquid.

It works by relaxing blood vessels and increasing blood flow, and it needs sexual stimulation to produce an erection.

It is a prescription medicine, and the NHS notes it may also be supplied after a consultation with a pharmacist.

The NHS says people with erection problems are usually told to take 1 tablet about 1 hour before sex. The effect can last up to 36 hours, and you should not take more than 1 tablet a day. Doses for pulmonary arterial hypertension are different and are set by the specialist.

Food matters: a large meal can delay the effect, and drinking too much alcohol reduces how well it works.

• For pulmonary hypertension, if you miss a dose take it when you remember unless the next is near; skip it otherwise, and never double up.

• If you take more than prescribed, call NHS 111, or 999 for an ambulance if you are seriously unwell.

Sildenafil is not usually prescribed together with nitrates, such as those used for chest pain, because the combination can drop blood pressure dangerously. Tell any clinician treating you that you have taken it.

The NHS also lists anticonvulsants, certain blood pressure medicines, the antibiotics rifampicin, erythromycin and clarithromycin, antifungals and the herbal remedy St John's wort as interacting. Poppers and illegal stimulants such as amphetamines and cocaine increase the risk of serious effects. Do not have grapefruit or grapefruit juice.

Sildenafil may be unsuitable if you have had a recent heart attack or stroke, low or uncontrolled high blood pressure, liver or kidney disease, a blood cancer, sickle cell anaemia, previous loss of vision from reduced blood supply to the optic nerve, or a bend in the penis that makes erections painful.

It is not usually used during pregnancy or breastfeeding, although a doctor may recommend it for pulmonary hypertension if the benefits outweigh the risks.

Common effects include headache, dizziness, a blocked nose, stomach upset, anxiety and temporary changes in vision. If your vision is affected, do not drive or use machinery.

Serious effects are rare: allergic reaction, blistering skin, heart attack, irregular heartbeat, seizures, fainting, and a painful erection lasting longer than 4 hours (priapism). Call 999 for anaphylaxis, heart attack symptoms or a seizure, and NHS 111 for other serious effects.

Suspected side effects can be reported through the Yellow Card scheme.

Erection problems can be the first sign of conditions such as heart disease or diabetes. The NHS advises a check-up with a doctor within 6 months of starting sildenafil if you did not get it on prescription.

The guide on erectile dysfunction covers causes and assessment. Tadalafil is a related medicine with its own guide, and the NHS gives a similar 36-hour duration for both.

Whatever the supply route, the medicine is only one part of managing erection problems. Blood pressure, cholesterol, blood sugar, smoking, alcohol, weight and stress can all contribute, and addressing them is often as important as the tablet. Anyone who develops chest pain, severe dizziness or a sudden change in vision after taking sildenafil should stop and seek urgent help.