Sildenafil and Warfarin: Can You Take Them Together?

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Sildenafil and Warfarin: Understanding the Interaction

Published: 16 October 2026

Many men taking warfarin wonder whether sildenafil and warfarin can be used together safely. The good news is that sildenafil (Viagra) is not contraindicated with warfarin, but there are important considerations your prescriber will check before treatment. This page explains how these medicines interact, what monitoring may be needed, and practical steps to use erectile dysfunction treatment safely while on anticoagulation therapy.

What Are Sildenafil and Warfarin?

Sildenafil is a prescription medicine used to treat erectile dysfunction (ED) in men. It belongs to a group of medicines called PDE5 inhibitors, which work by relaxing blood vessels in the penis, allowing increased blood flow when you are sexually aroused [3]. Sildenafil is the active ingredient in Viagra and is available in 25mg, 50mg, and 100mg tablets. It is typically taken about one hour before sexual activity, and its effects can last for around four to six hours. Sexual stimulation is needed for the medicine to work.

Warfarin is an anticoagulant (blood thinner) prescribed to prevent blood clots in conditions such as atrial fibrillation, deep vein thrombosis, pulmonary embolism, or after heart valve replacement. It works by blocking vitamin K-dependent clotting factors in the liver, which slows down the blood's ability to clot. Because warfarin affects clotting, people taking it require regular blood tests (INR monitoring) to ensure the dose is correct and the blood is not too thin or too thick.

Both medicines are widely prescribed in the UK, and many men take warfarin for cardiovascular conditions while also experiencing erectile dysfunction. Understanding how these medicines interact is essential for safe and effective treatment [1].

Do Sildenafil and Warfarin Interact?

Sildenafil and warfarin do not have a major direct drug interaction that would make them unsafe to take together. Unlike medicines such as nitrates, nicorandil, or riociguat—which must never be combined with sildenafil due to the risk of a dangerous drop in blood pressure—warfarin does not share this mechanism. However, there are still important considerations.

Sildenafil can have a mild blood-thinning effect of its own by inhibiting platelet aggregation (the clumping together of blood cells). This effect is generally small and not clinically significant in most people, but when combined with warfarin, there is a theoretical risk of slightly increased bleeding [3]. This does not mean the combination is unsafe, but it does mean your prescriber and pharmacist will want to know you are taking warfarin before prescribing sildenafil.

Additionally, if you are taking warfarin, you may already have underlying cardiovascular disease. Your prescriber will assess whether your heart and circulation are healthy enough for sexual activity, as this can place extra demands on the heart. Erectile dysfunction itself can be an early warning sign of heart disease or diabetes, so it is always worth discussing new ED with your GP [1][2].

The Mechanism in Plain English

Warfarin stops your blood from clotting as quickly as it normally would by interfering with clotting factors. Sildenafil, on the other hand, relaxes blood vessels and may slightly reduce the stickiness of platelets (blood cells involved in clotting). When used together, the overall effect on bleeding is usually minimal, but your prescriber will want to ensure your INR (a measure of how thin your blood is) remains stable and that you are not at increased risk of bruising or bleeding.

What Your Prescriber Checks Before Prescribing Sildenafil and Warfarin Together

When you request sildenafil through an online consultation at Cured Pharmacy, our UK-registered prescribers review your full medical history and current medicines. If you are taking warfarin, the prescriber will check several key points to ensure sildenafil is safe and appropriate for you.

First, they will confirm that your INR is stable and that you are being monitored regularly by your GP or anticoagulation clinic. Unstable INR or recent bleeding problems may require further investigation before starting ED treatment. Second, they will assess your cardiovascular health, including any history of heart attack, stroke, angina, heart failure, or very low blood pressure. Sexual activity can increase heart rate and blood pressure, so it is important that your heart is healthy enough for this level of exertion [2].

Third, the prescriber will review all your other medicines to check for interactions. For example, if you are also taking an alpha-blocker (such as doxazosin or tamsulosin) for prostate symptoms, you will need to start on the lowest dose of sildenafil to avoid dizziness or fainting. If you are on other blood-pressure medicines, the prescriber will consider the combined effect on your blood pressure. Finally, they will ensure you are not taking any medicines that are absolutely contraindicated with sildenafil, such as nitrates, nicorandil, riociguat, or another PDE5 inhibitor [3].

Practical Advice for Taking Sildenafil with Warfarin

If your prescriber approves sildenafil while you are on warfarin, there are some practical steps you can take to use the medicine safely. Always take sildenafil exactly as prescribed—typically 25mg, 50mg, or 100mg about one hour before sexual activity, and no more than once in 24 hours. Do not increase the dose without consulting your prescriber.

Continue your regular INR monitoring as advised by your GP or anticoagulation clinic. If you notice any unusual bruising, bleeding gums, blood in your urine or stools, or prolonged bleeding from cuts, contact your doctor promptly. These could be signs that your blood is too thin, although they are not necessarily caused by the combination of medicines.

