“Sildenafil (Viagra).” NHS. https://www.nhs.uk/medicines/sildenafil-viagra. Accessed 3 Aug. 2026.
If you've been looking into treatment for erectile dysfunction (ED), you've almost certainly come across sildenafil.
It's one of the most commonly prescribed ED medications in the world. Sildenafil has helped millions of men get erections, and it’s been available for more than 25 years.
You might know it better by its brand name, Viagra. But today, sildenafil is widely available as a generic medicine that works in exactly the same way, usually at a lower cost.
Key takeaways
- Sildenafil is the active ingredient in Viagra.
- It helps you get and keep an erection by increasing blood flow to the penis.
- It usually starts working within 30 to 60 minutes and lasts for around four hours.
- For most men, it's a safe and effective first treatment for erectile dysfunction.
What is sildenafil?
If you've ever heard of Viagra, you've already heard of sildenafil. Viagra is simply the brand name, while sildenafil is the generic version.
Sildenafil comes as a tablet that's taken before sex to help you get and keep an erection that’s firm enough for sex.
It belongs to a group of medicines called PDE5 (phosphodiesterase type 5) inhibitors, which help increase blood flow to your penis when you're sexually aroused.
Sildenafil doesn't increase your sex drive or cause an automatic erection. Sexual stimulation is still needed for it to work.
What is sildenafil used for?
The main use of sildenafil is to treat erectile dysfunction. It helps men who have difficulty getting or maintaining an erection that's firm enough for sex.
Sildenafil is also prescribed under different brand names and doses to treat pulmonary hypertension, a condition that affects the blood vessels in the lungs.
What is the difference between Viagra and sildenafil?
Viagra and sildenafil contain exactly the same active ingredient, so they work in the same way and are equally effective. The main difference is just in the name.
Viagra is Pfizer's original branded version, while sildenafil is the generic medicine that's now available from several manufacturers after the original patent expired.
If you're wondering whether Viagra is "stronger" than sildenafil, the answer is no. A 50mg sildenafil tablet is exactly the same medication as the 50mg Viagra tablet.
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How to take sildenafil
Taking sildenafil correctly gives it the best chance of working. Its effects usually last for around four hours.
- Take it around 30 to 60 minutes before sex.
- Take sildenafil on an empty stomach. Taking it after a large or high-fat meal can slow down how quickly it starts working.
- Swallow the tablet whole with water (don’t take it with grapefruit juice, as this can increase the risk of side effects).
Don’t forget that you won’t get an instant erection. Sexual stimulation is still needed for it to work. Don't take more than one dose in 24 hours.
What sildenafil strengths are available?
Sildenafil comes in several strengths. Sildenafil 50mg is the standard starting dose for treating erectile dysfunction, but sometimes other doses are prescribed.
- 25mg: Often taken by men who are older, have certain health conditions or experience side effects.
- 50mg: The usual starting dose for most men
- 100mg: Men who need a stronger effect after clinician review
If 50mg isn't effective enough, your clinician may increase the dose to 100mg. If you experience side effects, they may reduce it to 25mg instead.
The maximum recommended dose of sildenafil for erectile dysfunction is 100mg once a day. Taking more won't necessarily improve your erections and may increase your risk of side effects.
What are the side effects of sildenafil?
Most men tolerate sildenafil well. When sildenafil side effects do happen, they're usually mild and temporary.
Common side effects include:
- Headache
- Flushing
- Indigestion
- A blocked nose
- Dizziness
- Blurred vision
Serious side effects are rare, but you should seek urgent medical attention if you develop chest pain, sudden vision loss or an erection lasting more than four hours (known as priapism).
Who should not take sildenafil?
Sildenafil isn't suitable for everyone.
You shouldn't take sildenafil if you:
- take nitrate medicines for angina.
- have been advised not to have sex because of a heart condition.
- have very low blood pressure.
- have certain rare inherited eye conditions.
Your clinician will also want to know about any other medications you're taking before prescribing sildenafil.
If you have underlying heart disease or other long-term health conditions, they can help decide whether sildenafil is safe for you.
Is sildenafil right for you?
For many men, sildenafil is the first treatment they try for erectile dysfunction. It's well-studied, highly effective, and has been used safely by millions of people worldwide.
That said, it's not the right choice for everyone. The best ED treatment depends on your overall health, any other medicines you take, and whether you'd prefer an on-demand tablet like sildenafil or a longer-lasting option such as tadalafil (which you can also take as a daily pill).
And if ED has come on alongside symptoms like a low sex drive, fatigue or reduced strength, it could be a sign of low testosterone. In that case, treating the underlying hormone deficiency with testosterone replacement therapy may help improve your symptoms.
Our experts can help you choose the option that best fits your lifestyle and medical history. Take Voy’s quick quiz to get started.
