Sildenafil

What is sildenafil?

Sildenafil is part of a group of medicines that are known as phosphodiesterase type 5 (PDE5) inhibitors. They are mainly used for erectile dysfunction or pulmonary arterial hypertension. Sildenafil is sold under different brand names, including Viagra (for ED) and Revatio (for PAH)[1]. 

What is sildenafil used for?

Sildenafil is prescribed for two conditions. With the first, erectile dysfunction (or ED), sildenafil can help men with ED achieve and maintain an erection sufficient for sexual activity. It achieves this by increasing blood flow to the penis in response to sexual stimulation[1]. 

With the second condition, pulmonary arterial hypertension (PAH - a specific type of high blood pressure affecting the lungs), sildenafil helps relax and widen the blood vessels in the lungs, lowering the blood pressure there. This can improve the ability to exercise and reduce PAH symptoms[1]. 

How does sildenafil work?

Sildenafil works by blocking an enzyme called PDE5, which breaks down a substance called cyclic GMP. This substance helps relax smooth muscle cells[1].  

In ED, this action enhances blood flow to the penis during sexual arousal, helping to achieve and maintain an erection. In PAH, sildenafil relaxes the blood vessels in the lungs, reducing pressure and improving oxygen delivery[1]. 

Who can take sildenafil?

Sildenafil can be taken by adults aged 18 and over. 

However, sildenafil is contraindicated (must not be taken) by people who [1]: 

• Take any form of nitrate medication (e.g., for angina/heart failure - tablets, sprays, patches, or recreational 'poppers' like amyl nitrite) 

• Have severe heart,unstable angina, or severe liver impairment  

• Have recently had a stroke or heart attack 

• Have low blood pressure (hypotension) 

• Have hereditary retinal disorders such as retinitis pigmentosa (a rare condition) 

• Have sickle cell anaemia 

• Have a deformed or curved penis 

• Have a stomach ulcer 

• Are men for whom sexual activity is inadvisable due to underlying cardiovascular problems 

• Have bleeding problems like haemophilia 

• Have severe kidney problems requiring dialysis 

• Have a history of non-arteritic anterior ischaemic optic neuropathy (NAION - a specific type of sudden vision loss) 

Always speak to your GP or specialist before starting sildenafil, especially if you have any medical conditions or take other medication. 

Please tell your doctor or pharmacist about any conditions you may have before starting sildenafil, and always follow the instructions in the Patient Information Leaflet[3].

How do you take sildenafil?

You can take sildenafil with a glass of water, with or without food, usually between 30 minutes to 1 hour before sexual activity. Avoiding heavy or high-fat meals beforehand can improve results, as these can significantly delay the onset of action[1]. 

For those taking sildenafil for ED, sexual stimulation is still required for the medicine to work. For PAH, doses should be taken at evenly spaced intervals three times a day. 

What is the typical dosage of sildenafil?

For ED, the usual starting dose of sildenafil is 50mg. This is taken around one hour before sexual activity. The dose may be amended to 25mg or 100mg, depending on how well it works. It may also be adjusted if you experience side effects. Do not take more than one dose in 24 hours[1]. 

For pulmonary arterial hypertension (PAH), the typical dose is 20mg three times a day (doses must be taken approximately every 4 to 6 hours (e.g., three times a day at roughly the same times), exactly as prescribed.). However, this may change by person – always follow your specialist’s guidance[1]. The maximum recommended dose for PAH is usually 20mg three times a day (total 60mg daily). 

Can you stop taking sildenafil suddenly?

If you are taking sildenafil for ED, you can stop taking it at any time if it is no longer needed[1]. For PAH, it’s important not to stop taking it without medical advice, as this could cause a dangerous increase in pulmonary artery pressure. 

What should you do if you take too much sildenafil?

Taking too much sildenafil can cause side effects, such as[1]: 

• Dizziness or fainting 

• Changes in your vision 

• Chest pains 

• Priapism (a prolonged and painful erection) 

Priapism (an erection lasting more than 4 hours) is a medical emergency requiring immediate treatment to prevent permanent damage. Symptoms of overdose can also include a severe drop in blood pressure. If you suspect you’ve taken an overdose, seek urgent medical attention or call 999. 

