Sildenafil is one of the most widely used medicines for erectile dysfunction (ED). In the UK, sildenafil is available in a number of authorised medicines, including branded and generic products.
It is also the active ingredient associated with Viagra and Kamagra products. However, products containing sildenafil can have very different manufacturers, formulations and regulatory statuses.
This guide explains what sildenafil is, how it works, commonly used strengths, how it is taken for erectile dysfunction, possible side effects, medicine interactions and the relationship between sildenafil, Viagra and Kamagra.
What Is Sildenafil?
Sildenafil citrate is a medicine used primarily to treat erectile dysfunction in adult men. It belongs to a group of medicines known as phosphodiesterase type 5 inhibitors, usually shortened to PDE5 inhibitors.
Erectile dysfunction means being unable to achieve or maintain an erection sufficient for satisfactory sexual activity.
Sildenafil is also used medically for pulmonary arterial hypertension, although the doses and treatment arrangements for that condition are different from its use for erectile dysfunction.
How Does Sildenafil Work for Erectile Dysfunction?
During sexual stimulation, the body releases chemical signals that cause blood vessels supplying the penis to relax. This allows more blood to enter the erectile tissue.
Sildenafil inhibits the PDE5 enzyme and supports this natural process, helping improve blood flow to the penis.
Sildenafil does not automatically cause an erection. Sexual stimulation is still required for the medicine to have its intended erectile effect.
It also does not directly increase sexual desire or libido.
Sildenafil in the UK
Sildenafil is an established medicine in the United Kingdom and is available in a range of authorised products.
Depending on the specific product and circumstances, sildenafil may be supplied on prescription or through an appropriate pharmacy service.
UK-authorised sildenafil medicines are subject to medicines regulation covering areas such as product quality, manufacturing standards, approved information and safety monitoring.
This is important because the presence of sildenafil as an ingredient does not automatically mean that every sildenafil-containing product has UK marketing authorisation.
Sildenafil 25mg, 50mg and 100mg
For erectile dysfunction, authorised sildenafil tablets are commonly available in 25mg, 50mg and 100mg strengths.
A commonly recommended starting dose for many adult men is 50mg taken when required before sexual activity. Depending on effectiveness and tolerability, a healthcare professional may consider a lower or higher dose.
The maximum recommended dose for many authorised sildenafil products used for ED is 100mg, with a maximum recommended dosing frequency of once per day.
A higher dose should not automatically be considered better. Increasing the amount of sildenafil can also increase the likelihood of unwanted effects.
When Is Sildenafil Taken?
Sildenafil for erectile dysfunction is commonly taken approximately one hour before sexual activity.
The exact onset can vary between individuals. Factors such as food intake, general health and the particular medicine can affect how quickly the effect becomes noticeable.
Patients should follow the instructions supplied with their particular medicine rather than relying on a single general timing rule.
Does Food Affect Sildenafil?
Yes. Taking sildenafil with food can delay its onset compared with taking it in a fasted state.
A substantial or high-fat meal can therefore mean that sildenafil takes longer to produce its intended effect.
This does not necessarily mean that the medicine will not work, but the timing can be different.
Does Sildenafil Require Sexual Stimulation?
Yes. Sildenafil supports the body’s natural erectile response but does not create an erection independently of sexual stimulation.
This is an important distinction because sildenafil is not a sexual stimulant and does not directly increase sexual desire.
Sildenafil and Viagra
Viagra is a well-known branded medicine containing sildenafil citrate.
Sildenafil is therefore the active ingredient, while Viagra is a particular brand of sildenafil medicine.
Generic sildenafil medicines contain the same active pharmaceutical ingredient but are marketed under the ingredient name or another brand.
For a direct comparison involving another sildenafil-containing brand, read our Kamagra vs Viagra guide.
Sildenafil and Kamagra
Kamagra products are also associated with sildenafil citrate.
This means the relationship is similar to the distinction between an ingredient and a finished branded product:
- Sildenafil is the active pharmaceutical ingredient.
- Kamagra is a brand associated with products containing sildenafil.
However, Kamagra does not have UK marketing authorisation, whereas a number of sildenafil medicines are authorised for use in the UK.
Read our Kamagra and Sildenafil guide for a detailed explanation of this distinction.
Sildenafil Tablets and Kamagra Tablets
Conventional sildenafil medicines are commonly supplied as tablets. Kamagra is also commonly encountered in a tablet formulation associated with 100mg sildenafil.
Two tablets that state that they contain sildenafil should not automatically be treated as identical products.
Differences can include:
- Manufacturer
- Marketing authorisation
- Manufacturing and quality controls
- Tablet formulation
- Packaging
- Patient information
- Regulatory oversight
For information specifically about the Kamagra tablet formulation, see our Kamagra Tablets UK 100mg Sildenafil page.
Sildenafil and Oral Jelly
Sildenafil is most commonly associated with tablets, but Kamagra is also encountered in an Oral Jelly formulation.
Kamagra Oral Jelly is supplied in individual sachets rather than as a conventional solid tablet.
The formulation may be different, but sildenafil remains the active ingredient. The usual sildenafil contraindications, interactions and potential side effects therefore remain relevant.
See our Kamagra Oral Jelly UK page for more information.
Common Sildenafil Side Effects
Like all medicines, sildenafil can cause side effects, although not everyone experiences them.
Commonly reported side effects can include:
- Headache
- Dizziness
- Blocked or stuffy nose
- Indigestion or stomach discomfort
- Nausea
- Temporary changes in vision
Some people experience no significant side effects, while others may find a particular dose or medicine unsuitable.
