Sildenafil, the active ingredient in Viagra, is one of the most commonly used treatments for erectile dysfunction in the UK. For many men, it is well tolerated when used as prescribed. However, it is still a prescription only medicine, so it is important to understand when it may not be suitable and the precautions you should take before using it.
This guide explains what to avoid, who should not take sildenafil, and the warning signs that mean you should seek medical advice. The information is based on NHS and MHRA guidance to help you make an informed decision about your treatment.
The most important safety message is this: Sildenafil should never be taken with nitrate medicines, as this combination can cause a dangerous drop in blood pressure.
Quick answer (featured snippet style): Sildenafil may be suitable for many men when it is prescribed following a clinical assessment. However, it should never be taken with nitrate medicines or recreational drugs known as "poppers." It may also not be suitable for people with certain heart conditions, very low blood pressure, or some rare eye conditions. While alcohol is not strictly prohibited, drinking too much alcohol may make it more difficult to get or maintain an erection. A clinical assessment by a qualified prescriber is the best way to determine whether sildenafil is appropriate for you.
Is sildenafil safe? The short answer
For many men who do not have any contraindications, sildenafil has a well established safety profile and has been used since it was first licensed in the late 1990s.
Whether sildenafil is suitable for you depends on several factors, including:
- The medicines you are currently taking.
- Your overall health and medical history.
- Using a genuine, licensed medicine at the dose recommended by your prescriber.
Most safety concerns can be avoided by taking sildenafil exactly as prescribed, avoiding medicines that interact with it, and only obtaining it from a regulated pharmacy.
Treatment suitability always depends on an individual clinical assessment. Your prescriber will determine whether sildenafil is an appropriate treatment for you.
What is sildenafil and how does it work?
Sildenafil belongs to a group of medicines known as PDE5 inhibitors.
It works by relaxing the blood vessels in the penis, allowing more blood to flow into the area when you are sexually aroused. Sexual stimulation is still needed for sildenafil to work, as it does not increase sexual desire on its own.
Sildenafil is available as both a generic medicine and the original branded version, Viagra. It also belongs to the same family of medicines as other erectile dysfunction treatments, such as tadalafil.
Generic sildenafil contains the same active ingredient as branded Viagra and is required to meet the same UK quality, safety, and effectiveness standards. Your prescriber will help determine which treatment option is most appropriate for your individual needs.
Medicines you should never combine with sildenafil
Some medicines can interact with sildenafil and may not be safe to take together. Before starting treatment, always tell your prescriber and pharmacist about all the medicines you take, including prescription medicines, over the counter products, supplements, and any recreational drugs.
Nitrates and poppers
The most important interaction is with nitrate medicines. These are commonly prescribed for chest pain (angina) and include medicines such as glyceryl trinitrate (GTN), isosorbide mononitrate, and isosorbide dinitrate. Recreational drugs known as poppers (such as amyl nitrite) should also never be taken with sildenafil.
Both sildenafil and nitrates can lower blood pressure. Taking them together can cause a sudden and potentially dangerous drop in blood pressure. For this reason, the NHS advises that sildenafil should never be taken with nitrates or poppers.
Riociguat and medicines for pulmonary hypertension
Sildenafil should also not be taken with riociguat, a medicine used to treat certain types of pulmonary hypertension. Combining these medicines may also lead to a significant drop in blood pressure.
If you take any medication for high blood pressure in the lungs, let your prescriber know before considering sildenafil.
Alpha blockers and other medicines that may interact
Some medicines may affect how sildenafil works or increase the likelihood of side effects.
These include:
- Alpha blockers used to treat high blood pressure or an enlarged prostate.
- Some blood pressure medicines.
- Certain antibiotics and antifungal medicines.
- Some medicines used to treat HIV.
Taking one of these medicines does not always mean sildenafil is unsuitable. However, your prescriber will review your medications and decide whether sildenafil is appropriate or whether any adjustments are needed.
Who should not take sildenafil (Viagra)?
