Sildenafil and Efavirenz: Can They Be Taken Together?
Learn how these medicines interact and how our UK pharmacist-led consultation ensures safe, effective treatment.
Takes less than 2 minutes to complete 100% online
ED Treatments We Offer
Pricing Disclaimer: Prices on some pages may not be up to date — the live pricing table below and pricing shown during consultation are official current prices and take precedence over any other figures on the site.
Trusted by thousands
Real people, real results
“Fabulous service and quick delivery”
“Felt the effects of these after about 5 days. Erections are better and can get aroused and ready easier”
“Excellent value and brilliant customer service”
“I received excellent service and everything went well. The order arrived quickly and in perfect condition. It's working very well! Thank you to the whole team for the great work!”
“Delivery was good, the product lived up to expectations.”
How Our Online Service Works
Choose your treatment
Select from our recommended UK-licensed medications.
Answer quick questions
Select safe UK treatments. Quick answers.
Get it delivered fast
We will deliver direct to you as quickly as tomorrow.
Sildenafil and Efavirenz: Understanding the Interaction
Published: 16 October 2026
Many men taking efavirenz for HIV wonder whether sildenafil and efavirenz can be taken together safely. The good news is that sildenafil can usually be used alongside efavirenz, but the interaction requires careful dose adjustment because efavirenz reduces the effectiveness of sildenafil. This page explains how these medicines interact, what your prescriber will check during your consultation, and the practical steps to ensure safe and effective treatment for erectile dysfunction while managing your HIV therapy.
At a glance
What is sildenafil and efavirenz?
Sildenafil is a medicine used to treat erectile dysfunction (ED). It belongs to a group of medicines called PDE5 inhibitors, which work by relaxing blood vessels in the penis to improve blood flow during sexual stimulation [3]. Sildenafil is the active ingredient in Viagra and is available in 25mg, 50mg, and 100mg tablets. It is usually taken about one hour before sexual activity and works for around four to six hours. Sexual stimulation is still needed for the medicine to work.
Efavirenz is an antiretroviral medicine used to treat HIV infection. It belongs to a class of medicines called non-nucleoside reverse transcriptase inhibitors (NNRTIs). Efavirenz works by blocking an enzyme that HIV needs to multiply, helping to control the virus and protect the immune system. It is typically taken once daily as part of combination antiretroviral therapy. Efavirenz affects how the liver processes many other medicines, which is why interactions need careful consideration.
How do sildenafil and efavirenz interact?
Efavirenz is a CYP3A4 inducer, which means it speeds up the activity of an enzyme in the liver that breaks down sildenafil [3]. When efavirenz increases this enzyme's activity, sildenafil is removed from the body more quickly than usual. This can reduce the amount of sildenafil in your bloodstream and may make it less effective at treating erectile dysfunction. In plain English, efavirenz can weaken the effect of sildenafil, so you might not get the full benefit from a standard dose.
This interaction is well-recognised and manageable. The British National Formulary notes that enzyme inducers like efavirenz can reduce the plasma concentration of sildenafil [3]. Your prescriber will take this into account when deciding on the right dose for you. It does not mean the combination is unsafe, but it does mean that dose adjustments or alternative treatments may be needed to ensure the ED medicine works effectively.
What happens in your body
When you take sildenafil, it is absorbed into your bloodstream and then broken down (metabolised) mainly by an enzyme called CYP3A4 in your liver. Efavirenz increases the activity of this enzyme, so sildenafil is broken down faster. As a result, lower levels of sildenafil remain active in your body, and the medicine may not last as long or work as strongly. This is different from some other HIV medicines (such as ritonavir or cobicistat) that slow down the same enzyme and increase sildenafil levels, requiring a dose reduction. With efavirenz, the concern is reduced effectiveness rather than increased side effects.
What your prescriber checks before prescribing sildenafil and efavirenz together
During your online consultation at Cured Pharmacy, the prescriber will review your full medication list, including efavirenz and any other antiretroviral medicines you take. They will check for any other medicines or health conditions that might affect the safety or effectiveness of sildenafil. This includes asking about your blood pressure, heart health, and any history of chest pain, stroke, or eye problems [1][2]. Because efavirenz can reduce sildenafil's effectiveness, the prescriber may recommend starting with a higher dose than usual, or they may suggest an alternative PDE5 inhibitor that is less affected by enzyme induction.
