Sildenafil and Allopurinol: Can You Take Them Together?
Understand the interaction between these medicines and how our UK pharmacist-led consultation keeps you safe.
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Sildenafil and Allopurinol: Understanding the Interaction
Published: 16 October 2026
Many men taking allopurinol for gout or high uric acid levels wonder whether sildenafil and allopurinol can be taken together safely. The good news is that no major interaction between sildenafil (Viagra) and allopurinol is commonly reported in clinical practice [3]. However, every medicine you take—including those for gout—must be declared during your consultation, so your prescriber can review your full medical history and ensure sildenafil is suitable for you.
Declare Every Medicine You Take
Even if no interaction is expected, your prescriber must know about allopurinol and all other medicines to check for serious interactions (like nitrates) and ensure sildenafil is safe for your individual health.
What Is Sildenafil and How Does It Work?
Sildenafil is a prescription medicine used to treat erectile dysfunction (ED) in men [1][3]. It belongs to a group of medicines called phosphodiesterase type 5 (PDE5) inhibitors. When you are sexually stimulated, sildenafil helps increase blood flow to the penis, making it easier to get and keep an erection suitable for sexual activity.
Sildenafil is available in three strengths: 25 mg, 50 mg, and 100 mg tablets. You take it about one hour before sexual activity, and the effect can last for up to four hours [3]. It does not work without sexual stimulation, and you should not take it more than once in 24 hours. Sildenafil is the active ingredient in branded Viagra, but generic versions are widely available and work in exactly the same way.
Common side effects include headache, flushing, indigestion, and nasal congestion [3]. Most men tolerate sildenafil well, but it is not suitable for everyone—particularly those taking certain other medicines or with specific heart conditions. That is why a consultation with a UK-registered prescriber is essential before starting treatment.
What Is Allopurinol and Why Is It Prescribed?
Allopurinol is a prescription medicine used to prevent gout attacks and to lower high levels of uric acid in the blood (hyperuricaemia). Gout is a type of arthritis that causes sudden, severe joint pain, often in the big toe, and is caused by a build-up of uric acid crystals in the joints. Allopurinol works by reducing the amount of uric acid your body produces, which helps prevent gout flares and long-term joint damage.
Allopurinol is usually taken once daily, and it may take several weeks or months to reach its full effect. It is a long-term treatment, and you will typically continue taking it even when you are not experiencing gout symptoms. Common side effects include skin rashes (which should be reported to your doctor promptly), upset stomach, and headaches.
Because allopurinol is taken regularly for a chronic condition, many men on this medicine will also be managing other health issues—such as high blood pressure, diabetes, or heart disease—which can themselves contribute to erectile dysfunction [1]. It is important to tell your prescriber about all your current medicines and health conditions during your ED consultation.
When to get urgent help
- Erection lasting more than 4 hours (priapism)—go to A&E immediately to prevent permanent damage
- Sudden loss of vision in one or both eyes—stop sildenafil and seek urgent medical help
- Chest pain during or after sex—stop sexual activity, do not take more sildenafil, and call 999 if pain is severe or does not go away
- Sudden loss of hearing or ringing in the ears—stop sildenafil and see a doctor urgently
- Severe dizziness, fainting, or signs of a serious allergic reaction (swelling of face, lips, or throat, difficulty breathing)—call 999
Call 999 or go to A&E in an emergency, or contact NHS 111 for urgent advice.
Do Sildenafil and Allopurinol Interact?
There is no commonly reported major interaction between sildenafil and allopurinol [3]. These two medicines work through different mechanisms in the body: sildenafil affects blood flow to the penis by inhibiting the PDE5 enzyme, while allopurinol reduces uric acid production by inhibiting the enzyme xanthine oxidase. Because they do not share metabolic pathways or target the same systems, they are not expected to interfere with each other's effectiveness or significantly increase the risk of side effects when taken together.
