Tadalafil and Ritonavir: Can They Be Taken Together?

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Tadalafil and Ritonavir: Understanding the Interaction

Published: 25 October 2026

Tadalafil and ritonavir can be taken together, but ritonavir significantly increases tadalafil levels in the body, so dose adjustments are essential. Ritonavir is a potent CYP3A4 inhibitor used in HIV treatment and in Paxlovid (for COVID-19), and it slows down how quickly your liver breaks down tadalafil, meaning the erectile dysfunction medicine stays in your system longer and at higher concentrations [4]. This interaction is well-recognised and manageable with the right precautions, but it must be reviewed by a prescriber during your consultation.

What is tadalafil and ritonavir?

Tadalafil is a phosphodiesterase type 5 (PDE5) inhibitor licensed to treat erectile dysfunction and, in some formulations, benign prostatic hyperplasia [4]. It works by relaxing blood vessels in the penis, allowing increased blood flow when you are sexually aroused. 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 requires sexual stimulation to work and does not cause spontaneous erections.

Ritonavir is an antiretroviral medicine primarily used in combination therapy for HIV infection [12]. It is also a component of Paxlovid, the antiviral treatment for COVID-19. Ritonavir is often used at low doses not for its own antiviral effect, but because it 'boosts' other medicines by inhibiting the liver enzyme CYP3A4, which breaks down many drugs [12]. This boosting effect is useful in HIV treatment but means ritonavir can significantly raise the levels of other medicines metabolised by the same enzyme, including tadalafil.

Both medicines are prescription-only in the UK. If you are taking ritonavir (or any other potent CYP3A4 inhibitor), you must declare this during your online consultation so the prescriber can adjust your tadalafil dose appropriately and check for any other contraindications.

How do tadalafil and ritonavir interact?

Ritonavir inhibits CYP3A4, the main liver enzyme responsible for breaking down tadalafil [4]. When you take ritonavir and tadalafil together, tadalafil is cleared from your body much more slowly, leading to higher blood levels and a longer duration of action. In clinical studies, ritonavir increased tadalafil exposure significantly, meaning the same dose of tadalafil has a much stronger and longer-lasting effect when combined with ritonavir [4][12].

This interaction is not a reason to avoid tadalafil altogether, but it does mean your prescriber must reduce the tadalafil dose to avoid side effects such as headache, flushing, indigestion, back pain, and—most importantly—a drop in blood pressure that could cause dizziness or fainting [4]. The British National Formulary advises a maximum tadalafil dose of 10 mg in 72 hours if you are taking a potent CYP3A4 inhibitor such as ritonavir [4]. For daily tadalafil, the dose may need to be reduced or avoided entirely, depending on your individual circumstances.

Your prescriber will review your full medication list, including the dose and frequency of ritonavir, to determine the safest tadalafil regimen for you. Never adjust your tadalafil or ritonavir dose yourself, and do not take tadalafil more frequently than advised, as this increases the risk of serious side effects.

Why CYP3A4 inhibition matters

CYP3A4 is a liver enzyme that metabolises a wide range of medicines, including all PDE5 inhibitors (sildenafil, tadalafil, vardenafil, and avanafil) [3][4][5][6]. When ritonavir blocks this enzyme, tadalafil accumulates in your bloodstream. Think of it like a sink with a slower drain: the same amount of water (tadalafil) takes much longer to empty, so the level rises higher. This is why dose reduction is essential—it keeps tadalafil levels within a safe and effective range.

What your prescriber checks before prescribing tadalafil with ritonavir

During your online consultation at Cured Pharmacy, the prescriber will ask detailed questions about your current medicines, including ritonavir and any other antiretrovirals or treatments for COVID-19 [2]. They will check the dose and frequency of ritonavir, as well as any other CYP3A4 inhibitors you may be taking (such as cobicistat, ketoconazole, itraconazole, or clarithromycin), because these can have a similar effect on tadalafil levels [4][12].

The prescriber will also review your cardiovascular health, blood pressure, and any history of heart disease, stroke, or angina, because tadalafil can lower blood pressure and sexual activity itself places demands on the heart [2][4]. If you are taking other blood-pressure medicines (including alpha-blockers such as tamsulosin or doxazosin), the prescriber must ensure you are stable on those treatments and that the combination is safe [4]. Tadalafil must never be taken with nitrates (such as GTN spray or isosorbide mononitrate), nicorandil, riociguat, or recreational drugs known as 'poppers', as this combination can cause a dangerous drop in blood pressure [4].

You will also be asked about any history of priapism (prolonged erection), retinal disorders, or sudden vision or hearing loss, as these are contraindications or require special caution [4]. The prescriber may recommend a lower starting dose of tadalafil (for example, 10 mg on-demand rather than 20 mg) and will advise you on the maximum frequency of use while you are taking ritonavir.

Recommended tadalafil doses when taking ritonavir

If you are taking ritonavir, the British National Formulary recommends a maximum tadalafil dose of 10 mg, and this dose should not be taken more frequently than once every 72 hours (three days) [4]. This allows time for tadalafil to be cleared from your system before the next dose. Some prescribers may start you on an even lower dose, such as 5 mg, to see how you tolerate the combination, especially if you are also taking other medicines that affect blood pressure.

