Tadalafil and Darunavir: Can They Be Taken Together?

Tadalafil and Darunavir: Understanding the Interaction

Published: 19 October 2026

Tadalafil and darunavir can interact because darunavir (an HIV protease inhibitor usually boosted with ritonavir or cobicistat) significantly increases tadalafil levels in the body. This does not mean the combination is unsafe, but it does require careful dose adjustment and prescriber oversight to minimise the risk of side effects such as headache, flushing, or low blood pressure [4]. If you are taking darunavir for HIV and experiencing erectile dysfunction, effective treatment is available—your prescriber will tailor the tadalafil dose to ensure both safety and effectiveness.

What is tadalafil and darunavir?

Tadalafil is a phosphodiesterase type 5 (PDE5) inhibitor licensed in the UK to treat erectile dysfunction (ED) [4]. It works by relaxing blood vessels in the penis, allowing increased blood flow during sexual stimulation. Tadalafil is available as on-demand tablets (10 mg or 20 mg taken at least 30 minutes before sex, with effects lasting up to 36 hours) or as a daily low-dose option (2.5 mg or 5 mg) [4]. It is the active ingredient in branded Cialis and various generic versions.

Darunavir is an antiretroviral medicine used to treat HIV infection [12]. It belongs to a class called protease inhibitors, which block an enzyme HIV needs to replicate. Darunavir is almost always prescribed alongside a 'booster' drug—either ritonavir or cobicistat—that increases darunavir levels and makes the treatment more effective. These boosters are potent inhibitors of the liver enzyme CYP3A4, which is also responsible for breaking down tadalafil and other PDE5 inhibitors.

Because darunavir is boosted with ritonavir or cobicistat, it significantly slows the breakdown of tadalafil, leading to higher and longer-lasting tadalafil levels in the bloodstream [4]. This interaction is well recognised and manageable through dose adjustment, but it must be accounted for during prescribing to avoid excessive side effects.

How do tadalafil and darunavir interact?

The interaction between tadalafil and darunavir occurs through the CYP3A4 enzyme system in the liver. Tadalafil is metabolised (broken down) primarily by CYP3A4 [4]. Darunavir, when boosted with ritonavir or cobicistat, is a potent inhibitor of this enzyme, meaning it slows down the breakdown of tadalafil considerably. As a result, tadalafil stays in the body longer and reaches higher concentrations than it would on its own.

In practical terms, this means that a standard dose of tadalafil could produce stronger effects—and potentially more side effects—when taken alongside darunavir. Common side effects of tadalafil include headache, indigestion, back pain, muscle aches, flushing, and nasal congestion [4]. At higher levels, there is also an increased risk of low blood pressure, particularly if you are taking other medicines that lower blood pressure or if you drink alcohol heavily.

This interaction does not mean you cannot use tadalafil if you are on darunavir. It simply means your prescriber will need to adjust the tadalafil dose and monitor you more carefully. The British National Formulary provides clear guidance on dose limits for tadalafil when used with potent CYP3A4 inhibitors like ritonavir and cobicistat [4], and your prescriber will follow these recommendations during your consultation.

Why the booster matters

Darunavir on its own has some effect on CYP3A4, but the major interaction comes from the booster—ritonavir or cobicistat—that is co-prescribed with it. Both of these boosters are very potent CYP3A4 inhibitors. If you are taking darunavir without a booster (which is uncommon), the interaction with tadalafil is less pronounced, but you should still declare all your medicines during your consultation so your prescriber can assess your individual situation.

What your prescriber will check

Before prescribing tadalafil to someone taking darunavir, a UK prescriber will review your full medication list, including the exact formulation and dose of your HIV treatment. They will confirm whether your darunavir is boosted with ritonavir or cobicistat, as this determines the degree of interaction and the maximum safe dose of tadalafil [4].

Your prescriber will also ask about your cardiovascular health, blood pressure, and any other medicines you take that might interact with tadalafil—such as alpha-blockers (e.g. tamsulosin, doxazosin) or other blood-pressure-lowering drugs. They will check that you are not taking nitrates, nicorandil, riociguat, or recreational drugs known as 'poppers', as these combinations can cause a dangerous drop in blood pressure and must never be used with tadalafil [4].

Your HIV specialist or GP will usually be aware of your darunavir treatment, and it is important that any prescriber of ED medication is also informed. At Cured Pharmacy, our online consultation asks detailed questions about all current medicines, and a UK-registered prescriber reviews every request before approving a prescription. If there is any uncertainty about the safety of the combination, the prescriber may contact your GP or HIV clinic for further information, or suggest an alternative ED treatment.

