Information
What is Sildenafil?
Medically reviewed by Wajahet Ali Bhatti, GPhC Registration: 2203737 | Last updated: 22/01/2026
Sildenafil is a popular treatment used to help those with erectile dysfunction (ED) get and keep an erection. It’s a tablet that’s taken as needed before sex.
This was the first ED medication discovered in 1998, under the brand Viagra. Since then, non-branded (generic) forms are now available, simply called sildenafil. These are generally more affordable.
Belonging to the PDE-5 inhibitor family, sildenafil improves blood flow which can help with an erection.
How does it work?
By inhibiting the PDE-5 enzyme, blood vessels in the body relax, particularly in the penis, allowing more blood in.
This extra blood fills chambers within the penis, resulting in an erection hard enough for sex. Whilst effective, the erection does not spontaneously occur. It is important to be sexually stimulated for the treatment to work.
Sildenafil usually takes 30-60 minutes to start working, though some may notice effects after just 12 minutes. Highest concentrations of sildenafil in the blood occur 1 hour after taking. For this reason, it's recommended to take your dose approximately 1 hour before anticipated sexual activity.
How effective is sildenafil?
A large clinical trial involving over 8000 men assessed how effective sildenafil really is. The trial found 74% of men taking the 50mg dose reported improved erections. This increased to 82% with the 100mg dose and 62% with the 25mg dose.
When compared to placebo, which only improved erections in 25% of men, it can be seen sildenafil is indeed effective.
Although most effective for psychologically driven ED, the study found it also helped men with symptoms as a result of:
- Spinal cord injuries
- Depression
- Surgery
- Diabetes
- Heart disease
- High blood pressure
- Mixed causes
- Other physical causes
Sildenafil doses
Sildenafil is available in 3 different dose strengths:
- 25mg
- 50mg
- 100mg
Patients would usually start at the 50mg dose, taken no more than once daily, approximately 1 hour before anticipated sexual activity. Sexual stimulation is required for the treatment to work.
The dose can be increased to 100mg if the current dose fails to achieve satisfactory results on 4 or more occasions.
Similarly, the dose can be reduced to 25mg for those who are more prone to side effects, have kidney or liver problems, take certain other medication or have health conditions that affect how the drug is metabolised.
| Pack size | 25mg(price per tablet) | 50mg(price per tablet) | 100mg(price per tablet) |
|---|---|---|---|
| 4 pack | £2.50 | £2.50 | £2.50 |
| 8 pack | £1.87 | £1.87 | £1.87 |
| 16 pack | £1.56 | £1.56 | £1.56 |
| 32 pack | £1.25 | £1.25 | £1.25 |
How to take sildenafil
Always take sildenafil as instructed by your prescriber. You should not take sildenafil more than once a day.
Take one tablet approximately 1 hour before you plan to have sex, swallowing the tablet whole with water. Whilst the treatment can be taken with or without food, when taken alongside a high fat meal, it may take longer to work.
People react differently to sildenafil and the amount of time needed for it to work varies. Typically, it starts to take effect after 30-60 minutes.
For the treatment to work, sexual stimulation is required. Sildenafil will not spontaneously give you an erection.
Who can take sildenafil?
Sildenafil is suitable for men who are struggling to achieve or maintain an erection hard enough for satisfactory sexual performance and may include men who:
- May get an erection but not one that's strong enough for sex
- Struggle to maintain the erection long enough for satisfying intercourse
- Cannot get an erection at all.
Sildenafil is a suitable treatment for those who anticipate sex up to a few times a week. Those who are having sex more often or have a more spontaneous sex life may benefit from a daily medicine like tadalafil.
Who shouldn’t take sildenafil?
Whilst a generally safe treatment, sildenafil should not be taken by:
- Women
- Those who take other ED medicines
- Anyone allergic to any of the ingredients listed in the packaging.
It is also contraindicated in men who:
- Take nitrates ( for chest pain/angina)
- Use poppers (amyl nitrite)
- Take riociguat (for high blood pressure in the lungs)
- Have severe liver problems
- Have severe heart problems or who’ve had a recent heart attack
- Have had a recent stroke
- Have low blood pressure
- Have inherited eye diseases
- Have a specific eye condition called non-arteritic anterior ischaemic optic neuropathy (NAION)
It’s important to speak to your prescriber before taking sildenafil. You may need an alternative dose, or it may not be suitable in some cases, including if you:
- Have sickle cell disease
- Have leukaemia or multiple myeloma
- Have Peyronie’s disease (a penis condition)
- Have heart problems
- Have a current stomach ulcer
- Have bleeding disorders
- Have kidney or liver problems
- Are taking certain heart failure medicines
- Are taking certain HIV treatments
- Are taking alpha blockers (for high blood pressure or an enlarged prostate)
- Are allergic to lactate
Sildenafil side effects
Most men take sildenafil without any issues, but like other medications it can come with side effects. The most common are headaches, with over 10% of men experiencing them.