Daily low-dose tadalafil (2.5 mg or 5 mg) is generally not recommended when you are taking potent CYP3A4 inhibitors such as ritonavir, because the accumulation of tadalafil over consecutive days can lead to excessive blood levels and side effects [4]. If daily treatment is preferred for your circumstances, your prescriber will discuss alternative options or a carefully adjusted regimen.

Do not take more than the prescribed dose, and do not take tadalafil more often than advised, even if you feel the effect is wearing off sooner than expected. If you are also prescribed another PDE5 inhibitor (such as sildenafil or vardenafil), do not take them together or on the same day, as this increases the risk of side effects and low blood pressure [3][4][5]. Always follow your prescriber's instructions and report any side effects promptly.

What if I'm taking Paxlovid?

Paxlovid (nirmatrelvir with ritonavir) is a short-term treatment for COVID-19, usually taken for five days [12]. If you are prescribed Paxlovid and you are already taking tadalafil, you should stop tadalafil for the duration of the Paxlovid course and for at least 72 hours after the last dose, to allow ritonavir to clear from your system [4][12]. Your prescriber or pharmacist will advise you on when it is safe to restart tadalafil. Do not take tadalafil and Paxlovid together without explicit guidance from a healthcare professional.

Side effects and warning signs with tadalafil and ritonavir

Common side effects of tadalafil include headache, flushing, indigestion, nasal congestion, back pain, and muscle aches [4]. These are usually mild and short-lived, but they may be more noticeable or last longer when tadalafil is combined with ritonavir, because tadalafil levels are higher. Dizziness or light-headedness, especially when standing up, can occur due to a drop in blood pressure, and this risk is increased if you are also taking other blood-pressure medicines or alpha-blockers [4].

Seek urgent medical help (call 999 or go to A&E) 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 [4]. These are rare but serious side effects that require immediate treatment. If you develop chest pain during or after sexual activity, stop and seek medical advice urgently, as this may indicate a heart problem [4].

If you experience severe dizziness, fainting, or a very fast or irregular heartbeat, stop taking tadalafil and contact your GP or call 111 for advice. Do not drive or operate machinery if you feel dizzy or light-headed. Report any new or worsening side effects to your prescriber, especially if you have recently started ritonavir or changed the dose of either medicine.

Practical advice for taking tadalafil and ritonavir safely

Always take tadalafil exactly as prescribed, and do not exceed the recommended dose or frequency [4]. If you are taking ritonavir, remember that tadalafil will stay in your system longer, so you must wait at least 72 hours between doses of on-demand tadalafil (and follow your prescriber's specific advice). Keep a record of when you take each dose to avoid accidental doubling up.

Avoid drinking large amounts of alcohol when taking tadalafil, as alcohol can worsen erectile dysfunction, increase the risk of side effects such as dizziness and low blood pressure, and make it harder for the medicine to work [4]. Grapefruit juice can also raise tadalafil levels and should be avoided or limited [4]. If you are prescribed a new medicine (including over-the-counter or herbal remedies such as St John's wort), tell your doctor or pharmacist that you are taking tadalafil and ritonavir, as further dose adjustments may be needed.

If you develop an infection or other illness that requires a short course of another CYP3A4 inhibitor (such as clarithromycin or itraconazole), contact your prescriber for advice, as you may need to pause tadalafil temporarily [4][12]. Never stop or change your ritonavir or antiretroviral treatment without consulting your HIV specialist or GP, as this could affect your viral load and overall health. Erectile dysfunction can be an early sign of cardiovascular disease or diabetes, so if you have not already done so, see your GP for a general health check [1][2].

When to contact your prescriber

Contact your prescriber if tadalafil is not working as expected, if you experience side effects that concern you, or if your ritonavir dose or regimen changes. Also get in touch if you are prescribed a new medicine for any condition, or if you develop new symptoms such as chest pain, shortness of breath, or unexplained swelling. Your prescriber can review your treatment and adjust your tadalafil dose or suggest an alternative approach if needed.

Scientific References

1. NHS. (n.d.). Erectile dysfunction (impotence). NHS website. https://www.nhs.uk/conditions/erection-problems-erectile-dysfunction/ [accessed 25 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 25 October 2026]
3. Joint Formulary Committee. (n.d.). Sildenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/sildenafil/ [accessed 25 October 2026]
4. Joint Formulary Committee. (n.d.). Tadalafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/tadalafil/ [accessed 25 October 2026]
5. Joint Formulary Committee. (n.d.). Vardenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/vardenafil/ [accessed 25 October 2026]
6. Joint Formulary Committee. (n.d.). Avanafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/avanafil/ [accessed 25 October 2026]
7. Electronic Medicines Compendium. (n.d.). Summaries of Product Characteristics and Patient Information Leaflets. https://www.medicines.org.uk/emc/ [accessed 25 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.