Sildenafil, widely recognised by the brand name Viagra, is a phosphodiesterase type 5 (PDE5) inhibitor prescribed for erectile dysfunction and pulmonary arterial hypertension. Whilst generally well-tolerated, some patients wonder: does sildenafil give you diarrhoea? Diarrhoea is classified as an uncommon side effect, affecting between 0.1% and 1% of users according to UK regulatory data. Understanding the relationship between sildenafil and gastrointestinal symptoms helps patients make informed decisions about their treatment. This article examines the evidence, frequency, mechanisms, and practical management strategies for diarrhoea associated with sildenafil use, aligned with MHRA and NHS guidance.

Can Sildenafil Cause Diarrhoea?

Sildenafil, commonly known by the brand name Viagra, can cause diarrhoea as a side effect, though it is not one of the most frequently reported adverse reactions. Sildenafil belongs to a class of medications called phosphodiesterase type 5 (PDE5) inhibitors, primarily prescribed for erectile dysfunction and pulmonary arterial hypertension. Whilst the drug is generally well-tolerated, gastrointestinal disturbances including diarrhoea have been documented in clinical trials and post-marketing surveillance data.

According to the Summary of Product Characteristics (SmPC) approved by the Medicines and Healthcare products Regulatory Agency (MHRA), diarrhoea is classified as an uncommon side effect of sildenafil, occurring in between 0.1% and 1% of patients. Other gastrointestinal side effects include dyspepsia (indigestion) and nausea. It is important to note that experiencing diarrhoea does not necessarily mean the medication is unsuitable for you, as individual responses to medicines vary considerably.

The mechanism by which sildenafil may cause gastrointestinal symptoms is related to its pharmacological action. While sildenafil selectively targets PDE5 enzymes, these enzymes are also present in the smooth muscle of the gastrointestinal tract. This distribution of PDE5 throughout various tissues may contribute to digestive disturbances in some individuals.

If you experience diarrhoea after taking sildenafil, it is worth considering other potential causes such as dietary factors, concurrent medications, or underlying gastrointestinal conditions. Certain medicines that interact with sildenafil, particularly strong CYP3A4 inhibitors (such as ketoconazole, ritonavir or erythromycin), can increase sildenafil levels in your blood and potentially increase the risk of side effects. Keeping a symptom diary can help identify patterns and determine whether the diarrhoea is genuinely related to sildenafil use. Should symptoms persist or cause concern, consulting your GP or prescribing clinician is advisable to discuss alternative treatment options or management strategies.

You can report any suspected side effects to the MHRA through the Yellow Card Scheme at www.mhra.gov.uk/yellowcard or via the Yellow Card app.

How Common Is Diarrhoea with Sildenafil?

Diarrhoea is classified as an uncommon side effect of sildenafil, occurring in between 0.1% and 1% of people who take the medication. According to the UK Summary of Product Characteristics (SmPC) for sildenafil products, gastrointestinal side effects are generally less frequent than other recognised adverse reactions such as headache, facial flushing, or nasal congestion.

In clinical trials and post-marketing surveillance, the most commonly reported side effects of sildenafil were:

• Headache (very common: may affect more than 1 in 10 people)

• Facial flushing (common: may affect up to 1 in 10 people)

• Dyspepsia or indigestion (common: may affect up to 1 in 10 people)

• Nasal congestion (common: may affect up to 1 in 10 people)

• Visual disturbances (uncommon: may affect up to 1 in 100 people)

Diarrhoea is reported less frequently than these effects, categorised as uncommon (affecting between 0.1% and 1% of patients). The incidence of side effects may vary depending on the dose administered, with higher doses potentially associated with an increased risk compared to lower doses.

It is also worth noting that the frequency of side effects can differ between individuals based on various factors including age, concurrent medical conditions, other medications being taken, and individual sensitivity to the drug. Older adults and those with multiple health conditions may experience a different side effect profile. The risk of side effects may also increase if you take sildenafil alongside medicines that inhibit the CYP3A4 enzyme system, such as certain antibiotics, antifungals or HIV medications.

If you are concerned about the likelihood of experiencing diarrhoea or other side effects, discussing your individual risk factors with your prescriber can provide personalised guidance and help set realistic expectations about treatment.

The MHRA encourages patients to report any suspected side effects through the Yellow Card Scheme (www.mhra.gov.uk/yellowcard or via the Yellow Card app), which helps monitor the safety of medicines in the UK.