According to NHS guidance, sildenafil may not be suitable for everyone. You should speak to your prescriber if you:
- Take nitrate medicines or riociguat.
- Have previously had an allergic reaction to sildenafil or a similar medicine.
- Have serious heart or liver disease.
- Have recently had a stroke, heart attack, or a serious heart rhythm problem.
- Have low blood pressure or high blood pressure that is not well controlled.
- Have a rare inherited eye condition, such as retinitis pigmentosa.
- Have a blood disorder, such as sickle cell anaemia, leukaemia, or multiple myeloma.
- Have a stomach ulcer or a bleeding disorder, such as haemophilia.
- Have a physical condition affecting the shape of the penis, such as Peyronie's disease.
These are some of the reasons why sildenafil should only be taken after a clinical assessment. Your prescriber will review your medical history to help determine whether the treatment is suitable for you.
Who should speak to a prescriber first?
Some people may still be able to take sildenafil but should have a more detailed assessment before starting treatment.
This includes people with cardiovascular disease, as sexual activity places additional demands on the heart. Your prescriber will assess whether it is safe for you to use sildenafil based on your overall health.
You should also speak to your prescriber if you have reduced kidney or liver function, as these conditions can affect how sildenafil is processed by the body.
The most important thing to remember is that sildenafil is not a one size fits all treatment. A qualified prescriber will assess your individual circumstances and decide whether it is appropriate for you. Sildenafil should only be used under the supervision of an appropriately qualified prescriber.
Sildenafil and alcohol
A common question is whether you can drink alcohol while taking sildenafil. The answer is that you do not usually need to avoid alcohol completely. Drinking a small amount is unlikely to stop sildenafil from working for most people.
However, drinking too much alcohol can make it more difficult to get or maintain an erection. Both alcohol and sildenafil can also lower blood pressure, which may increase the chance of feeling dizzy or lightheaded.
If you are planning to take sildenafil, drinking alcohol in moderation is generally the best approach. Everyone responds differently, so your experience may vary.
Food, grapefruit juice, and caffeine
Grapefruit juice
It is best to avoid grapefruit and grapefruit juice when taking sildenafil. Grapefruit can affect the way your body processes the medicine, which may increase the amount of sildenafil in your bloodstream and make side effects more likely.
Caffeine
There is no well established interaction between sildenafil and caffeine. For most people, having a normal cup of tea or coffee is not expected to cause any problems.
However, consuming large amounts of caffeine may affect sleep or increase feelings of anxiety, which could have an impact on sexual performance. As with most things, moderation is recommended.
Food and heavy meals
You can eat and drink as normal when taking sildenafil. However, eating a very large or high fat meal before taking your tablet may slow down how quickly it is absorbed, which could delay when it starts to work.
If you are planning ahead, a lighter meal may be a more convenient option.
Sildenafil strengths and the one dose in 24 hours rule
Sildenafil tablets for erectile dysfunction are available in 25mg, 50mg, and 100mg strengths. While many people ask about 100mg sildenafil, the highest strength is not necessarily the best option for everyone. Your prescriber will recommend the most appropriate strength based on your individual needs.
One important safety rule applies to everyone: Do not take sildenafil more than once in any 24 hour period.
Taking more than one dose within 24 hours will not improve its effectiveness and may increase the risk of side effects. Always follow the instructions provided by your prescriber and the Patient Information Leaflet.
Sildenafil, illness, and fever
Some people wonder whether it is safe to take sildenafil if they have a fever or are feeling unwell.
Sildenafil does not usually cause a fever. It can cause flushing, which is a warm or red feeling in the face and neck. This is a common side effect and can sometimes be mistaken for a fever, but it is different from having a raised body temperature.
If you have a high temperature, are dehydrated, or are feeling generally unwell, it may be sensible to wait until you have recovered before taking sildenafil. Illness and dehydration can affect your blood pressure and may change how you respond to the medication.
If you feel unwell after taking sildenafil or you are unsure whether it is appropriate to take it, speak to your prescriber or pharmacist for advice.