The prescriber will also ensure that you are not taking any medicines that are contraindicated with sildenafil, such as nitrates (GTN spray, isosorbide mononitrate), nicorandil, riociguat, or recreational drugs called poppers [3]. These combinations can cause a dangerous drop in blood pressure and must be avoided. If you take an alpha-blocker for prostate symptoms (such as tamsulosin or doxazosin), the prescriber will check that you are stable on that medicine and may start you on a lower dose of sildenafil to reduce the risk of dizziness. All of these checks are part of the safe prescribing process and help to tailor the treatment to your individual needs.
What you need to know
- Sildenafil can usually be taken with efavirenz, but efavirenz may reduce its effectiveness.
- Your prescriber may recommend a higher dose of sildenafil to compensate for the interaction.
- Never stop your HIV medication; continue efavirenz exactly as prescribed by your specialist.
- Declare all medicines during your consultation so the prescriber can check for interactions.
- Seek urgent help for erections lasting over four hours or sudden vision or hearing loss.
Practical advice for taking sildenafil with efavirenz
If your prescriber approves sildenafil while you are taking efavirenz, follow the dosing instructions carefully. You may be prescribed a higher starting dose than someone not taking efavirenz, to compensate for the reduced effectiveness. Take sildenafil about one hour before you plan to have sex, and remember that sexual stimulation is needed for it to work [3]. Do not take more than one dose in 24 hours, and do not increase the dose yourself without speaking to your prescriber or pharmacist first.
Keep taking your efavirenz and other HIV medicines exactly as prescribed by your HIV specialist. Do not stop or change your antiretroviral therapy because of the interaction with sildenafil. If you find that sildenafil is not working well enough, contact your prescriber. They may adjust the dose, suggest trying a different PDE5 inhibitor such as tadalafil or vardenafil, or explore other treatment options. Avoid drinking large amounts of alcohol, as this can worsen erectile dysfunction and increase the risk of dizziness or low blood pressure when combined with sildenafil.
Alternative ED treatments
If sildenafil does not work well enough with efavirenz, other PDE5 inhibitors may be considered. Tadalafil (Cialis) is also affected by enzyme inducers, but its longer duration of action (up to 36 hours) may offer more flexibility [4]. Vardenafil and avanafil are other options, though they too can be affected by efavirenz [5][6]. Your prescriber will discuss which medicine is most suitable for you based on your response, lifestyle, and other medications. Non-tablet treatments, such as alprostadil cream or injections, are not affected by efavirenz and may be an option if oral medicines are not effective [7].
Warning signs and when to seek help with sildenafil and efavirenz
Most men tolerate sildenafil well, even when taking efavirenz. Common side effects of sildenafil include headache, flushing, indigestion, and a stuffy nose [3]. These are usually mild and short-lived. However, you should seek urgent medical help if you experience an erection lasting more than four hours (priapism), sudden loss of vision in one or both eyes, or sudden decrease or loss of hearing. If you develop chest pain during or after sex, stop sexual activity and seek medical advice immediately, as this could indicate a heart problem.
Dizziness or feeling faint when standing up can occur, especially if you also take blood pressure medicines or alpha-blockers. If this happens, sit or lie down until you feel better, and mention it to your prescriber at your next review. Do not drive or operate machinery if you feel dizzy. If sildenafil seems less effective than expected, do not simply take more tablets. Contact your prescriber to discuss dose adjustment or alternative treatments. Never take sildenafil with nitrates, nicorandil, riociguat, or poppers, as this combination can cause a dangerous fall in blood pressure [3].
When to get urgent help
- Erection lasting more than 4 hours (priapism) – go to A&E immediately
- Sudden loss of vision in one or both eyes – seek urgent medical help
- Sudden decrease or loss of hearing – contact a doctor straight away
- Chest pain during or after sex – stop activity and call 999 if severe
- Severe dizziness or fainting – sit down and seek same-day GP advice if it persists
Call 999 or go to A&E in an emergency, or contact NHS 111 for urgent advice.
