Sildenafil and St John's Wort: Do They Interact?

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At a glance

What is sildenafil and how does it work?

Sildenafil is a prescription medicine used to treat erectile dysfunction (ED) in men. It's the active ingredient in Viagra and is also available as a lower-cost generic medicine. Sildenafil belongs to a group of medicines called PDE5 inhibitors, which work by relaxing blood vessels in the penis, allowing more blood to flow in when you're sexually aroused [3].

Sildenafil is typically taken about one hour before sexual activity, and its effects can last for around four to five hours. It's available in three strengths: 25mg, 50mg, and 100mg tablets. The medicine only works when you're sexually stimulated – it won't cause an erection on its own. You should take sildenafil a maximum of once a day [3].

At Cured Pharmacy, sildenafil is available after an online consultation reviewed by a UK-registered prescriber. The prescriber will check your medical history, current medicines, and overall health to make sure sildenafil is safe and suitable for you. This includes checking for any interactions with other medicines or supplements you're taking, including herbal remedies like St John's wort.

What is St John's wort and why do people take it?

St John's wort (Hypericum perforatum) is a herbal remedy commonly used to treat mild depression and low mood. It's available over the counter in pharmacies, health food shops, and supermarkets without a prescription. Many people choose St John's wort because they prefer a 'natural' treatment or want to avoid prescription antidepressants [12].

While St John's wort is widely used, it's important to understand that 'natural' doesn't always mean safe or free from side effects and interactions. St John's wort affects the way your liver processes many prescription medicines, which can make them less effective or, in some cases, increase the risk of side effects. This is why it's essential to tell your doctor, pharmacist, or prescriber about any herbal remedies you're taking.

If you're considering St John's wort for depression or low mood, it's worth seeing your GP first. Depression and anxiety can themselves contribute to erectile dysfunction, and your GP can help you find the most appropriate treatment – whether that's talking therapy, prescription medication, or lifestyle changes – while also ensuring any ED treatment you're using will work effectively.

St John's wort and prescription medicines

St John's wort is known to interact with a wide range of prescription medicines, including the contraceptive pill, warfarin, some antidepressants, anticonvulsants, and immunosuppressants. It can also reduce the effectiveness of medicines used to treat HIV, cancer, and heart conditions. If you're taking any prescription medicine, always check with your pharmacist or doctor before starting St John's wort [12].

How do sildenafil and St John's wort interact?

The interaction between sildenafil and St John's wort happens because of the way your liver breaks down medicines. Sildenafil is processed by an enzyme in your liver called CYP3A4. St John's wort increases the activity of this enzyme, which means sildenafil is broken down and removed from your body more quickly than it should be [3].

When sildenafil is broken down faster, lower levels of the medicine remain in your bloodstream. This means the medicine may not work as well – you might find it doesn't help your erections as much as expected, or it doesn't work at all. The interaction doesn't cause dangerous side effects or a sudden drop in blood pressure (unlike the serious interactions with nitrates or riociguat), but it does reduce the effectiveness of your ED treatment.

The effect of St John's wort on liver enzymes can last for several weeks after you stop taking it. If you've recently stopped St John's wort and are starting sildenafil, or if you're planning to stop the herbal remedy while taking sildenafil, tell your prescriber. They may need to adjust your dose or timing to make sure the treatment works properly.

What you need to know

• St John's wort can make sildenafil less effective by speeding up how your body breaks it down.
• Always tell your prescriber if you're taking St John's wort, even though it's available without prescription.
• Don't stop St John's wort suddenly if you're using it for depression – speak to your GP first.
• The interaction doesn't cause dangerous side effects, but sildenafil may not work as well as expected.
• Other ED medicines like tadalafil and vardenafil are also affected by St John's wort in the same way.

What your prescriber checks before prescribing sildenafil

When you request sildenafil through Cured Pharmacy's online consultation, a UK-registered prescriber reviews your answers to make sure the medicine is safe and suitable for you. They'll ask about your current medicines, including over-the-counter products, herbal remedies, and supplements. It's essential to list St John's wort if you're taking it, even though it's available without a prescription.

