Sildenafil and Erythromycin: Can They Be Taken Together?

Sildenafil and Erythromycin: Understanding the Interaction

Published: 16 October 2026

Sildenafil and erythromycin can interact because erythromycin affects how your body breaks down sildenafil, potentially increasing its levels in your bloodstream. This doesn't mean you cannot take them together, but it does mean your prescriber needs to adjust your sildenafil dose or timing to keep treatment safe and effective. Understanding this interaction helps you use both medicines confidently when treating erectile dysfunction alongside a bacterial infection.

What Are Sildenafil and Erythromycin?

Sildenafil is a prescription medicine used to treat erectile dysfunction (ED). It belongs to a group called PDE5 inhibitors, which work by relaxing blood vessels in the penis, allowing increased blood flow when you're sexually aroused [3]. Sildenafil is the active ingredient in Viagra and is available in 25mg, 50mg, and 100mg tablets. You typically take it about one hour before sexual activity, and it works only when you're sexually stimulated.

Erythromycin is a macrolide antibiotic prescribed to treat bacterial infections such as chest infections, skin infections, and certain sexually transmitted infections [12]. It works by stopping bacteria from growing and multiplying. Erythromycin is usually taken two to four times daily, depending on the infection being treated and the formulation prescribed. Like many antibiotics, it's important to complete the full course even if you start feeling better.

Both medicines are metabolised (broken down) in your liver by enzymes, which is where the potential for interaction arises. Your prescriber will consider this when deciding whether you can take them together and what dose adjustments might be needed.

How Do Sildenafil and Erythromycin Interact?

The interaction between sildenafil and erythromycin happens in your liver. Sildenafil is broken down by an enzyme system called CYP3A4. Erythromycin is a moderate inhibitor of this enzyme, meaning it slows down how quickly your body clears sildenafil from your system [3][12]. When erythromycin blocks this enzyme, sildenafil stays in your bloodstream for longer and reaches higher levels than it would on its own.

Higher sildenafil levels can increase both the beneficial effects and the risk of side effects. Common side effects of sildenafil include headaches, flushing, indigestion, and nasal congestion. When levels are raised by erythromycin, these effects may be more noticeable or last longer. More significantly, sildenafil lowers blood pressure slightly, and higher levels could cause a more pronounced drop, leading to dizziness or light-headedness.

This interaction is well-recognised and manageable. It does not mean the combination is unsafe, but it does require careful prescribing. Your doctor or pharmacist will typically recommend starting with a lower dose of sildenafil if you're taking erythromycin, or they may advise timing the doses carefully to minimise the overlap.

Why CYP3A4 Matters

CYP3A4 is one of the most important enzyme systems in your liver, responsible for breaking down around half of all medicines. When a medicine like erythromycin inhibits CYP3A4, it can affect many other drugs you take, not just sildenafil. This is why your prescriber reviews all your current medicines during a consultation. Other macrolide antibiotics such as clarithromycin are even stronger CYP3A4 inhibitors and require more significant dose adjustments or may be contraindicated with some ED treatments [3][12].

What Your Prescriber Checks Before Combining These Medicines

When you request sildenafil through an online consultation at Cured Pharmacy, a UK-registered prescriber reviews your medical history and current medicines. If you're taking erythromycin, they will check the dose and duration of your antibiotic course, your overall health, and whether you've taken sildenafil before. They'll also ask about your blood pressure, heart health, and any other conditions that might affect how you respond to sildenafil [1][2].

Your prescriber will consider starting you on a lower dose of sildenafil—often 25mg instead of 50mg—to account for the raised levels caused by erythromycin. They may also advise you to wait until you've finished your antibiotic course before starting sildenafil, especially if the infection is short-term. If you're already taking sildenafil and are then prescribed erythromycin by your GP, it's important to let both prescribers know so they can adjust your treatment plan.

The consultation will also cover other medicines that can interact with sildenafil. For example, if you're taking nitrates for angina, alpha-blockers for prostate problems, or other medicines that lower blood pressure, these must be declared. Sildenafil must never be taken with nitrates, nicorandil, riociguat, or recreational drugs known as poppers, as the combination can cause a dangerous drop in blood pressure [3].

Practical Advice for Taking Sildenafil and Erythromycin Together

If your prescriber approves the combination, take both medicines exactly as directed. Do not increase your sildenafil dose without consulting your prescriber, even if you feel the effect is weaker than expected. The interaction means sildenafil is likely working more strongly, not less. If you experience side effects such as severe headache, dizziness when standing, or prolonged flushing, contact your prescriber or pharmacist for advice.