However, it is essential to understand that 'no major interaction' does not mean you can skip the consultation. Your prescriber needs to know about allopurinol—and every other medicine you take—to assess your overall health, check for any less common interactions, and ensure sildenafil is safe for you. For example, if you are taking allopurinol because of kidney problems or heart disease, those underlying conditions may affect whether sildenafil is suitable [1][3].
During your online consultation at Cured Pharmacy, you will be asked to list all your current medicines, including allopurinol, over-the-counter products, and herbal supplements. A UK-registered prescriber will review your answers and decide whether sildenafil is appropriate for you. Never stop or change your allopurinol (or any prescribed medicine) without speaking to your doctor first.
Why You Must Declare All Your Medicines
Even when no major interaction is expected, your prescriber must see the full picture of your health. Some medicines that do interact seriously with sildenafil—such as nitrates for angina, nicorandil, or riociguat—can be life-threatening if combined with ED treatment [3]. Other medicines, like alpha-blockers for prostate problems or blood pressure tablets, require careful dose adjustments and monitoring. By declaring allopurinol and all other medicines, you help your prescriber keep you safe and choose the most suitable treatment and dose for your individual circumstances.
What Your Prescriber Checks Before Approving Sildenafil
When you request sildenafil through Cured Pharmacy, a UK-registered prescriber will review your medical history and current medicines to ensure the treatment is safe for you [1][2]. They will check for any conditions that make sildenafil unsuitable, such as recent heart attack or stroke, unstable angina, severe heart failure, very low blood pressure, or a history of sudden vision loss (non-arteritic anterior ischaemic optic neuropathy, or NAION) [3].
Your prescriber will also look for medicines that interact with sildenafil. You must never take sildenafil with nitrates (such as GTN spray, isosorbide mononitrate, or isosorbide dinitrate), nicorandil, riociguat, or recreational drugs called 'poppers' (amyl or alkyl nitrites), as the combination can cause a dangerous drop in blood pressure [3]. If you take alpha-blockers like tamsulosin or doxazosin, you may still be able to use sildenafil, but you will need to start on the lowest dose and be stable on your alpha-blocker first.
Certain medicines that affect liver enzymes—such as ritonavir, cobicistat, ketoconazole, itraconazole, or clarithromycin—can increase sildenafil levels in your blood, requiring a lower starting dose [3]. Conversely, medicines like rifampicin, carbamazepine, phenytoin, or St John's wort can reduce sildenafil's effectiveness. Your prescriber will consider all these factors, along with your allopurinol and any other treatments, to decide on the safest option for you.
When Sildenafil May Not Be Suitable
Sildenafil is not suitable for everyone. If your doctor has advised you to avoid sexual activity—for example, because of severe heart disease—you should not take sildenafil or any other ED medicine [1][3]. Men with certain inherited eye conditions (such as retinitis pigmentosa) or a history of NAION should discuss alternative treatments with their doctor. If you have severe liver or kidney problems, your prescriber may need to adjust your dose or recommend a different medicine. Always answer the consultation questions honestly and fully so your prescriber can make the right decision for your health.
At a glance
Practical Advice for Taking Sildenafil and Allopurinol Together
If your prescriber approves sildenafil and you are already taking allopurinol, you can take both medicines as directed without needing to separate them by a specific time interval. Allopurinol is usually taken once daily, often in the morning, while sildenafil is taken about one hour before you plan to have sex [3]. There is no need to skip or delay your allopurinol dose on days when you use sildenafil.
Continue taking your allopurinol exactly as your doctor prescribed, even if you are not currently experiencing gout symptoms. Stopping allopurinol suddenly can lead to a flare-up of gout. If you experience any new or unusual symptoms after starting sildenafil—such as severe dizziness, chest pain during sex, a prolonged erection lasting more than four hours, or sudden loss of vision or hearing—stop taking sildenafil and seek urgent medical help [3].