Other common side effects that impact between 1-10% of men include:
- Dizziness
- Changes in colour vision, blurred vision, other visual changes
- Flushing
- Blocked nose
- Feeling sick
- Indigestion
A serious but uncommon side effect is an allergic reaction. Signs include difficulty breathing, wheezing, dizziness and swelling of the face, lips, throat or eyes. This is a medical emergency requiring immediate medical attention. If these symptoms occur, you should call 999.
Other uncommon side effects, affecting fewer than 1% of men include:
- Nose inflammation, causing a runny, stuffy, itchy nose with sneezing
- Sleepiness
- Reduced sensation
- Eye pain, sensitivity to light, visual flashes
- Eye redness
- Tear duct issues and/or conjunctivitis
- Tinnitus (ringing in the ears)
- Vertigo (feeling like the room is spinning)
- Abnormal or fast heartbeat
- High or low blood pressure
- Nose bleeds
- Sinus congestion
- Acid reflux
- Being sick
- Dry mouth
- Stomach pain
- Rashes
- Muscle pain or pain in arms/legs
- Blood in your urine
- Chest pain
- Fatigue
- Feeling hot
More serious side effects are even rarer, impacting fewer than 1 in 1000 people taking sildenafil. These include things like:
- Seizures
- Stroke/mini stroke
- Fainting
- Serious eye problems like visual changes, swelling, bleeding, retinal detachment
- Deafness
- Heart attack
- Abnormal heart rhythm
- Severe drug reactions affecting the skin, with blistering, rashes, and sores
- Bleeding of the penis
- Priapism (an erection lasting more than 4 hours)
Many of these require urgent medical attention so it’s important you speak to a doctor. A full list of side effects can be found in the information leaflet enclosed with the medication.
If you’re considering sildenafil treatment, speak to our clinical team at Quick Meds, who will be happy to talk through any concerns you may have, ensuring the treatment is safe before prescribing.
Erectile dysfunction
What is erectile dysfunction?
Erectile dysfunction (ED) is a condition where a man cannot get an erection, or keep this erection long enough for sex. It affects around 50% of men aged 40-70, but is also becoming more common in younger men.
For an erection to occur, nerve signals from the brain and functioning blood vessels are needed to fill the penis with blood. ED occurs when one of these pathways is affected and can occur gradually or suddenly, causing stress and concern.
Anyone suffering with high blood pressure, high cholesterol, heart disease or diabetes are at risk of developing ED as these can disrupt blood flow to the penis.
Similarly, psychological factors can interfere with the nerve signals from the brain and cause ED symptoms.
If ED is caused by an underlying physical condition, it's important to address this first, but where there isn’t a clear cause, medication such as sildenafil can be used.
How does sildenafil help with ED?
Sildenafil belongs to a class of drugs known as phosphodiesterase 5 (PDE5) inhibitors. By blocking this enzyme, blood vessels relax, allowing blood to freely flow into the penis.
This increased blood flow which is then trapped in the penis chambers, can help achieve harder erections. This is not spontaneous and stimulation is required.
As with any other ED tablets, without sexual arousal or stimulation, the treatment will not work. Once stimulated, the medication takes over, allowing blood flow and the resulting erection.
If ED is caused by stress or other psychological factors, the good news is sildenafil can still work. Clinical trials show sildenafil helped 84% of men with psychological ED improve their erections.
Alternatives to sildenafil
Whilst an effective treatment for erectile dysfunction, sildenafil is not the only option. Common alternatives include:
Viagra
Viagra is probably the most well-known ED medication. It contains the same active ingredient as sildenafil, but some men prefer to choose Viagra as they know the brand.
Cialis
Cialis is another branded medication. This treatment contains a different drug, tadalafil, and may take slightly less time to work than Viagra or sildenafil.
Tadalafil
The generic form of Cialis, tadalafil, is often cheaper. This medication can also be taken as a low-dose daily pill. This is a good option for those having sex often or with a more spontaneous sex life.
| ED medications (PDE 5 inhibitors) | Viagra | Sildenafil | Cialis | Tadalafil |
|---|---|---|---|---|
| Active ingredient | Sildenafil | Sildenafil | Tadalafil | Tadalafil |
| How quickly does it work? | 1 hour | 1 hour | 30 minutes+ | 30 minutes+ |
| How effective is it at improving erections? | 82% | 82% | 86% | 86% |
| Doses | 25, 50, or 100mg | 25, 50, or 100mg | 5, 10, or 20mg | 2.5, 5, 10, or 20mg |
| When is it taken? | As needed before sex | As needed before sex | As needed before sex | As needed before sex or daily (tadalafil daily) |
| Branded? | Yes | No | Yes | No |
Buying ED medication online
ED medications are most often only available on prescription. This means it needs to be prescribed by a healthcare professional after they’ve checked it’s suitable for you.
Here at Quick Meds, patients submit a short online form for medical review. This form asks questions about health and use of other medication to determine suitability.
Once approved, orders from Quick Meds can be dispatched on the same day for delivery as soon as by the next day.