Timing can help manage the interaction. Erythromycin is usually taken with or just after food to reduce stomach upset, while sildenafil can be taken with or without food (though a heavy, fatty meal may delay its effect). Your prescriber may suggest taking sildenafil at a time when erythromycin levels are lower, though this depends on your dosing schedule. Always follow the specific advice given during your consultation.

Avoid grapefruit juice while taking both medicines, as it also inhibits CYP3A4 and can further raise sildenafil levels [3]. Limit alcohol, as heavy drinking can worsen erectile dysfunction and add to the blood-pressure-lowering effect of sildenafil. If you develop a new infection or are prescribed a different antibiotic, inform your prescriber so they can reassess whether your sildenafil dose remains appropriate.

When to Seek Advice

Contact your GP or pharmacist if you notice unusual side effects, such as visual disturbances, sudden hearing loss, or chest pain during sexual activity. If you develop an erection lasting more than four hours (priapism), seek urgent medical help by attending A&E or calling 999, as this requires immediate treatment to prevent lasting damage [3]. These situations are rare but require prompt attention.

Other Antibiotics and ED Medication: What You Should Know

Erythromycin is not the only antibiotic that can interact with sildenafil and other PDE5 inhibitors. Clarithromycin, another macrolide antibiotic, is a potent CYP3A4 inhibitor and requires even greater caution. With clarithromycin, your prescriber may recommend a significantly reduced sildenafil dose or suggest avoiding the combination altogether, depending on your individual circumstances [3][12].

Azithromycin, a related macrolide, has a much weaker effect on CYP3A4 and is less likely to cause a significant interaction with sildenafil. However, all medicines you take should be declared during your consultation so your prescriber can make an informed decision. Some antibiotics, such as rifampicin, have the opposite effect: they speed up the breakdown of sildenafil, potentially making it less effective [3].

If you're prescribed an antibiotic for a sexually transmitted infection and also need ED treatment, your prescriber will choose the safest combination. Many antibiotics have no significant interaction with sildenafil, but it's essential to provide a complete list of your current and recent medicines, including over-the-counter treatments and supplements.

Alternatives to Sildenafil When Taking Erythromycin

If the interaction between sildenafil and erythromycin is a concern, your prescriber may suggest an alternative PDE5 inhibitor. Tadalafil (Cialis) is also affected by CYP3A4 inhibitors, but the dose adjustment is different: the maximum recommended dose is reduced rather than the starting dose [4]. Tadalafil has a longer duration of action—up to 36 hours—which may suit some men better, and it can also be taken daily at a lower dose.

Vardenafil (Levitra) and avanafil (Spedra) are other PDE5 inhibitors licensed in the UK. However, both are more sensitive to CYP3A4 inhibitors than sildenafil or tadalafil. With potent inhibitors like clarithromycin, vardenafil and avanafil are often contraindicated or require significant dose reductions [5][6]. Erythromycin, being a moderate inhibitor, may still allow their use, but your prescriber will assess this carefully.

In some cases, your prescriber may recommend waiting until you've completed your antibiotic course before starting or resuming ED treatment. Most antibiotic courses last between five and fourteen days, and erythromycin is cleared from your system within a few days of finishing the course. This approach avoids the interaction entirely and may be the simplest solution if your ED treatment is not urgent.

Non-Oral ED Treatments

If oral ED medicines are not suitable while you're taking erythromycin, other options exist. Eroxon is a topical gel that works locally and does not interact with antibiotics. Vitaros (alprostadil cream) and Caverject (alprostadil injection) are prescription-only treatments that also work locally and are not affected by CYP3A4 inhibitors [7]. Your prescriber can discuss these alternatives if needed.

Scientific References

1. NHS. (n.d.). Erectile dysfunction (impotence). NHS website. https://www.nhs.uk/conditions/erection-problems-erectile-dysfunction/ [accessed 16 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 16 October 2026]
3. Joint Formulary Committee. (n.d.). Sildenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/sildenafil/ [accessed 16 October 2026]
4. Joint Formulary Committee. (n.d.). Tadalafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/tadalafil/ [accessed 16 October 2026]
5. Joint Formulary Committee. (n.d.). Vardenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/vardenafil/ [accessed 16 October 2026]
6. Joint Formulary Committee. (n.d.). Avanafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/avanafil/ [accessed 16 October 2026]
7. Joint Formulary Committee. (n.d.). Alprostadil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/alprostadil/ [accessed 16 October 2026]
8. Electronic Medicines Compendium. (n.d.). Summaries of Product Characteristics and Patient Information Leaflets. https://www.medicines.org.uk/emc/ [accessed 16 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.