Lifestyle factors can also affect how well sildenafil works. Heavy alcohol can worsen erectile dysfunction and add to the blood-pressure-lowering effect of sildenafil, increasing the risk of dizziness [3]. Grapefruit juice can raise sildenafil levels in your blood, so it is best to avoid it. Maintaining a healthy weight, staying active, managing stress, and controlling conditions like high blood pressure and diabetes can all improve your response to ED treatment [1][2].
Other ED Treatments and Allopurinol
If sildenafil is not suitable for you, or if you would prefer a different option, there are other licensed ED treatments available in the UK [2]. Tadalafil (Cialis) is another PDE5 inhibitor that can be taken on-demand (10 mg or 20 mg at least 30 minutes before sex, with effects lasting up to 36 hours) or as a daily low-dose tablet (2.5 mg or 5 mg) [4]. Vardenafil (Levitra) and avanafil (Spedra) are also PDE5 inhibitors, each with slightly different timing and duration [5][6].
Like sildenafil, these medicines have no major reported interaction with allopurinol, but the same safety checks apply: your prescriber must review your full medical history and current medicines before approving any ED treatment. All PDE5 inhibitors share the same contraindications—they must never be taken with nitrates, nicorandil, riociguat, or poppers [4][5][6].
For men who cannot use PDE5 inhibitors, other options include alprostadil cream (Vitaros) or alprostadil injections (Caverject), which work locally rather than throughout the body [7]. Eroxon is a topical gel medical device available without prescription. Your GP or a specialist can discuss these alternatives if tablets are not suitable. At Cured Pharmacy, our online consultation allows a UK prescriber to recommend the most appropriate ED treatment for your individual circumstances, taking into account your allopurinol and any other medicines you take.
Why See Your GP About New Erectile Dysfunction
If you are experiencing erectile dysfunction for the first time, it is important to see your GP for a full assessment [1][2]. ED can be an early warning sign of underlying health problems such as heart disease, high blood pressure, or diabetes. Your GP can check your cardiovascular health, review your medicines (including allopurinol), and identify any treatable causes. Addressing these underlying issues can improve your overall health and may also improve your erections. Once your GP has assessed you, an online consultation for ED treatment can be a convenient and discreet way to access ongoing care.
In summary
- No major interaction is reported between sildenafil and allopurinol—they can usually be taken together safely.
- Always declare allopurinol and all other medicines during your consultation for a full safety review.
- Never take sildenafil with nitrates, nicorandil, riociguat, or poppers—this combination is dangerous.
- See your GP if you have new erectile dysfunction, as it can signal heart disease or diabetes.
- Prescription ED treatment requires an online consultation reviewed by a UK-registered prescriber.
Scientific References
- NHS. (n.d.). Erectile dysfunction (impotence). NHS website. https://www.nhs.uk/conditions/erection-problems-erectile-dysfunction/ [accessed 16 October 2026]
- National Institute for Health and Care Excellence. (n.d.). Erectile dysfunction. Clinical Knowledge Summaries. https://cks.nice.org.uk/topics/erectile-dysfunction/ [accessed 16 October 2026]
- Joint Formulary Committee. (n.d.). Sildenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/sildenafil/ [accessed 16 October 2026]
- Joint Formulary Committee. (n.d.). Tadalafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/tadalafil/ [accessed 16 October 2026]
- Joint Formulary Committee. (n.d.). Vardenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/vardenafil/ [accessed 16 October 2026]
- Joint Formulary Committee. (n.d.). Avanafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/avanafil/ [accessed 16 October 2026]
- Joint Formulary Committee. (n.d.). Alprostadil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/alprostadil/ [accessed 16 October 2026]
Information on this page is for educational purposes only and is not medical advice. Prescription treatments require clinical assessment by a UK-registered prescriber. Always consult a qualified healthcare professional before starting or changing any medication.
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Medically reviewed by
Tarun Kumar, Superintendent Pharmacist (GPhC 2233073)