Tadalafil and Sirolimus: Can They Be Taken Together?

Tadalafil and Sirolimus: Understanding the Interaction

Published: 25 October 2026

If you're prescribed sirolimus (an immunosuppressant) and considering tadalafil for erectile dysfunction, you may wonder whether tadalafil and sirolimus can be taken together safely. The short answer is that they can interact because sirolimus affects how your body processes tadalafil, potentially raising its levels in your bloodstream. This doesn't mean the combination is forbidden, but it does require careful prescriber assessment, dose adjustment, and monitoring to minimise the risk of side effects.

What is tadalafil and sirolimus used for?

Tadalafil is a phosphodiesterase type 5 (PDE5) inhibitor licensed in the UK to treat erectile dysfunction (ED) [3][4]. It works by relaxing blood vessels in the penis, allowing increased blood flow when you're sexually aroused. Tadalafil is available on-demand (usually 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 (2.5 mg or 5 mg). It requires sexual stimulation to work and is only supplied on prescription after a clinical consultation [2].

Sirolimus (also known as rapamycin) is an immunosuppressant medicine prescribed to prevent organ rejection after transplant surgery, or occasionally to treat certain rare conditions. It works by dampening your immune system to stop it attacking the transplanted organ. Sirolimus is taken daily, usually as a tablet, and blood levels are monitored closely by your transplant or specialist team to keep you within a safe and effective range [12].

How do tadalafil and sirolimus interact?

The interaction between tadalafil and sirolimus happens through a liver enzyme system called cytochrome P450 3A4 (CYP3A4). Tadalafil is broken down (metabolised) mainly by CYP3A4, while sirolimus is both metabolised by CYP3A4 and acts as a moderate inhibitor of this enzyme [4][12]. When sirolimus slows down CYP3A4, tadalafil stays in your bloodstream longer and at higher concentrations than it would on its own.

Higher tadalafil levels can increase the risk of side effects such as headache, flushing, indigestion, back pain, muscle aches, and dizziness. More importantly, tadalafil lowers blood pressure slightly, and higher levels may amplify this effect, particularly if you're also taking other medicines that affect blood pressure. The good news is that this interaction is predictable and manageable: your prescriber can adjust the tadalafil dose (often starting lower or using a reduced maximum dose) and monitor you for any problems [4].

It's important to note that sirolimus itself does not directly cause the dangerous blood-pressure drop seen with nitrates or riociguat, so the combination is not absolutely contraindicated. However, every patient's situation is unique, and your transplant team and prescriber will need to work together to ensure both medicines are used safely.

Why CYP3A4 matters

CYP3A4 is one of the most important enzymes in your liver for processing medicines. Many drugs—including tadalafil, sirolimus, and others—rely on it to be broken down and cleared from your body. When one medicine inhibits (slows down) CYP3A4, other medicines that depend on it can build up. This is why your prescriber needs a complete list of all your medicines, including over-the-counter products and herbal supplements, before prescribing tadalafil [12].

What your prescriber will check before prescribing tadalafil and sirolimus together

Before prescribing tadalafil to someone taking sirolimus, a UK prescriber will review your full medical history and current medicines. They will check that you're not taking any medicines that are absolutely contraindicated with tadalafil—such as nitrates (GTN spray, isosorbide mononitrate or dinitrate), nicorandil, riociguat, or amyl/alkyl nitrites (poppers)—because combining tadalafil with these can cause a dangerous fall in blood pressure [3][4].

Your prescriber will also ask about your cardiovascular health, including blood pressure, any history of heart attack, stroke, or angina, and whether sexual activity is safe for you. They will want to know about other medicines that affect blood pressure (such as alpha-blockers like tamsulosin or doxazosin, or other antihypertensives), as these can add to tadalafil's blood-pressure-lowering effect [2][4].

Crucially, they will consider the sirolimus interaction and may start you on a lower tadalafil dose or recommend a reduced maximum dose. For example, the Summary of Product Characteristics for tadalafil advises caution and dose reduction when used with moderate CYP3A4 inhibitors [4]. Your transplant team should be informed, as they monitor your sirolimus levels and overall health closely. Never adjust or stop sirolimus without your transplant doctor's advice, as this could risk organ rejection.

Practical advice for taking tadalafil and sirolimus safely

If your prescriber approves tadalafil while you're on sirolimus, follow their dosing instructions exactly. You will likely be started on a lower dose than usual—for instance, 10 mg on-demand rather than 20 mg, or a lower daily dose if that regimen is chosen. Do not take more than the prescribed dose, and do not take tadalafil more often than once a day if using the on-demand form [4].