Kamagra Oral Jelly and unlicensed sildenafil: An important warning
Kamagra, including Kamagra Oral Jelly, is often marketed as a treatment for erectile dysfunction that contains sildenafil. However, it is not licensed for use in the UK.
This is an important distinction because unlicensed products have not been assessed by the MHRA for their quality, safety, or effectiveness. As a result, there is no guarantee that they contain the stated amount of sildenafil or that they are free from other undeclared ingredients.
The MHRA has repeatedly warned against buying erectile dysfunction medicines from unregulated sources. Illegal products seized in the UK have sometimes contained incorrect amounts of the active ingredient or other unexpected substances.
Because the contents of unlicensed products cannot be guaranteed, combining products such as Kamagra Oral Jelly with alcohol or other medicines may increase the risk of unwanted effects.
The safest option is to use a licensed sildenafil medicine supplied by a regulated UK pharmacy after a clinical assessment by a qualified prescriber.
Side effects and warning signs to look out for
Like all medicines, sildenafil can cause side effects, although not everyone experiences them. Most side effects are mild, temporary, and improve as the medicine wears off.
Common side effects include:
- Headache
- Flushing (a warm or red feeling in the face)
- Indigestion
- A blocked or runny nose
- Mild changes in vision, such as a blue tinge or blurred vision
While these side effects are usually not serious, some symptoms require immediate medical attention.
Seek urgent medical help straight away if you experience:
- An erection that lasts longer than 4 hours (priapism), as this is a medical emergency.
- Sudden loss of vision or a significant reduction in vision in one or both eyes.
- Sudden hearing loss or a noticeable decrease in hearing.
- Chest pain during or after sexual activity. If this happens, do not take a nitrate medicine and seek emergency medical assistance immediately.
- Signs of a serious allergic reaction, such as swelling of the face, lips, tongue, or throat, or difficulty breathing.
Being aware of these warning signs before starting treatment can help you use sildenafil safely and know when to seek medical advice promptly.
How to get sildenafil safely in the UK
The safest way to use sildenafil is through a UK regulated, GPhC registered pharmacy that requires a clinical assessment. You can complete a confidential online consultation and, if appropriate, an independent prescriber will approve treatment. You can start that process through the erectile dysfunction treatment assessment, which takes only a few minutes.
A regulated service gives you three things an unregulated seller cannot: a prescriber who checks your medicines and medical history, a genuine licensed product, and aftercare if you have questions or side effects. Our prescribing decisions are overseen by our clinical team, and treatment is dispensed with discreet, tracked delivery. Pricing is shown clearly before you order, with no hidden prescription fee.
Key takeaways
- Never combine sildenafil with nitrates, poppers or riociguat, as this can cause a dangerous drop in blood pressure.
- Sildenafil is not suitable for everyone, including some people with heart disease, low blood pressure or certain eye and blood disorders.
- Alcohol is not banned, but heavy drinking reduces the benefit and increases side effects.
- Avoid grapefruit juice, and never take more than one dose in 24 hours.
- Avoid unlicensed products such as Kamagra oral jelly, and use a regulated UK pharmacy with a clinical assessment.
Medical disclaimer
This article is provided for general information only and should not be considered medical advice, a diagnosis, or a treatment recommendation. It is not a substitute for a consultation with a qualified healthcare professional.
Sildenafil is a prescription only medicine and should only be used following an individual clinical assessment and under the supervision of an appropriately qualified prescriber. It is not suitable for everyone and should never be taken with nitrate medicines or poppers.
Do not start, stop, or change any medication based on the information on this page alone. Always read the Patient Information Leaflet supplied with your medicine and speak to your prescriber or pharmacist if you have questions about your treatment, other medicines you take, or any side effects you experience.
If you develop chest pain, an erection lasting more than 4 hours, sudden loss of vision or hearing, or signs of a serious allergic reaction, seek urgent medical attention immediately.
Suspected side effects can also be reported through the MHRA Yellow Card Scheme.
This content is intended to reflect current NHS and MHRA guidance and should be reviewed by a qualified clinician before publication.
