Tadalafil and Ondansetron: Can They Be Taken Together?

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Tadalafil and Ondansetron: Understanding the Interaction

Published: 24 October 2026

Many men wonder whether tadalafil and ondansetron can be taken together safely. The good news is that no major interaction is commonly reported between tadalafil (Cialis) and ondansetron (Ondemet, Zofran), an anti-sickness medicine often prescribed for nausea and vomiting. However, every medicine you take—including over-the-counter and prescription treatments—must be declared during your consultation, so your prescriber or pharmacist can review your full medication list and ensure tadalafil is suitable for you.

What is tadalafil and ondansetron used for?

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 when you are sexually aroused. Tadalafil is available on-demand (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 tablet (2.5 mg or 5 mg) for men who prefer spontaneity [4]. Tadalafil is the generic form of Cialis and requires a prescription after a clinical assessment by a UK-registered prescriber.

Ondansetron is a 5-HT3 receptor antagonist used to prevent and treat nausea and vomiting, particularly after surgery, chemotherapy, or radiotherapy [12]. It is available as tablets, orodispersible tablets, and injections, and is prescribed when other anti-sickness medicines are unsuitable or ineffective. Ondansetron does not directly affect erectile function or blood pressure in the same way tadalafil does, which is why the two medicines are not known to have a clinically significant interaction.

Is there an interaction between tadalafil and ondansetron?

No major interaction is commonly reported between tadalafil and ondansetron. They work through different mechanisms: tadalafil acts on blood vessels in the penis to improve erectile function, while ondansetron blocks serotonin receptors in the gut and brain to reduce nausea [4][12]. Neither medicine is known to significantly alter the blood levels or effects of the other, and they are not listed among the medicines that should not be taken with tadalafil.

That said, both medicines can, in rare cases, affect heart rhythm (a condition called QT prolongation), particularly at higher doses or in people with existing heart conditions [12]. Your prescriber will review your full medical history and current medicines during your consultation to ensure it is safe for you to take tadalafil. If you are prescribed ondansetron regularly or have any heart concerns, it is important to mention this so your prescriber can assess your individual risk.

Remember, you must never take tadalafil with nitrates (GTN spray, isosorbide mononitrate or dinitrate), nicorandil, riociguat, or recreational drugs called 'poppers' (amyl or alkyl nitrites), as this combination can cause a dangerous drop in blood pressure [4]. Always declare every medicine you take, including over-the-counter treatments, herbal remedies, and supplements, during your online consultation.

How tadalafil works in your body

Tadalafil belongs to a group of medicines called PDE5 inhibitors. When you are sexually aroused, your body releases a chemical called nitric oxide, which triggers the production of cyclic GMP—a substance that relaxes smooth muscle in the penis and allows blood to flow in [4]. An enzyme called PDE5 breaks down cyclic GMP, ending the erection. Tadalafil blocks PDE5, so cyclic GMP levels stay higher for longer, making it easier to get and keep an erection when you are sexually stimulated. Tadalafil does not cause an erection on its own; sexual stimulation is still needed [4].

What does your prescriber check before prescribing tadalafil?

Before prescribing tadalafil, your prescriber will review your medical history, current medicines, and any conditions that might make tadalafil unsuitable [2][4]. They will check whether you have heart problems (such as recent heart attack or stroke, unstable angina, or severe heart failure), very low or uncontrolled high blood pressure, liver or kidney disease, blood disorders, stomach ulcers, or a history of priapism (prolonged erection). They will also ask about any previous sudden loss of vision (a rare condition called non-arteritic anterior ischaemic optic neuropathy, or NAION) or inherited retinal disorders, as tadalafil is not suitable in these cases [4].

Your prescriber will specifically check whether you are taking any medicines that interact with tadalafil. These include nitrates, nicorandil, riociguat, alpha-blockers (such as doxazosin or tamsulosin), and medicines that affect how your liver processes tadalafil, such as potent CYP3A4 inhibitors (e.g. ritonavir, ketoconazole, clarithromycin) [4]. If you take an alpha-blocker for prostate symptoms or high blood pressure, you should be stable on that medicine before starting tadalafil, and your prescriber will usually recommend the lowest dose of tadalafil to reduce the risk of dizziness or low blood pressure [4].

If you are taking ondansetron or any other anti-sickness medicine, your prescriber will note this during your consultation. While no major interaction is expected, they will consider your overall medication list and any heart conditions to ensure tadalafil is safe for you. Never stop or change a prescribed medicine without advice from your doctor or pharmacist.

Practical advice for taking tadalafil safely

If you are prescribed tadalafil, take it exactly as directed by your prescriber. For on-demand use, take one 10 mg or 20 mg tablet at least 30 minutes before you plan to have sex; the effect can last up to 36 hours, so you do not need to rush [4]. For daily use, take one 2.5 mg or 5 mg tablet at the same time each day, regardless of when you plan to have sex. Swallow the tablet whole with water; you can take it with or without food, although a very heavy or high-fat meal may delay how quickly it works [4].

Do not take more than one dose of on-demand tadalafil in 24 hours, and do not take on-demand and daily tadalafil together. Avoid drinking large amounts of alcohol, as this can make it harder to get an erection and increase the risk of dizziness or low blood pressure [4]. Grapefruit juice can raise tadalafil levels in your blood, so it is best to avoid it while taking this medicine. If you are taking ondansetron for nausea, continue to take it as prescribed by your doctor; there is no need to separate the doses, but if you have any concerns, speak to your pharmacist.