Be alert for side effects, especially headache, flushing, dizziness, or muscle aches. These are common with tadalafil but may be more noticeable if levels are higher due to the sirolimus interaction. If side effects are troublesome or persistent, contact your prescriber—they may adjust your dose. Avoid drinking large amounts of alcohol, as this can worsen erectile dysfunction and add to the blood-pressure-lowering effect of tadalafil [1][2].

Do not drink grapefruit juice while taking tadalafil and sirolimus, as grapefruit is a potent CYP3A4 inhibitor and can further raise the levels of both medicines [4][12]. Keep all your medical teams informed: your GP, transplant specialist, and the pharmacist or prescriber managing your ED treatment should all know about every medicine you take. If you're prescribed any new medicine (including antibiotics like clarithromycin or antifungals like itraconazole, which are strong CYP3A4 inhibitors), tell the prescriber you're on tadalafil and sirolimus so they can check for further interactions.

When to seek urgent help

Seek immediate medical help (call 999 or go to A&E) if you experience an erection lasting more than four hours (priapism), sudden loss of vision or hearing, or chest pain during sex. If you feel very dizzy, faint, or notice a rapid or irregular heartbeat after taking tadalafil, stop and seek same-day medical advice. These are rare but serious side effects that require prompt attention [3][4].

Other medicines that interact with tadalafil and sirolimus

Because both tadalafil and sirolimus are processed by CYP3A4, any other medicine that affects this enzyme can alter the levels of one or both drugs. Potent CYP3A4 inhibitors—such as ritonavir, cobicistat (found in some HIV medicines), ketoconazole, itraconazole, clarithromycin, and Paxlovid—can significantly raise tadalafil levels and may also affect sirolimus [4][12]. If you need one of these medicines, your prescriber may reduce your tadalafil dose further, delay treatment, or choose an alternative.

Conversely, CYP3A4 inducers—such as rifampicin, carbamazepine, phenytoin, and St John's wort—speed up the breakdown of both tadalafil and sirolimus, potentially making them less effective. St John's wort, a herbal remedy sometimes used for low mood, is particularly problematic for transplant patients because it can lower sirolimus levels and risk rejection; it should be avoided [12].

Alpha-blockers (tamsulosin, doxazosin, alfuzosin) are often prescribed for urinary symptoms and can interact with tadalafil by adding to its blood-pressure-lowering effect, causing dizziness or fainting. If you take an alpha-blocker, you should be stable on it before starting tadalafil, and your prescriber will usually start you on the lowest PDE5 inhibitor dose [3][4]. Always declare every medicine, supplement, and herbal product during your consultation, even if you think it's unrelated.

Can I use other ED treatments if I'm on sirolimus?

All licensed oral ED medicines in the UK are PDE5 inhibitors and are metabolised by CYP3A4, so the same interaction principles apply. Sildenafil (Viagra), vardenafil (Levitra), and avanafil (Spedra) will all have their levels raised by sirolimus, requiring dose adjustment or caution [3][5][6]. Vardenafil and avanafil are particularly sensitive to CYP3A4 inhibition, and the Summary of Product Characteristics for these medicines advises avoiding them with potent inhibitors; your prescriber will assess whether sirolimus (a moderate inhibitor) is compatible and adjust accordingly [5][6].

Tadalafil is often preferred in patients taking interacting medicines because it has a longer half-life and more flexible dosing options, but the choice depends on your individual circumstances. Never take two PDE5 inhibitors together, as this can cause a dangerous drop in blood pressure [2][3]. Non-oral options such as Vitaros (alprostadil cream) or Caverject (alprostadil injection) work locally rather than systemically and do not interact with sirolimus via CYP3A4, but they require a separate prescription and specialist assessment [7].

Eroxon, a topical gel medical device available without prescription, may be an option for some men and does not have the same drug interactions, though it works differently and may be less effective for moderate to severe ED [2]. Discuss all options with your prescriber to find the safest and most suitable treatment for you.

Talking to your transplant team

If you're considering ED treatment while on sirolimus, it's good practice to let your transplant team know. They monitor your sirolimus levels and overall health closely and can advise whether any additional precautions are needed. Erectile dysfunction is common after transplant surgery and can be related to the underlying condition, medicines, or psychological factors; your team will understand and can support you in finding a safe solution [2].

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. Joint Formulary Committee. (n.d.). Alprostadil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/alprostadil/ [accessed 25 October 2026]
8. 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.