Avoid heavy alcohol when taking sildenafil, as it can worsen erectile dysfunction, increase the risk of side effects such as dizziness or headache, and add to the blood-pressure-lowering effect. Grapefruit juice can raise sildenafil levels in the blood and should be avoided. If you experience chest pain during or after sex, stop and seek urgent medical help—do not take GTN spray or any nitrate medicine, as this can cause a dangerous drop in blood pressure when combined with sildenafil [3].

If you develop an erection lasting more than four hours (priapism), sudden loss of vision or hearing, or severe dizziness, seek emergency medical attention. Always inform any healthcare professional treating you that you are taking both warfarin and sildenafil.

Starting Dose and Monitoring

Most men will start on sildenafil 50mg, but if you are taking other medicines that affect blood pressure or if you are older or have kidney or liver problems, your prescriber may recommend starting with 25mg. There is no need to change your warfarin dose simply because you are starting sildenafil, but do continue your regular INR checks and report any new symptoms to your GP or anticoagulation team.

Warning Signs and When to Seek Help with Sildenafil and Warfarin

While most men tolerate sildenafil well, it is important to know the warning signs that require urgent medical attention. If you experience chest pain, severe dizziness, or fainting during or after sexual activity, call 999 or go to A&E immediately. Do not take any nitrate medicine (such as GTN spray) if you have used sildenafil in the past 24 hours, as this combination can be life-threatening.

An erection lasting more than four hours (priapism) is a medical emergency and requires immediate treatment to prevent permanent damage to the penis. Sudden loss of vision in one or both eyes, or sudden decrease or loss of hearing (sometimes with ringing in the ears), are rare but serious side effects that need urgent assessment [3].

If you notice signs of increased bleeding—such as unusual bruising, nosebleeds that do not stop, blood in your urine or stools, vomiting blood, or coughing up blood—contact your GP or anticoagulation clinic the same day. These symptoms may indicate that your INR is too high, and your warfarin dose may need adjusting. While sildenafil is unlikely to be the sole cause, it is important to report any new symptoms when taking multiple medicines.

Common side effects of sildenafil include headache, flushing, indigestion, and nasal congestion. These are usually mild and temporary. If side effects are troublesome or persistent, speak to your prescriber, who may adjust the dose or suggest an alternative ED treatment such as tadalafil or vardenafil [2][3].

Alternative ED Treatments If You Take Warfarin

If sildenafil is not suitable or well tolerated, there are other licensed ED treatments available in the UK that can also be used with warfarin, subject to the same prescriber checks. Tadalafil (Cialis) is another PDE5 inhibitor that can be taken on-demand (10mg or 20mg at least 30 minutes before sex, with effects lasting up to 36 hours) or as a daily low dose (2.5mg or 5mg). The longer duration of tadalafil can offer more spontaneity for some men [4].

Vardenafil (Levitra) and avanafil (Spedra) are also PDE5 inhibitors with slightly different onset times and durations. Vardenafil is taken 25 to 60 minutes before sex, while avanafil can work in as little as 15 to 30 minutes. All PDE5 inhibitors share the same contraindications (nitrates, nicorandil, riociguat, poppers) and similar precautions regarding cardiovascular health and other medicines [5][6].

For men who cannot use PDE5 inhibitors, alternatives include Vitaros (alprostadil cream applied to the tip of the penis) or Caverject (alprostadil injection into the penis). These work by a different mechanism and do not have the same cardiovascular contraindications, but they require careful instruction and are not suitable for everyone. Eroxon is a topical gel medical device available without prescription that may help some men. Your prescriber or GP can discuss the full range of options with you [7].

At Cured Pharmacy, all prescription ED treatments are supplied only after an online consultation reviewed by a UK-registered prescriber. This ensures that the medicine is safe and appropriate for your individual circumstances, including your use of warfarin and any other health conditions or medicines.

Choosing the Right Treatment

The choice of ED treatment depends on your preferences, lifestyle, other medicines, and how quickly you want the medicine to work. Sildenafil is a well-established, cost-effective option for many men. Tadalafil offers longer-lasting effects, which some men prefer. Your prescriber will help you choose the most suitable option based on your consultation answers and medical history.

Scientific References

1. NHS. (n.d.). Erectile dysfunction (impotence). NHS website. https://www.nhs.uk/conditions/erection-problems-erectile-dysfunction/ [accessed 16 October 2026]
2. 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]
3. Joint Formulary Committee. (n.d.). Sildenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/sildenafil/ [accessed 16 October 2026]
4. Joint Formulary Committee. (n.d.). Tadalafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/tadalafil/ [accessed 16 October 2026]
5. Joint Formulary Committee. (n.d.). Vardenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/vardenafil/ [accessed 16 October 2026]
6. Joint Formulary Committee. (n.d.). Avanafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/avanafil/ [accessed 16 October 2026]
7. 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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