Viagra vs sildenafil: What’s the difference between the two

If you've been reading about ED (erectile dysfunction) treatments, trying to figure out the similarities and differences between Viagra and sildenafil, it’s normal.

We know what you are wondering: If sildenafil costs a couple of pounds a tablet and Viagra costs several times that, surely something's been left out of the generic version, right?

Not really. That's really the whole story, and the rest of this guide is just the detail behind it.

Read on to understand why the same medicine costs different, how they are different, and which one is more suited for you.

Before you begin: A little bit about OnlineClinic

OnlineClinic has been operating since 2011 and has treated more than 2.1 million patients.

The pharmacy is registered with the General Pharmaceutical Council (GPhC), our prescribers are registered with the General Medical Council (GMC), and the service is regulated by the Care Quality Commission (CQC).

As of August 2026, on Trustpilot, the service rating is 4.6 out of 5 based on 6,715 reviews.

Yes. Both medicines have the same active ingredient: Sildenafil citrate.

Both medicines are available as 25mg, 50mg, and 100mg tablets.

Both medicines are licensed in the UK for the same treatment, and both carry the same small print: They help the erection happen, but you still need to be aroused for anything to work.

The bit nobody tends to explain is what a generic manufacturer has to go through first.

They can't just make a tablet and put sildenafil on the label. They have to test it against real Viagra in volunteers and prove that two things match:

• How much of the drug gets into the bloodstream
• How quickly the drug gets into the bloodstream

Both have to land inside a narrow band, between 80% and 125% of the branded product, before the MHRA will license it.

The factories themselves are held to the same manufacturing and inspection standards as Pfizer's.

Viagra and sildenafil are not physically identical, so it's worth knowing where the differences lie.

• The colour and shape. The blue diamond is Pfizer's branding, not part of the medicine. Generics tend to be white and round or oval.
• The bits that aren't the drug. Fillers, coatings and binders vary between manufacturers.
• Lactose. Some sildenafil tablets contain a little lactose. One commonly dispensed UK 50mg version has 3.7mg in it, and its leaflet advises to avoid it if you have certain rare inherited sugar intolerances.
• The box and the pack size.
• The price. Our Viagra price starts at £28.99, while our sildenafil prices start at £8.99.

If you know you react to something in a particular tablet, say so in your consultation, and the prescriber will pick around it.

Sildenafil doesn't cause an erection. It's worth saying plainly, because it's the single biggest misunderstanding.

Here's what's going on.

When you're aroused, your body releases something that widens the blood vessels in the penis.

There's an enzyme that shuts that signal off again fairly quickly.

Sildenafil blocks the enzyme, so the signal lasts for longer and the erection holds.

No arousal, no signal, nothing for the tablet to work with. Taking one and waiting for something to happen won't get you anywhere.

What to expect:

• Take it about an hour before sex
• It usually peaks somewhere between 30 minutes and two hours, with an hour being the usual timeframe
• You've then got a useful window of roughly 4 to 5 hours
• A big fatty meal pushes the peak back by about an hour
• Alcohol works against you regardless, because it interferes with erections all on its own

The trial that started all of this was published in the New England Journal of Medicine back in 1998.

In it, 69% of attempts at sex were successful on sildenafil, against 22% on placebo.

Everything since has pointed the same way.

Pooling the trials, somewhere around 75% to 80% of men get a meaningful improvement, compared with roughly 25% on a placebo.

One review of 16 trials found men on sildenafil were about 3.75 times as likely to report better erections.

A network meta-analysis published in the Journal of Sexual Medicine in 2026 put sildenafil 100mg at the top of the group when the on-demand treatments were compared against each other.

Most men don’t experience a lot of side effects on Viagra or sidelnafil, or maybe they experience mild side effects.

Here are some of the side effects from taking Viagra or sildenafil:

Common side effects

• Headache
• Flushing (sudden reddening of face, neck, or the upper chest, caused by widening of the blood vessels)
• Indigestion
• Blocked or runny nose
• Dizziness
• Blurred vision, or a temporary blue tinge to things

That last side effect happens because sildenafil has a mild effect on a related enzyme in the eye. It doesn’t last for long, and it's more likely at 100mg.

Rare side effects, but get help straight away

• An erection lasting more than four hours
• Sudden loss or reduction of hearing
• Sudden loss of vision in one eye
• A serious allergic reaction

There are some situations, medical conditions, and medicines that can interact with or affect the absorption of sildenafil. It is wise to stay off sildenafil in those circumstances.

