Can You Take Tadalafil and Trimethoprim Together?

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Tadalafil and Trimethoprim: Understanding the Interaction

Published: 26 October 2026

Many men wonder whether tadalafil and trimethoprim can be taken together safely. The good news is that no major interaction is commonly reported between tadalafil (Cialis) and trimethoprim, a widely used antibiotic for urinary tract infections. However, your prescriber will still review all your current medicines during your consultation to ensure the combination is appropriate for you.

What is tadalafil and trimethoprim used for?

Tadalafil is a prescription medicine used to treat erectile dysfunction (ED) in men [4]. It belongs to a group of medicines called PDE5 inhibitors, which work by relaxing blood vessels in the penis, allowing increased blood flow when you are sexually aroused. Tadalafil is available as the branded medicine Cialis or as a generic version, and it can be taken either on-demand (10mg or 20mg at least 30 minutes before sex, with effects lasting up to 36 hours) or as a daily low dose (2.5mg or 5mg) [4].

Trimethoprim is an antibiotic commonly prescribed for bacterial infections, particularly urinary tract infections (UTIs) and some respiratory infections. It works by stopping bacteria from producing folic acid, which they need to grow and multiply. A typical course lasts 3 to 7 days, depending on the type and severity of infection. Trimethoprim is not related to erectile dysfunction treatment and works through a completely different mechanism in the body.

Because these two medicines work in different ways and are metabolised differently, they do not typically interfere with each other's effectiveness or safety profile. However, it is essential that your prescriber knows about all medicines you are taking, including antibiotics, to ensure there are no individual factors that might affect you.

Do tadalafil and trimethoprim interact with each other?

No major interaction between tadalafil and trimethoprim is commonly reported in clinical practice or drug interaction databases. Trimethoprim does not significantly affect the liver enzymes (CYP3A4) that break down tadalafil, so it is unlikely to raise or lower tadalafil levels in your bloodstream [4]. Similarly, tadalafil does not interfere with trimethoprim's antibacterial action or how your body processes the antibiotic.

This means that, for most men, taking tadalafil while on a short course of trimethoprim for a urinary infection should not pose additional risks beyond the usual side effects of each medicine individually. However, individual responses can vary, and some men may experience side effects from either medicine that could be mistaken for an interaction—for example, headache or dizziness can occur with both tadalafil and, less commonly, trimethoprim.

It is important to remember that your prescriber or pharmacist will check every current medicine during your online consultation at Cured Pharmacy. This includes antibiotics, over-the-counter medicines, and herbal supplements. You should never stop or change a prescribed antibiotic course without medical advice, as incomplete treatment can lead to antibiotic resistance or infection recurrence.

How the interaction check works

When you complete an online consultation for tadalafil at Cured Pharmacy, you will be asked to list all medicines you are currently taking, including recent or ongoing antibiotic courses. A UK-registered prescriber reviews this information alongside your medical history and current health conditions. They check for known interactions, contraindications, and any individual factors (such as kidney or liver function) that might affect how your body handles either medicine. This thorough review ensures that tadalafil is safe and appropriate for you, even if you are taking trimethoprim or another antibiotic at the same time.

What your prescriber checks before approving tadalafil and trimethoprim together

Even though no major interaction is expected, your prescriber will assess several important factors before approving tadalafil while you are taking trimethoprim. First, they will confirm that you do not have any contraindications to tadalafil itself, such as recent heart attack or stroke, unstable angina, severe heart failure, very low blood pressure, or use of nitrate medicines (GTN spray, isosorbide mononitrate or dinitrate), nicorandil, riociguat, or recreational drugs known as 'poppers' [4]. These combinations can cause a dangerous drop in blood pressure and must be avoided.

Your prescriber will also review any other medicines you take regularly. For example, if you are on alpha-blockers for prostate symptoms (such as tamsulosin or doxazosin), you should be stable on that treatment and start tadalafil at the lowest dose to minimise the risk of dizziness or low blood pressure [4]. If you take potent CYP3A4 inhibitors—such as ritonavir, cobicistat, or certain antifungals like ketoconazole—your tadalafil dose may need to be reduced, as these medicines can raise tadalafil levels in the blood [4].

Finally, your prescriber will consider your overall health, including kidney and liver function. Trimethoprim is mainly cleared by the kidneys, and tadalafil dosing may also be adjusted in men with significant kidney or liver impairment [4]. By reviewing all these factors together, your prescriber ensures that taking tadalafil alongside trimethoprim is safe and suitable for your individual circumstances.

Practical advice when taking tadalafil with trimethoprim

If your prescriber approves tadalafil while you are on trimethoprim, there are a few practical steps to help you use both medicines safely and effectively. First, take each medicine exactly as prescribed: trimethoprim is usually taken once or twice daily with or without food, while tadalafil on-demand is taken at least 30 minutes before sexual activity (or daily tadalafil is taken at the same time each day) [4]. Do not double up on doses or change the timing without consulting your prescriber.