Recommended tadalafil doses with darunavir

When tadalafil is used on-demand (taken as needed before sexual activity) in someone taking darunavir with ritonavir or cobicistat, the British National Formulary recommends a maximum single dose of 10 mg within any 72-hour period [4]. This is lower than the standard maximum of 20 mg, reflecting the increased tadalafil levels caused by the interaction. Some prescribers may start with an even lower dose to assess tolerability.

For daily tadalafil (a lower dose taken every day to allow spontaneity), the interaction also requires dose adjustment. The usual starting dose is 2.5 mg once daily, which may be increased to 5 mg depending on response and tolerability, but higher daily doses are generally avoided when potent CYP3A4 inhibitors are present [4].

These dose adjustments are based on pharmacokinetic studies showing that ritonavir and cobicistat can increase tadalafil exposure several-fold [4]. Your prescriber will select the appropriate dose for you based on your HIV regimen, other medications, and your response to treatment. Never adjust the dose yourself or take tadalafil more frequently than prescribed, as this increases the risk of side effects.

What if I need a higher dose?

If the lower dose of tadalafil is not effective enough, discuss this with your prescriber rather than increasing the dose on your own. They may consider switching to a different PDE5 inhibitor with a different interaction profile, or they may explore other factors contributing to erectile dysfunction, such as blood sugar control, blood pressure, psychological factors, or lifestyle changes [1][2]. Erectile dysfunction in men with HIV is multifactorial and often benefits from a holistic approach.

Practical advice for taking tadalafil with darunavir

Take tadalafil exactly as prescribed. If you are using on-demand tadalafil, take it at least 30 minutes before sexual activity (effects can last up to 36 hours), and do not take another dose within 72 hours if you are on darunavir with a booster [4]. If you are on daily tadalafil, take it at the same time each day, regardless of when you plan to have sex.

Avoid drinking large amounts of alcohol, as this can increase the risk of side effects such as dizziness, headache, and low blood pressure. Grapefruit juice can also raise tadalafil levels further and should be limited. Sexual stimulation is still required for tadalafil to work—it does not cause an automatic erection [4].

Be aware of potential side effects. Headache, flushing, indigestion, and nasal congestion are common and usually mild [4]. If you experience dizziness or feel faint, sit or lie down and avoid driving or operating machinery. If you develop chest pain during or after sex, do not use nitrate medicines (such as GTN spray)—seek urgent medical help and tell the healthcare professional that you have taken tadalafil.

Keep your HIV specialist or GP informed that you are using tadalafil, especially if your HIV treatment changes. If a new medicine is added to your regimen, check with your prescriber or pharmacist whether it might interact with tadalafil. Do not stop taking your HIV medication without medical advice.

When to seek medical help

Seek urgent medical attention if you experience an erection lasting more than four hours (priapism), as this can cause permanent damage to the penis if not treated promptly [4]. Priapism is rare but is a medical emergency.

Contact a doctor immediately if you have sudden loss of vision in one or both eyes, as this may be a sign of non-arteritic anterior ischaemic optic neuropathy (NAION), a rare but serious condition [4]. Similarly, sudden hearing loss or ringing in the ears requires prompt medical assessment.

If you develop chest pain, severe dizziness, or fainting after taking tadalafil, call 999 or go to A&E. Do not take nitrate medicines if you have used tadalafil in the past 48 hours, as the combination can cause a life-threatening drop in blood pressure [4].

If you experience side effects that are troublesome but not immediately dangerous—such as persistent headache, severe indigestion, or muscle pain—contact your prescriber. They may adjust your dose or suggest an alternative treatment. Remember that erectile dysfunction can be an early warning sign of cardiovascular disease or diabetes, so if ED is a new problem, it is important to see your GP for a full health check [1][2].

Ongoing monitoring

Your HIV care team will monitor your overall health, including blood pressure and cardiovascular risk factors, as part of routine HIV management. Make sure they are aware you are using tadalafil. If your HIV treatment changes—for example, if you switch to a different antiretroviral regimen—let your ED treatment prescriber know, as this may affect the interaction and the appropriate tadalafil dose.

Scientific References

1. NHS. (n.d.). Erectile dysfunction (impotence). NHS website. https://www.nhs.uk/conditions/erection-problems-erectile-dysfunction/ [accessed 19 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 19 October 2026]
3. Joint Formulary Committee. (n.d.). Sildenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/sildenafil/ [accessed 19 October 2026]
4. Joint Formulary Committee. (n.d.). Tadalafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/tadalafil/ [accessed 19 October 2026]
5. Joint Formulary Committee. (n.d.). Vardenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/vardenafil/ [accessed 19 October 2026]
6. Joint Formulary Committee. (n.d.). Avanafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/avanafil/ [accessed 19 October 2026]
7. Electronic Medicines Compendium. (n.d.). Summaries of Product Characteristics and Patient Information Leaflets. https://www.medicines.org.uk/emc/ [accessed 19 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.