Persistent or troublesome symptoms should be discussed with a pharmacist or doctor.
Serious Sildenafil Side Effects
Serious adverse reactions are less common but can require urgent medical attention.
Medical help should be sought for severe or unexpected symptoms such as serious allergic reactions, significant cardiovascular symptoms, seizures or a painful erection lasting several hours.
Anyone who develops concerning symptoms should not simply take another dose in an attempt to improve the response.
Sildenafil and Nitrate Medicines
One of the most important sildenafil interactions involves nitrate medicines, which may be used to treat chest pain or angina.
Sildenafil and nitrates should not normally be used together because the combination can cause a dangerous fall in blood pressure.
This warning applies to sildenafil as an active ingredient regardless of whether it appears in Viagra, a generic sildenafil medicine or another sildenafil-containing product.
Sildenafil and Poppers
Recreational substances commonly known as poppers can contain nitrates or nitrites.
Combining these substances with sildenafil can produce a significant and potentially dangerous reduction in blood pressure.
This is an important sildenafil safety consideration regardless of the brand or formulation involved.
Sildenafil and Other Medicines
Sildenafil can interact with medicines other than nitrates.
Additional caution or medical advice may be required for people using certain medicines for:
- Heart or blood-pressure conditions
- Enlarged prostate
- Pulmonary hypertension
- HIV
- Fungal infections
- Some bacterial infections
- Epilepsy
Anyone taking regular medication should check whether sildenafil is suitable before using it.
Who May Need Extra Care With Sildenafil?
Sildenafil is not suitable for every person.
Medical assessment may be particularly important for people with significant cardiovascular disease, very low blood pressure, serious liver or kidney problems, certain eye conditions or a history of serious cardiovascular events.
The suitability of sildenafil depends on the individual rather than simply on age or the presence of erectile dysfunction.
Why Persistent Erectile Dysfunction Should Be Checked
Erectile dysfunction can sometimes be associated with an underlying health condition rather than being an isolated sexual problem.
Possible contributing factors include:
- Cardiovascular disease
- Diabetes
- High blood pressure
- High cholesterol
- Hormonal conditions
- Anxiety or depression
- Smoking
- Alcohol
- Some medicines
Persistent ED can therefore be a reason to have a broader health assessment, even if an ED medicine appears to improve erections.
Sildenafil vs Tadalafil
Sildenafil and tadalafil are both PDE5 inhibitors, but they are not identical medicines.
One important difference is the treatment window. Tadalafil can remain useful for considerably longer than sildenafil, which can make the two medicines suitable for different preferences and circumstances.
Food can also affect the medicines differently.
Neither medicine should automatically be considered better for every patient. Individual response, health conditions, other medication and treatment preferences all matter.
Sildenafil vs Vardenafil
Vardenafil is another PDE5 inhibitor used for erectile dysfunction.
Like sildenafil, it is generally taken before anticipated sexual activity and requires sexual stimulation to have its intended effect.
The two medicines have differences in dosing, pharmacology and product information, so they should not be treated as interchangeable without considering the individual circumstances.
For a broader comparison, return to our ED Medicines by Ingredient guide.
Sildenafil and UK Medicine Regulation
The regulatory status of a medicine applies to the particular finished product rather than simply to its active ingredient.
This is why it is possible for sildenafil to be an authorised and widely used active ingredient while a particular sildenafil-containing brand does not have UK marketing authorisation.
Understanding this distinction is especially important when comparing licensed sildenafil medicines with Kamagra.
For more detail, see our Kamagra UK Safety & Regulatory Information page.
Sildenafil Frequently Asked Questions
What is sildenafil used for?
Sildenafil is mainly used to treat erectile dysfunction. It is also used medically for pulmonary arterial hypertension under different treatment arrangements.
Is sildenafil the same as Viagra?
Sildenafil is the active ingredient in Viagra. Viagra is a particular branded sildenafil medicine.
Does Kamagra contain sildenafil?
Kamagra products are associated with sildenafil citrate as their active ingredient. However, Kamagra and authorised UK sildenafil medicines do not have the same regulatory status.
What strengths does sildenafil come in for ED?
Authorised sildenafil tablets for erectile dysfunction are commonly available in strengths such as 25mg, 50mg and 100mg, depending on the particular product.
How long before sexual activity is sildenafil taken?
Many authorised sildenafil products recommend taking the medicine approximately one hour before sexual activity. The exact instructions supplied with the particular medicine should always be followed.
Can sildenafil be taken more than once per day?
For many authorised sildenafil products used for erectile dysfunction, the maximum recommended dosing frequency is once per day.
Does sildenafil work without sexual stimulation?
No. Sexual stimulation is required for sildenafil to support the erectile response.
Can sildenafil be taken with nitrates?
Sildenafil should not normally be combined with nitrate medicines because the combination can cause a dangerous reduction in blood pressure.
More ED Medicine Information
Continue with our ED Medicines by Ingredient guide to compare sildenafil with tadalafil and vardenafil.
For information specifically related to Kamagra and sildenafil, see:
- Kamagra and Sildenafil: What Is the Difference?
- Kamagra vs Viagra
- Kamagra UK Safety & Regulatory Information
- Kamagra Tablets UK 100mg Sildenafil
- Kamagra Oral Jelly UK
For our broader site overview, visit the Kamagra UK information homepage.
This page provides general educational information about sildenafil and erectile dysfunction. It does not replace individual advice, diagnosis or treatment from a doctor, pharmacist or other qualified healthcare professional.