How to access sildenafil safely at Cured Pharmacy
Cured Pharmacy is a GPhC-registered online pharmacy (GPhC 9012511) offering a confidential, UK-based prescribing service for erectile dysfunction. To request sildenafil, you complete a short online consultation that asks about your health, current medicines (including efavirenz), and any conditions that might affect treatment. A UK-registered prescriber reviews your answers and, if appropriate, approves your prescription. Your medicine is then dispensed by our pharmacy team and delivered discreetly to your door.
We offer a range of ED treatments, including generic sildenafil from £0.45 per tablet (£0.36 with first-order code Ednew), tadalafil, vardenafil, and avanafil. Because you are taking efavirenz, it is especially important to declare this during your consultation so the prescriber can choose the right dose and check for any other interactions. If you have any questions about the interaction between sildenafil and efavirenz, our pharmacist team is available to help. Prescription medicines are only supplied after clinical assessment by a UK prescriber, ensuring your safety and the suitability of treatment.
Why see a GP for 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 conditions such as heart disease, diabetes, or high blood pressure. Your GP can arrange blood tests, check your cardiovascular health, and review your HIV treatment to ensure everything is well controlled. Once any underlying causes have been addressed, online services like Cured Pharmacy can provide convenient ongoing access to ED treatment. If you already have a diagnosis and are stable on your HIV therapy, an online consultation is a safe and discreet way to access sildenafil and other ED medicines.
Always declare efavirenz in your consultation
Efavirenz affects how sildenafil works in your body. Telling your prescriber about all your HIV medicines ensures you get the right dose and the safest, most effective treatment for erectile dysfunction.
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.
Your Questions Answered
Can I take sildenafil if I am on efavirenz for HIV?
Yes, sildenafil can usually be taken with efavirenz, but efavirenz may reduce its effectiveness. Your prescriber will adjust the dose accordingly and check for any other interactions during your consultation.
Will efavirenz make sildenafil less effective?
Efavirenz is an enzyme inducer and can lower sildenafil levels in your bloodstream, which may reduce its effectiveness. Your prescriber may recommend a higher dose or an alternative PDE5 inhibitor to ensure good results.
Do I need to stop my HIV medication to take sildenafil?
No, never stop or change your HIV treatment without advice from your HIV specialist. Sildenafil can be prescribed alongside efavirenz with appropriate dose adjustments, and your antiretroviral therapy must continue as prescribed.
What dose of sildenafil should I take if I am on efavirenz?
Your prescriber will decide the right dose based on your individual circumstances. Because efavirenz can reduce sildenafil levels, you may be started on a higher dose than usual, but this will be determined during your consultation.
Are there other ED treatments that work better with efavirenz?
Tadalafil, vardenafil, and avanafil are also affected by efavirenz to varying degrees. Your prescriber will discuss which PDE5 inhibitor is most suitable, or may suggest non-oral treatments such as alprostadil if needed.
Can I take sildenafil with ritonavir or cobicistat?
Ritonavir and cobicistat are potent enzyme inhibitors that increase sildenafil levels, so a much lower dose is required (maximum 25mg in 48 hours). Your prescriber must check the specific combination and adjust the dose to avoid side effects.
What should I do if sildenafil does not work well enough?
Contact your prescriber to discuss dose adjustment or trying a different PDE5 inhibitor. Do not increase the dose yourself, and do not take more than one tablet in 24 hours.
Is it safe to buy sildenafil online if I take efavirenz?
Yes, as long as you use a registered UK pharmacy like Cured Pharmacy (GPhC 9012511) and declare all your medicines, including efavirenz, during the online consultation. A UK prescriber will review your case and ensure safe prescribing.
Get Discreet ED Treatment Online
Start your journey to better health today.
- Reviewed by UK-based prescribers
- Takes under 3 minutes
- Discreet Packaging
Start Your Journey To Better Health Today
Once you complete a short online assessment you will be able to choose your preferred treatment from safe medically approved options tailored to you.
Start My Assessment Log in to Your Account
Medically reviewed by
Tarun Kumar, Superintendent Pharmacist (GPhC 2233073)