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Sildenafil is a tablet that helps you get and keep an erection strong enough for sex. It’s better known as Generic Viagra.

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Generic Sildenafil is like Viagra, but without the branding. It works in exactly the same way, and gives very similar results.

A cheaper alternative to Viagra. In Sildenafil generic tablets there’s an ingredient called sildenafil citrate. You also get it in Viagra. It’s a type of medicine that helps to relax blood vessels.

Erectile dysfunction happens when not enough blood can get into the penis. The culprit is tight blood vessels, stopping blood from flowing in. An enzyme called PDE5 is what causes this.

Sildenafil works by countering the effects of this enzyme, so blood can flow into the penis. And then, hey presto – a firmer and longer-lasting erection.

Because Sildenafil is a generic, it’s cheaper than Viagra. But it’s the same drug and shouldn’t be any less effective.

Sildenafil tablets come in 25mg, 50mg and 100mg doses. So pretty much (well, exactly) the same as Viagra.

If you’re taking it for the first time, you’ll normally start on Sildenafil 50mg. If you take this dose and it works well but you get mild side effects, a doctor will suggest lowering your dose to 25mg.

Men taking Sildenafil pills for the first time but who also have risk factors that make them more prone to side effects (like diabetes for example) may be recommended Sildenafil 25mg to start with.

Sildenafil 100mg is only for when you’ve tried 50mg and it hasn’t been strong enough. 100mg is the absolute max dose.

It’s a question that gets asked a lot. The main difference between a generic (Sildenafil) and a branded medicine (Viagra) is that a branded drug is one pharma company’s baby. When a lab discovers a drug, there is normally a period of exclusivity, where only that lab can legally produce and market the drug before other labs can make their own versions (generics).

Viagra is a Pfizer drug. They launched it in 1998 and still own the rights to the name today. But in 2013, this exclusivity on the drug itself expired, so other labs were free to produce it as a generic (Sildenafil).

There are a few different versions of Sildenafil marketed by different pharma companies (for example Sildenafil Teva and Sildenafil Actavis) and Pfizer even have their own version of it too.

Viagra and Sildenafil pills look different, and come in different packaging. But clinically, the drugs are the same. Sildenafil should work just as well as its branded daddy.

Page reviewed by:
Dr Daniel Atkinson
Dr Daniel Atkinson
GP Clinical Lead
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Last updated 24/09/2021

What you need to know about Sildenafil pills

What is erectile dysfunction?

ED or male impotence is when a man can’t get an erection that stays hard enough for sex. Most men will encounter the problem at least once in their lives, and it's understood to affect around half of all men aged between 40 and 70 to some extent.[1] It can sometimes just happen sporadically, and resolve itself. But at other times it can persist and become a problem for couples when they want to have sex.

There are lots of reasons why it can happen. Physical causes can often be behind ED. Any medical condition that interferes with circulation, such as high cholesterol or diabetes, or with the transmission of nerve signals from the brain, can prevent a man from getting an erection. Lifestyle factors like smoking, drinking too much alcohol, being overweight or using recreational drugs can trigger ED too.

Emotions and psychology can also play a part. For example, men who are having sex with a new partner for the first time can sometimes feel eager to impress and put pressure on themselves to perform well. Stress is sometimes a factor as well, and this isn’t always necessarily related to sex.

How does Sildenafil work?

Does Sildenafil need a prescription?

How to buy Sildenafil online

How to take Sildenafil

Take Sildenafil one hour before you plan to have sex, and swallow your tablet whole with water. It should take around 30-60 minutes to work. Never take Sildenafil more than once in any 24 hour period.

Stick to the dose prescribed for you by the doctor. If you take more Sildenafil tablets than you should, seek medical attention.

If you get any serious side effects, go to your nearest emergency room as soon as possible. This includes any signs of an allergic reaction, chest pain, or an erection that lasts for four hours or more and won’t go away on its own.

How long does Sildenafil last?

Sildenafil with food and alcohol

Choosing the right Sildenafil dose

How effective is Sildenafil?

What are the most common Sildenafil side effects?

The side effect reported most is headache (1 in 10 or more users). Facial flushing, stuffy nose, blurred vision and an upset stomach are other common side effects (1 in 10 or fewer users).

Sticking as closely as you can to the instructions will help to lower the risk of side effects.

What should I do if I get side effects?

When should I see a doctor?

Is there any difference between Viagra and Sildenafil?

Sildenafil is a generic medication, so it will be less expensive. Some people call Sildenafil ‘cheap Viagra’ for this reason, but it does the same thing in the body and is usually just as effective.

Viagra is a branded medication from Pfizer, and the tablets have a distinct blue appearance and some in a recognisable box. Sildenafil isn’t always made by Pfizer, so the appearance of Sildenafil pills can vary between manufacturers. 

Some men prefer Viagra over the Sildenafil generic. But the two drugs are clinically bioequivalent, which means there’s little to no difference between them in how well they work.

What should I do if Sildenafil doesn’t work?

Sildenafil vs Tadalafil: Which is better?

Can you get Sildenafil for women?


Always read the patient information leaflet that comes with your tablet before taking this treatment.

You can find more safety information about Sildenafil at the link below.

More info

Sildenafil side effects

Some people get side effects when they take Sildenafil. If you notice any serious side effects, you should go to your nearest emergency room or talk to your doctor as soon as possible.

This includes: an erection that lasts for four hours or longer; any sudden loss of vision or hearing; any signs of an allergic reaction, like swelling of the face or breathing problems; chest pain (sit up and try to relax if this happens); or any type of fit or seizure.

Very common side effects (1 in 10 people or more):


Common side effects (1 in 10 people or less):

Flushing in the face, digestion problems, vision problems such as blurred vision, colour tinges in vision or being sensitive to light, stuffy nose, feeling dizzy.

Uncommon side effects (1 in 100 people or less):

Being sick, skin rash, bleeding at the back of the eye or bloodshot eyes, irritation or pain in the eyes, seeing double, strange sensation in the eye, watery eyes, palpitations or increased heartbeat, muscular pain, sleepiness, loss of sense of touch, vertigo, ringing sound in the ears, feeling sick, dry mouth, bleeding from the penis, finding blood in the semen or urine, tiredness, chest pain. 

Rare side effects (1 in 1,000 people or less):

Rise or drop in blood pressure, fainting, stroke, changes in heartbeat, heart attack, bleeding from the nose, sudden loss of hearing.

Frequency unknown:

Drop in blood flow to parts of the brain, sudden death, unstable angina.

Sildenafil warnings


[1] NHS Inform. 2020. Erectile dysfunction (impotence). [Accessed 23 February 2021].

[2] Wagner, G., Montorsi, F., Auerbach, S., Collins, M. 2001. Sildenafil Citrate (VIAGRA) Improves Erectile Function in Elderly Patients With Erectile Dysfunction: A Subgroup Analysis. Oxford Academic. [online]. The Journals of Gerontology: Series A, Volume 56, Issue 2, 1 February 2001, Pages M113–M119. [Accessed 23 February 2021].