What to Do If You Experience Diarrhoea While Taking Sildenafil

If you develop diarrhoea after taking sildenafil, the first step is to assess the severity and duration of symptoms. Mild, self-limiting diarrhoea that resolves quickly may not require specific intervention beyond supportive measures. However, persistent or severe diarrhoea warrants medical attention, particularly if accompanied by other concerning symptoms.

Practical self-care measures include:

• Maintaining adequate hydration – Drink plenty of clear fluids such as water, diluted squash, or oral rehydration solutions to replace lost fluids and electrolytes

• Avoiding caffeine and alcohol – These can exacerbate dehydration and gastrointestinal irritation

• Eating small, light meals – Avoid rich, spicy or fatty foods until symptoms improve

• Monitoring symptom patterns – Note when diarrhoea occurs in relation to sildenafil doses to help identify a potential link

For adults with uncomplicated diarrhoea and no red flag symptoms, over-the-counter loperamide may help with short-term symptom control. However, check with a pharmacist first, especially if you take other medications, as interactions are possible.

You should contact your GP or NHS 111 if:

• Diarrhoea persists for more than 7 days

• You experience severe abdominal pain or cramping

• There is blood or mucus in your stools

• You develop signs of dehydration (dizziness, reduced urination, extreme thirst, confusion)

• You have a high temperature (fever) alongside diarrhoea

• You have persistent vomiting

• Symptoms significantly impact your daily activities or quality of life

If you are elderly, immunocompromised, or have significant underlying health conditions, seek medical advice sooner.

Your doctor may recommend temporarily discontinuing sildenafil to determine whether it is the causative factor. Alternative PDE5 inhibitors such as tadalafil or vardenafil may be considered, as individual tolerability can vary between different medications within the same class. In some cases, adjusting the dose or timing of sildenafil administration may help minimise gastrointestinal side effects.

Do not stop taking sildenafil without consulting your prescriber if it has been prescribed for pulmonary arterial hypertension, as abrupt discontinuation could have serious consequences. Always seek medical advice before making changes to your prescribed treatment regimen.

Remember to report any suspected side effects to the MHRA through the Yellow Card Scheme at www.mhra.gov.uk/yellowcard or via the Yellow Card app.

Why Does Sildenafil Sometimes Cause Digestive Side Effects?

Understanding the pharmacological mechanism of sildenafil helps explain why digestive side effects can occur. Sildenafil works by selectively inhibiting the enzyme phosphodiesterase type 5 (PDE5), which is predominantly found in the smooth muscle of blood vessels in the corpus cavernosum of the penis and in pulmonary vasculature. By blocking PDE5, sildenafil prevents the breakdown of cyclic guanosine monophosphate (cGMP), a molecule that promotes smooth muscle relaxation and increased blood flow.

Whilst sildenafil is designed to be selective for PDE5, PDE5 enzymes are expressed in various tissues throughout the body, including the smooth muscle of the oesophagus, stomach, and intestines. This distribution of PDE5 in gastrointestinal tissues may contribute to alterations in gastrointestinal motility and function, potentially leading to symptoms such as diarrhoea, dyspepsia, or abdominal discomfort. At higher concentrations, sildenafil also has some activity against PDE6 (primarily associated with visual side effects), though this is less likely to explain gastrointestinal symptoms.

Additional factors that may contribute to digestive side effects include:

• Individual variation in drug metabolism – Genetic differences in cytochrome P450 enzymes (particularly CYP3A4) can affect how quickly sildenafil is metabolised, influencing both efficacy and side effect profiles

• Concurrent medications – Drug interactions, especially with potent CYP3A4 inhibitors like ketoconazole, erythromycin or ritonavir, can increase sildenafil blood levels and potentially increase the risk of side effects

• Underlying gastrointestinal sensitivity – Individuals with pre-existing conditions such as irritable bowel syndrome (IBS) may be more susceptible to medication-induced digestive disturbances

The British National Formulary (BNF) notes that sildenafil is primarily metabolised by the CYP3A4 pathway, and concomitant use of CYP3A4 inhibitors can significantly increase sildenafil exposure. Your prescriber should follow SmPC guidance regarding dose adjustments when these medications are used together.

Research continues to refine our understanding of PDE5 inhibitor side effects. If you experience troublesome gastrointestinal symptoms with sildenafil, discussing alternative treatment options with your healthcare provider is recommended.

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