Sildenafil and Trimethoprim: Can You Take Them Together?

Takes less than 2 minutes to complete 100% online

At a glance

What is sildenafil and trimethoprim used for?

Sildenafil is a prescription medicine licensed in the UK to treat erectile dysfunction (ED) in men [3]. It belongs to a group of medicines called PDE5 inhibitors, which work by relaxing blood vessels in the penis during sexual stimulation, helping you get and keep an erection firm enough for sex. Sildenafil is the active ingredient in branded Viagra and is available in 25mg, 50mg, and 100mg tablets. You typically take it about one hour before sexual activity, and it can work for up to four hours. Sexual stimulation is still needed for the medicine to work [1].

Trimethoprim is an antibiotic commonly prescribed for bacterial infections, particularly urinary tract infections (UTIs), chest infections, and some skin infections [12]. It works by stopping bacteria from making folic acid, which they need to grow and multiply. Trimethoprim is usually taken as a tablet once or twice daily for a short course, often three to seven days depending on the type and severity of infection. It is not effective against viral infections such as colds or flu.

Is there an interaction between sildenafil and trimethoprim?

No major interaction is commonly reported between sildenafil and trimethoprim [3][12]. Trimethoprim is not known to significantly affect the way sildenafil is broken down in the body, and sildenafil does not interfere with trimethoprim's ability to treat bacterial infections. This means that, for most men, taking both medicines at the same time is considered safe when prescribed appropriately.

However, it is still essential to tell your prescriber about all medicines you are taking, including antibiotics like trimethoprim, during your online consultation. Your prescriber will check your full medication list, medical history, and current health to ensure that sildenafil is suitable for you. Some men may experience side effects from either medicine, and your prescriber can advise on timing or monitoring if needed. Never start, stop, or change the dose of any prescribed medicine without first speaking to your doctor or pharmacist.

How sildenafil is processed in the body

Sildenafil is mainly broken down in the liver by an enzyme called CYP3A4 [3]. Medicines that strongly block this enzyme (such as ritonavir, ketoconazole, or clarithromycin) can raise sildenafil levels in the blood, increasing the risk of side effects. Trimethoprim does not significantly affect CYP3A4, which is why no major interaction occurs. Your prescriber will still review any other medicines you take that might interact with sildenafil, including those for blood pressure, heart conditions, or HIV treatment.

What your prescriber checks before prescribing sildenafil and trimethoprim

When you request sildenafil through Cured Pharmacy's online consultation, a UK-registered prescriber will review your answers to ensure the treatment is safe and suitable for you. They will check your current medicines, including trimethoprim and any other antibiotics, to identify potential interactions. They will also ask about your medical history, particularly any heart or blood-pressure conditions, recent stroke or heart attack, liver or kidney problems, and any allergies [2][3].

Your prescriber will confirm that you are not taking medicines that must never be combined with sildenafil, such as nitrates (GTN spray, isosorbide mononitrate), nicorandil, riociguat, or recreational drugs known as 'poppers' [3]. These combinations can cause a dangerous drop in blood pressure. If you take alpha-blockers for prostate symptoms or high blood pressure (such as tamsulosin or doxazosin), your prescriber will ensure you are stable on that medicine and may recommend starting sildenafil at a lower dose to reduce the risk of dizziness. All prescription medicines at Cured Pharmacy are only supplied after this clinical review.

What you need to know

• No major interaction is commonly reported between sildenafil and trimethoprim
• Your prescriber reviews all your medicines during consultation to ensure safety
• You can usually take both on the same day without separating doses
• Always declare antibiotics and other medicines when requesting sildenafil
• See your GP if you have new erectile dysfunction or symptoms of infection

Practical advice for taking sildenafil with trimethoprim

If your doctor has prescribed both sildenafil and trimethoprim, you can usually take them on the same day without concern. There is no need to separate the doses by a specific number of hours, although you may find it more convenient to take trimethoprim at a regular time (such as morning and evening) and sildenafil about an hour before you plan to have sex [3]. Remember that sildenafil works best on an empty stomach or after a light meal; a heavy or high-fat meal can delay its effect.