What should you do if you miss a dose of sildenafil?

If you are taking sildenafil for ED and miss a dose, skip the missed dose. Wait until your next planned dose before sexual activity[1]. 

If you miss a sildenafil dose for PAH, take the missed dose as soon as you remember. If it is nearly time for your next one, take that one as normal. Do not take a double dose to make up for the missed one. 

Which medicines should you avoid while taking sildenafil?

You should avoid taking sildenafil with other medicines, such as[1]: 

• Nitrate medications for chest pain (e.g. glyceryl trinitrate, isosorbide mononitrate) 

• Alpha-blockers used for high blood pressure or prostate problems 

• Certain antifungal medications (e.g. ketoconazole) 

• Some antibiotics (e.g. erythromycin) 

• Certain HIV protease inhibitors (e.g., ritonavir, saquinavir) 

Avoid consuming large amounts of grapefruit or grapefruit juice, as it can increase the level of sildenafil in your blood and the risk of side effects. 

Always inform your doctor or pharmacist about all medications and supplements you take before you start a new medicine, and follow the instructions in the Patient Information Leaflet[3].

What are the side effects of sildenafil?

Common side effects of sildenafil, which may affect more than 1 in 10 people, include:[1] 

• Headaches 

• Flushing 

• Indigestion 

• Nasal congestion 

• Dizziness 

Some less common side effects of sildenafil include: 

• Blurred vision or a blue tint to vision 

• Muscle pains 

• Rash and redness 

Important: This is not a complete list. Always read the leaflet in your medicine packet for full information[3]. 

Seek immediate medical attention (call 999 or go to A&E) if you experience:  

• Chest pain during or after sexual activity 

• Priapism (an erection lasting longer than four hours) 

• Sudden loss of vision or hearing 

• Sudden decrease or loss of hearing (sometimes with tinnitus or dizziness) 

• Signs of a severe allergic reaction (anaphylaxis) such as swelling of the face/lips/tongue/throat, severe difficulty breathing, or a rapid rash 

If you experience any of these, seek medical attention right away. 

Always inform your doctor or pharmacist about all medications and supplements you take before you start a new medicine, and follow the instructions in the Patient Information Leaflet[3].

Frequently asked questions about sildenafil

How long does sildenafil take to work? 

Sildenafil usually starts to work for ED within 30 to 60 minutes[1]. It can be taken up to four hours before sexual activity. It’s important to remember that sildenafil alone does not cause an erection – it must be combined with sexual arousal. 

Can women take sildenafil? 

Sildenafil is not licensed for treating any kind of sexual dysfunction in women[1]. However, it may be prescribed for PAH in both men and women under close specialist supervision. 

Can you take sildenafil for a long time? 

Yes, you can take sildenafil for weeks, months, or years without any lasting harmful effects[1]. Regular monitoring by your clinician is important, especially to check for any potential eye problems or cardiovascular effects. 

Is it safe to drink alcohol while taking sildenafil? 

Drinking a small amount (1-2 units) of alcohol is usually acceptable when you take sildenafil[1]. However, excessive alcohol can significantly reduce the effectiveness of the medicine. It may also increase the risk of some side effects and increase the risk of unsafe drops in blood pressure. 

Can sildenafil be taken daily? 

For ED, sildenafil is not usually taken every day unless prescribed as a lower daily dose. However, for PAH, it is generally taken three times a day as prescribed by your doctor or pharmacist[1]. 

Can I drive or ride a bike while taking sildenafil? 

Sildenafil does not make you sleepy, so it's usually safe to drive or ride a bike[1]. However, you should not drive, cycle, or operate machinery if it makes you feel dizzy or affects your eyesight. 

Can lifestyle changes help with erection problems? 

As well as using medication like sildenafil, erection problems like erectile dysfunction can often get better by making positive lifestyle changes, like[1]: 

• Maintaining a healthy weight 

• Quitting smoking 

• Cutting down on alcohol 

• Not taking recreational drugs 

• Exercising 

• Reducing stress 

• Managing underlying conditions like diabetes or high blood pressure 

For the official NHS guide to sildenafil, visit their website.