Tadalafil only works when you are sexually aroused, so it will not cause an erection on its own [4]. If you find that tadalafil is not effective, do not increase the dose yourself—speak to your prescriber, who may adjust your treatment or explore other options. Common side effects include headache, indigestion, back pain, muscle aches, flushing, and a stuffy or runny nose; these are usually mild and go away on their own [4].

When to seek urgent help

Seek immediate medical help if you experience an erection lasting more than four hours (priapism), as this can cause permanent damage to the penis [4]. Go to A&E or call 999 if you have sudden loss of vision or hearing, severe chest pain during sex, or signs of an allergic reaction (such as swelling of the face, lips, or throat, or difficulty breathing). If you feel dizzy, faint, or unwell after taking tadalafil, sit or lie down and seek medical advice. Do not drive or operate machinery if you feel dizzy or have visual disturbances.

Other medicines that interact with tadalafil

While tadalafil and ondansetron do not have a major interaction, there are several medicines you must never take with tadalafil. Do not take tadalafil if you use nitrates (such as GTN spray, isosorbide mononitrate, or isosorbide dinitrate) for angina, nicorandil for chest pain, riociguat for pulmonary hypertension, or recreational drugs called 'poppers' (amyl or alkyl nitrites) [4]. The combination can cause a dangerous drop in blood pressure that may be life-threatening. You must also not take tadalafil with another PDE5 inhibitor (such as sildenafil or vardenafil) at the same time.

Alpha-blockers (doxazosin, tamsulosin, alfuzosin, terazosin, prazosin, silodosin, indoramin) used for prostate symptoms or high blood pressure can add to the blood-pressure-lowering effect of tadalafil, increasing the risk of dizziness or fainting [4]. If you take an alpha-blocker, your prescriber will ensure you are stable on that medicine and will usually start you on the lowest dose of tadalafil. Potent CYP3A4 inhibitors (such as ritonavir, cobicistat, ketoconazole, itraconazole, or clarithromycin) can raise tadalafil levels in your blood, so your prescriber may reduce your tadalafil dose [4].

CYP3A4 inducers (such as rifampicin, carbamazepine, phenytoin, or St John's wort) can make tadalafil less effective by speeding up how quickly your body breaks it down [4]. Other blood-pressure medicines (such as ACE inhibitors, calcium-channel blockers, or diuretics) can add to the blood-pressure-lowering effect of tadalafil, but this is usually manageable; your prescriber will review your medicines during your consultation. Always tell your prescriber and pharmacist about every medicine you take, including over-the-counter treatments, herbal remedies, and supplements.

How to get tadalafil from Cured Pharmacy

Cured Pharmacy is a GPhC-registered online pharmacy (GPhC 9012511) offering a convenient, confidential service for erectile dysfunction treatment. To order tadalafil, complete a short online consultation that asks about your medical history, current medicines (including ondansetron or any other anti-sickness medicine), and any health conditions. A UK-registered prescriber will review your answers to ensure tadalafil is safe and suitable for you. If approved, your medicine is dispensed by our pharmacy team and delivered discreetly to your door.

We offer a range of ED treatments, including generic tadalafil at £7.95 for on-demand tablets (£6.36 with first-order code Ednew), branded Cialis at £12.00 (£9.60 with code Ednew), and Tadalafil 20 mg (Teva) at £9.99 (£7.99 with code Ednew). If tadalafil is not suitable, your prescriber may recommend an alternative PDE5 inhibitor such as sildenafil (from £0.45, or £0.36 with code Ednew), vardenafil (£16.95, or £13.56 with code Ednew), or avanafil (Spedra, £20.95, or £16.76 with code Ednew). All prices include the prescriber consultation and delivery.

If you are experiencing erectile dysfunction for the first time, we encourage you to see your GP as well, as ED can be an early sign of underlying health conditions such as heart disease or diabetes [1][2]. Treating the underlying cause can improve your erections and your overall health. Cured Pharmacy is here to provide convenient access to treatment once you and your doctor have discussed your symptoms and ruled out serious conditions.

Why choose Cured Pharmacy?

Cured Pharmacy combines the expertise of UK-registered prescribers and pharmacists with the convenience of online ordering. Every consultation is reviewed individually, and our team is available to answer your questions by phone, email, or live chat. We understand that ED can feel embarrassing, so we provide a discreet, non-judgemental service with plain packaging and fast, reliable delivery. Your privacy and safety are our top priorities, and we follow all GPhC and MHRA regulations to ensure you receive genuine, licensed medicines.

Scientific References

1. NHS. (n.d.). Erectile dysfunction (impotence). NHS website. https://www.nhs.uk/conditions/erection-problems-erectile-dysfunction/ [accessed 24 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 24 October 2026]
3. Joint Formulary Committee. (n.d.). Tadalafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/tadalafil/ [accessed 24 October 2026]
4. Electronic Medicines Compendium. (n.d.). Summaries of Product Characteristics and Patient Information Leaflets. https://www.medicines.org.uk/emc/ [accessed 24 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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