Your prescriber will also check for conditions that make sildenafil unsuitable, such as recent heart attack or stroke, severe heart failure, very low blood pressure, or if you're taking nitrates, nicorandil, or riociguat. They'll ask about other medicines that can interact with sildenafil, including alpha-blockers (used for prostate problems or high blood pressure) and medicines that affect the CYP3A4 enzyme [3].

If you're taking St John's wort, your prescriber may discuss whether it's worth continuing the herbal remedy or whether stopping it (under your GP's guidance, if you're using it for depression) might allow sildenafil to work more effectively. Alternatively, they might suggest a different approach to managing your erectile dysfunction, or recommend seeing your GP to review all your treatments together.

Other CYP3A4 inducers that affect sildenafil

St John's wort isn't the only medicine that speeds up the CYP3A4 enzyme. Prescription medicines including rifampicin (an antibiotic), carbamazepine and phenytoin (used for epilepsy), and some HIV medicines can also reduce sildenafil levels and make it less effective. Your prescriber will check for these during your consultation [3].

Practical advice if you're taking sildenafil and St John's wort

If you're currently taking both sildenafil and St John's wort, the most important step is to tell your prescriber or pharmacist. Don't stop taking St John's wort suddenly without speaking to your GP first, especially if you're using it to manage depression or anxiety. Stopping abruptly can cause withdrawal symptoms or a return of low mood.

Your prescriber may suggest stopping St John's wort (with your GP's support) and waiting a few weeks before reassessing how well sildenafil works. Because the enzyme-inducing effect can persist for some time after stopping the herbal remedy, you may notice sildenafil becomes more effective once St John's wort has fully cleared from your system.

If you prefer to continue St John's wort, discuss alternative treatments for erectile dysfunction with your prescriber. Other PDE5 inhibitors such as tadalafil, vardenafil, and avanafil are also affected by CYP3A4 inducers, so switching to a different ED tablet may not solve the problem. Your GP might suggest non-tablet treatments, addressing underlying causes of ED (such as improving cardiovascular health, managing diabetes, or reducing alcohol intake), or reviewing whether St John's wort is the best option for your mood.

When to get urgent help

• Erection lasting more than 4 hours (priapism) – go to A&E immediately
• Sudden loss of vision in one or both eyes – stop sildenafil and seek urgent medical advice
• Chest pain during or after sex – call 999; do not take more sildenafil
• Sudden decrease or loss of hearing, sometimes with ringing in the ears or dizziness – stop sildenafil and contact your GP or NHS 111 urgently

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

When to seek medical advice about sildenafil and St John's wort

If you've been taking sildenafil and it's not working as well as you expected, and you're also taking St John's wort, speak to your prescriber or GP. Reduced effectiveness could be due to the interaction, but it's also important to rule out other causes. Erectile dysfunction can be an early warning sign of cardiovascular disease or diabetes, so if ED is new or getting worse, see your GP for a full health check [1][2].

You should also seek advice if you're thinking of starting St John's wort while already taking sildenafil, or if you're planning to stop the herbal remedy. Your pharmacist or prescriber can help you understand how the change might affect your ED treatment and whether you need to adjust your dose or timing.

If you experience any side effects from sildenafil – such as severe headache, dizziness, chest pain during sex, sudden vision or hearing loss, or an erection lasting more than four hours – seek urgent medical help. These symptoms are not caused by the interaction with St John's wort, but they do require immediate attention. Call 999 or go to A&E if you have chest pain during sex or an erection that won't go down, and contact your GP or NHS 111 urgently for sudden changes in vision or hearing.

Alternatives to St John's wort for low mood

If you're taking St John's wort for depression or anxiety and it's interfering with your ED treatment, talk to your GP about alternatives. Prescription antidepressants, talking therapies such as cognitive behavioural therapy (CBT), and lifestyle measures like regular exercise and good sleep can all help with low mood. Your GP can tailor a treatment plan that works alongside your erectile dysfunction treatment [1].