Be aware of the side effects of each medicine so you can recognise them if they occur. Common side effects of tadalafil include headache, indigestion, back pain, muscle aches, flushing, and nasal congestion [4]. Trimethoprim can occasionally cause nausea, rash, or mild stomach upset. If you experience dizziness or light-headedness, sit or lie down until it passes, and avoid driving or operating machinery. These symptoms are more likely if you drink alcohol heavily or are dehydrated, so stay well hydrated and limit alcohol intake.

Complete your full course of trimethoprim, even if you start to feel better before it is finished. Stopping antibiotics early can allow bacteria to survive and develop resistance. If you develop any unusual symptoms—such as severe rash, difficulty breathing, chest pain during sex, or an erection lasting more than four hours—seek urgent medical help. Always inform any healthcare professional you see (GP, pharmacist, or hospital doctor) that you are taking both tadalafil and trimethoprim, especially if new medicines are being considered.

Timing your doses

There is no need to separate the timing of tadalafil and trimethoprim doses, as they do not interfere with each other's absorption or effectiveness. You can take them at whatever times suit your routine and the instructions on each medicine. For example, if you take trimethoprim in the morning and evening, you can still take on-demand tadalafil before planned sexual activity without waiting a specific interval. If you are on daily tadalafil, simply continue taking it at your usual time each day, regardless of when you take your antibiotic.

Other antibiotics and tadalafil: what you need to know

While trimethoprim does not interact significantly with tadalafil, it is worth knowing that some other antibiotics can affect how your body processes tadalafil. Macrolide antibiotics such as clarithromycin and erythromycin are potent inhibitors of the liver enzyme CYP3A4, which breaks down tadalafil [4]. If you are prescribed one of these antibiotics, your tadalafil dose may need to be reduced, or your prescriber may advise a temporary break from tadalafil during the antibiotic course.

On the other hand, some antibiotics like rifampicin (used for tuberculosis and some other infections) can speed up the breakdown of tadalafil, potentially making it less effective [4]. Your prescriber will check the specific antibiotic you are taking and adjust your ED treatment plan if necessary. This is why it is essential to declare every medicine—including short courses of antibiotics—during your online consultation.

Most commonly prescribed antibiotics for everyday infections (such as amoxicillin, doxycycline, or nitrofurantoin) do not have significant interactions with tadalafil. However, never assume a combination is safe without checking. The prescriber or pharmacist at Cured Pharmacy reviews every current medicine during your consultation to ensure your treatment is both safe and effective. If you are unsure about any medicine you are taking, ask before starting or stopping any treatment.

When to seek medical advice about tadalafil and trimethoprim

Most men tolerate tadalafil and trimethoprim together without problems, but you should seek medical advice if you experience any concerning symptoms. Contact your GP or pharmacist if you develop a new or worsening rash, severe headache, persistent dizziness, or any side effect that worries you. If you have chest pain or discomfort during sexual activity, stop and seek urgent medical help, as this could indicate a heart problem that needs immediate assessment [1][4].

Seek emergency help (call 999 or go to A&E) if you have an erection lasting more than four hours (priapism), sudden loss of vision in one or both eyes, or sudden decrease or loss of hearing. These are rare but serious side effects of tadalafil that require immediate treatment [4]. Also seek urgent help if you develop signs of a severe allergic reaction to either medicine, such as difficulty breathing, swelling of the face, lips, tongue or throat, or a widespread severe rash.

If your urinary infection symptoms do not improve after a few days of trimethoprim, or if they get worse, contact your GP. You may need a different antibiotic or further investigation. Remember, erectile dysfunction can sometimes be an early warning sign of underlying health conditions such as diabetes or heart disease [1][2]. If you are experiencing new or worsening ED, it is important to see your GP for a full health check, even if you are already using tadalafil successfully.

Ongoing support from Cured Pharmacy

Cured Pharmacy is a GPhC-registered online pharmacy (GPhC 9012511) offering convenient access to erectile dysfunction treatments after a thorough online consultation reviewed by a UK-registered prescriber. If you have questions about taking tadalafil with trimethoprim or any other medicine, our pharmacy team is here to help. You can contact us for advice, and we will ensure you have the information and support you need to use your medicines safely and confidently.

Scientific References

1. NHS. (n.d.). Erectile dysfunction (impotence). NHS website. https://www.nhs.uk/conditions/erection-problems-erectile-dysfunction/ [accessed 26 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 26 October 2026]
3. Joint Formulary Committee. (n.d.). Tadalafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/tadalafil/ [accessed 26 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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