It's important to source treatment from a registered supplier. Any website which attempts to bypass regulatory requirements such as not asking about your health should raise alarms.
Unregulated providers may be providing counterfeit medication, which may not contain the intended ingredients and may contain contaminants which can be harmful.
Quick Meds is a UK-based online pharmacy that’s regulated by the General Pharmaceutical Council (GPhC), so you know all medication is 100% legitimate. Our team of healthcare professionals has over 40 years’ collective experience and are always there to support you throughout your erectile dysfunction treatment.
FAQs
How does sildenafil work?
Sildenafil works by relaxing the blood vessels leading to the penis. This helps more blood flow into the chambers inside the penis, making it easier to get and keep an erection. For sildenafil to work, you’ll still need to be sexually stimulated.
Is it safe to drink alcohol with sildenafil?
Taking sildenafil and drinking alcohol isn’t necessarily dangerous, but drinking a lot of alcohol may interfere with your ability to get an erection. It’s best to avoid drinking alcohol while taking sildenafil.
Which ED medication is best?
Erectile dysfunction medications work differently for different people, and you may need to try a few different ones before you find one that works for you.
The most common ED medications, sildenafil (Viagra) and tadalafil (Cialis), are both effective for improving erections, with sildenafil improving erections in 82% of men and tadalafil in 86% at the highest dose.
Can I cut sildenafil tablets in half?
Yes, sildenafil tablets can generally be cut if you require a lower dose, though it's best to check with your pharmacist first. Use a proper pill-cutter and only for tablets which are scored. Coated tablets may crumble if cut as these are not designed to be cut.
What’s the difference between Viagra and sildenafil?
Viagra and the generic product both contain the same active ingredient: sildenafil citrate.
Viagra is a branded version that is created by Pfizer, the pharmaceutical company who first discovered the drug.
Generic sildenafil can be manufactured by any pharmaceutical company and as these companies aren’t discovering the drug, generic forms are usually more affordable.
Is sildenafil available on the NHS?
Yes, sildenafil can be prescribed on the NHS, though the quantity may be restricted. If you pay for your NHS prescription, you’ll pay the standard charge regardless of quantity prescribed.
Many find it easier and more affordable to buy sildenafil from a regulated online pharmacy like Quick Meds
Does sildenafil come with side effects?
Yes, like all medication sildenafil can come with side effects, although many men take it without any issues.
The most common side effect is headaches, and more serious side effects are much less common. Check out the patient information leaflet inside the medication for a full list of side effects.
Can I use sildenafil long term?
Yes, studies suggest that most men can take sildenafil safely for months or years without harmful effects.
Can I take sildenafil every day?
Yes, you can take sildenafil every day if needed, though ensure to not exceed more than one dose in 24 hours. If you find that you’re taking sildenafil very regularly, it may be better to switch to another medicine called tadalafil at a low dose everyday, which provides continuous coverage.
Why is sildenafil not working for me?
If sildenafil is not working, try the following steps: 1. Take the tablets on an empty stomach 2. Wait at least 1 hour before sexual activity 3. Ensure you are sexually aroused - sildenafil won’t work without stimulation 4. Avoid heavy alcohol 5. Try for about 4 times before deciding it's ineffective.
If, following the above steps, you still find the treatment is not working, speak to your pharmacist to increase your dose or suggest a suitable alternative medicine.
How long does sildenafil work for?
Sildenafil has a half-life between 3-5 hours, meaning half of the drug leaves your system in this time.
Based on this, sildenafil typically lasts between 4-6 hours at full effectiveness, with peak effects occurring 1-2 hours after taking. Some studies have even shown effectiveness for up to 12 hours.
How quickly does sildenafil start working?
Sildenafil typically starts working within 30-60 minutes though some may notice effects as early as 12 minutes. It's best to plan your dose and ideally, take it about 1 hour before anticipated sexual activity.
To prevent delays, it's best to take sildenafil on an empty stomach, as food can delay how long it takes to work by up to an hour.
Are there any natural alternatives to sildenafil?
Lifestyle changes like eating a healthy diet (especially one based on the Mediterranean diet), stopping smoking, exercising more, and managing stress are natural ways to reduce the risk of developing ED and improve symptoms in some cases.
However, there’s limited evidence about natural remedies and supplements that can help with erectile dysfunction, and none are recommended by healthcare professionals.
Can I buy sildenafil over the counter
The only over the counter version of sildenafil available is as Viagra Connect. This form is only available at the 50mg dose and does not require a prescription. You will still need to answer a few questions with a pharmacist, to ensure safe supply.
Why is Viagra more expensive than sildenafil
Viagra is a branded form of sildenafil. The company behind it, Pfizer, invested significant time and money into its initial discovery and marketing. For this reason, it generally costs more than its generic equivalent.
Can sildenafil help me last longer?
Sildenafil works by treating ED, allowing for more sustained erections. Whilst this can boost confidence and performance, it doesn't directly delay ejaculation. If you suffer with premature ejaculation, you should speak to your healthcare provider for a suitable treatment