Complete the full course of trimethoprim as prescribed by your doctor, even if you start to feel better before the course is finished. Stopping antibiotics early can allow the infection to return or contribute to antibiotic resistance. If you experience any unusual symptoms while taking both medicines—such as severe dizziness, chest pain, prolonged erection lasting more than four hours, or sudden vision or hearing loss—seek medical help immediately. These are rare but serious side effects that require urgent attention [1][3].

Common side effects to be aware of

Sildenafil can cause headaches, flushing, indigestion, nasal congestion, and dizziness in some men [3]. Trimethoprim may cause nausea, mild skin rash, or an upset stomach. If you notice side effects from either medicine, they are usually mild and short-lived. However, if side effects are severe or persistent, contact your GP or pharmacist for advice. Drinking plenty of water while taking trimethoprim can help reduce the risk of side effects and support your recovery from infection.

Other medicines that interact with sildenafil and trimethoprim

While sildenafil and trimethoprim do not interact with each other, both medicines can interact with other drugs. Sildenafil must never be taken with nitrates, nicorandil, riociguat, or another PDE5 inhibitor (such as tadalafil, vardenafil, or avanafil), as this can cause a dangerous fall in blood pressure [3]. Potent CYP3A4 inhibitors, including some HIV medicines (ritonavir, cobicistat), antifungals (ketoconazole, itraconazole), and the antibiotic clarithromycin, can raise sildenafil levels and require dose adjustments or avoidance.

Trimethoprim can interact with medicines that affect potassium levels, such as ACE inhibitors (ramipril, lisinopril) or potassium-sparing diuretics (spironolactone, amiloride), potentially leading to high potassium in the blood [12]. It may also interact with warfarin, increasing the risk of bleeding, and with methotrexate, raising the risk of side effects. Your prescriber and pharmacist will check your full medication list during the consultation to ensure all combinations are safe. Do not stop or change any prescribed medicine without professional advice.

When to get urgent help

• Erection lasting more than 4 hours (priapism) – go to A&E immediately
• Sudden loss of vision or hearing – seek urgent medical help
• Chest pain during or after sex – call 999
• Severe dizziness or fainting – seek same-day medical advice
• High fever, back pain, or blood in urine while on trimethoprim – contact your GP urgently

Call 999 or go to A&E in an emergency, or contact NHS 111 for urgent advice.

When to see your GP about erectile dysfunction or infections

If you are experiencing erectile dysfunction for the first time, it is important to see your GP for a face-to-face assessment [1][2]. ED can be an early warning sign of underlying health conditions such as heart disease, diabetes, or high blood pressure. Your GP can arrange blood tests, check your cardiovascular health, and discuss lifestyle factors such as smoking, alcohol, exercise, and stress. Treating the underlying cause can sometimes improve erectile function without the need for medication.

If you have symptoms of a urinary tract infection—such as pain or burning when you pass urine, frequent urination, cloudy or strong-smelling urine, or lower abdominal pain—contact your GP or use an NHS online service for assessment. Trimethoprim is effective for many bacterial infections, but your doctor will need to confirm the diagnosis and ensure the antibiotic is appropriate. If you develop a high fever, back pain, blood in your urine, or feel very unwell, seek urgent medical advice, as these may be signs of a more serious infection requiring immediate treatment [1][12].

How Cured Pharmacy can help

Cured Pharmacy is a GPhC-registered online pharmacy (GPhC 9012511) offering discreet, convenient access to erectile dysfunction treatments after a clinical consultation with a UK-registered prescriber. We stock a range of licensed ED medicines, including sildenafil (generic Viagra) from £0.45, tadalafil (generic Cialis) from £7.95, vardenafil (generic Levitra) at £16.95, and Spedra (avanafil) at £20.95. First-time customers can use code Ednew for 20% off. All treatments are dispensed by our qualified pharmacists and delivered discreetly to your door. Start your consultation today at curedpharmacy.com.

Choose your treatment

Select from our recommended UK-licensed medications.

Answer quick questions

Select safe UK treatments. Quick answers.

Get it delivered fast

We will deliver direct to you as quickly as tomorrow.

Start Your Assessment For Erectile Dysfunction

Start your journey to better health today.

• Reviewed by UK-based prescribers
• Takes under 3 minutes
• Discreet Packaging