Tell the prescriber, and expect a lower starting dose or a different plan, if you:

• Take an alpha-blocker for blood pressure or prostate symptoms
• Take ritonavir, or medicines like ketoconazole or erythromycin, which push sildenafil levels up
• Have significant kidney or liver problems
• Have a heart condition where you've been told to avoid exertion

Generic sildenafil suits most men:

• Same active ingredient, same strength
• Considerably cheaper per tablet, which adds up if you expect to use it regularly
• Available in all three strengths, so the dose can be adjusted
• It's what most men in the UK now take

Branded Viagra still makes sense for some:

• You've taken it before, it worked, and you'd rather not change anything
• A particular generic hasn't agreed with you
• You are familiar with Viagra and feel more confident in taking the branded version

Either is fine when:

• You're starting out and have no strong feeling either way
• The price difference doesn't matter much to you
• Your prescriber is happy with both

There's no rush on any of this. Most men take weeks to get to the point of ordering. Taking a little longer costs nothing.

This one confuses the comparison, so it's worth separating out.

Viagra Connect is sildenafil 50mg that the MHRA agreed could be sold without a prescription.

It was authorised in November 2017 and reached shelves in spring 2018.

You have a short chat with the pharmacist and can buy up to eight tablets, at that one strength.

Cialis Together, which is tadalafil, was also approved for sale without prescription in March 2023.

But here’s the catch:

• There's only one strength, so no moving up to 100mg or down to 25mg, depending on how you get on
• Going through a prescriber gets you all three strengths, larger quantities, and a record of the decision
• Per tablet, prescription sildenafil usually works out cheaper

The most useful recent news on this subject isn't about the medicine at all. It's about where men are getting it.

In February 2026, the MHRA reported that it had seized around 20 million doses of illegal erectile dysfunction medicine between 2021 and 2025, with 4.4 million of those in 2025 alone.

It warned that what turns up in these blisters may contain no active ingredient, the wrong dose, other drugs entirely, or something toxic.

Over the same period, the MHRA shut down more than 1,500 websites and social media accounts.

The regulator was blunt about why that market exists, and they're right.

Embarrassment is what's being sold. Sites promising no questions and no prescription are offering privacy and delivering an unknown tablet.

You can get the privacy without the gamble. Two checks take a minute:

• Look the pharmacy up on the GPhC register
• Look for the GPhC registered pharmacy logo

While we're on recent news, there's a story doing the rounds that sildenafil might protect against Alzheimer's.

One study in Neurology in 2024, found lower rates of diagnosis among men who'd taken these drugs.

Another study from the National Institutes of Health and a genetic analysis from Oxford, found nothing of the sort.

The evidence argues with itself, none of it comes from a proper randomised trial, and sildenafil isn't licensed for anything to do with memory.

Around a quarter of men don't get enough out of sildenafil even at the right dose, and some just don't get on with the side effects.

There are three other licensed alternatives to choose from:

Tadalafil's long window is why a lot of men end up moving to it. It takes the planning out of the evening.

It also comes as a low daily dose, which we compare with the on-demand approach in Cialis daily vs on demand.

If you'd rather try a couple before settling on one, that's what our trial pack is for.

Switching between these is routine, and it doesn't mean the first one failed you.

Erections depend on blood vessels, and problems with blood vessels tend to show up in the small ones before the big ones. So erectile trouble can be an early signal rather than a standalone problem.

The clearest evidence comes from a study in European Urology in 2003.

Among men who came in with chest pain and turned out to have coronary artery disease, two thirds had noticed erection problems first, on average around three years earlier.

A later meta-analysis in the Journal of the American College of Cardiology linked erectile dysfunction to higher rates of heart disease and stroke, largely regardless of the usual risk factors.

And a study of 95,038 men in PLOS Medicine found that the more severe the problem, the higher the risk.

None of that means an erection problem is an emergency.

It means it's a sensible moment to have someone look at blood pressure, blood sugar, cholesterol, sleep, alcohol, weight, and mood, plus anything you're already taking.

Antidepressants, some blood pressure medicines, and finasteride can all play a part.

Figure out what's causing it before treating it.

That's what the consultation is for. There's more on when it's worth a closer look at when to see a doctor about ED.

Our process is straightforward. From start to finish, this is what happens when you order with us:

1. Fill in the medical questionnaire. It takes 5 to 10 minutes. The questions are thorough on purpose
2. A GMC-registered prescriber reviews your answers. If they need more details, they will message you through your account
3. If treatment is approved, it is dispensed and dispatched. Next-day delivery, with the full patient information leaflet included.
4. If it is not approved, the prescriber explains why. You are not charged for treatment that is not issued

Our Viagra price starts at £28.99, while our sildenafil prices start at £8.99.

Every price includes the prescription, the consultation, the packaging, and delivery. There are no hidden fees.