Weight loss injections, particularly GLP-1 receptor agonists such as semaglutide (Wegovy) and liraglutide (Saxenda), are increasingly prescribed for obesity management in the UK. Whilst these medications demonstrate significant efficacy in achieving weight reduction, gastrointestinal side effects are amongst the most commonly reported adverse reactions. Diarrhoea is listed as a very common side effect in MHRA-approved product information, affecting more than 1 in 10 people. Understanding why these symptoms occur, how to manage them, and when to seek medical advice is essential for patients considering or currently using these treatments.

Understanding Weight Loss Injections and Gastrointestinal Side Effects

Weight loss injections, particularly GLP-1 receptor agonists such as semaglutide (Wegovy) and liraglutide (Saxenda), have become increasingly prescribed for obesity management in the UK. These medications work by mimicking naturally occurring hormones that regulate appetite and blood glucose levels. Whilst they demonstrate significant efficacy in achieving weight reduction, gastrointestinal side effects—including diarrhoea—are amongst the most commonly reported adverse reactions.

It's important to note that Wegovy (semaglutide 2.4 mg) and Saxenda (liraglutide 3 mg) are licensed for weight management in the UK, while Ozempic (semaglutide) is only licensed for type 2 diabetes treatment, not for weight management. NHS availability of these medications is guided by NICE technology appraisals (TA875 for semaglutide and TA664 for liraglutide).

The MHRA-approved product information for these medications clearly lists diarrhoea as a very common side effect, alongside nausea, vomiting, and constipation. Understanding that these gastrointestinal symptoms are a known consequence of the medication's mechanism of action can help patients and healthcare professionals anticipate and manage them appropriately. It is important to distinguish between expected, transient side effects and more serious complications requiring medical intervention.

Diarrhoea associated with weight loss injections typically occurs during the initial weeks of treatment or following dose escalation. For most patients, symptoms are mild to moderate in severity and tend to improve as the body adjusts to the medication. However, the impact on quality of life can be significant, and some individuals may require dose adjustments or additional supportive measures. Healthcare professionals should provide clear counselling about potential gastrointestinal effects before initiating treatment, ensuring patients have realistic expectations and know when to seek further advice.

Patients are encouraged to report any suspected side effects via the MHRA Yellow Card scheme.

Why GLP-1 Receptor Agonists Can Cause Diarrhoea

The mechanism by which GLP-1 receptor agonists cause diarrhoea relates to their pharmacological action on the gastrointestinal tract, though the precise mechanisms are multifactorial and not fully elucidated. GLP-1 (glucagon-like peptide-1) is an incretin hormone naturally produced in the intestines that influences multiple aspects of digestive function. When synthetic GLP-1 receptor agonists are administered, they bind to receptors throughout the gastrointestinal system, affecting gut motility, gastric emptying, and intestinal secretion.

These medications slow gastric emptying, which is beneficial for appetite suppression and glycaemic control, but this altered motility can disrupt normal bowel patterns. GLP-1 receptor activation also influences fluid and electrolyte secretion in the intestinal mucosa, which may contribute to changes in stool consistency.

The gastrointestinal effects are dose-dependent, which explains why symptoms often emerge or worsen when doses are increased according to standard titration schedules. Individual responses to GLP-1 receptor agonists vary considerably, with some patients experiencing minimal gastrointestinal disturbance while others have more pronounced symptoms.

Understanding these effects helps clinicians provide targeted advice about dietary modifications and symptom management strategies that may mitigate gastrointestinal disturbance whilst maintaining therapeutic efficacy. The product information for these medications (SmPCs) provides detailed information about their pharmacodynamic properties and associated side effects.

How Common Is Diarrhoea with Weight Loss Injections?

Clinical trial data and post-marketing surveillance provide evidence regarding the frequency of diarrhoea with GLP-1 receptor agonists. According to the STEP clinical trials for semaglutide 2.4 mg (Wegovy), diarrhoea was reported in approximately 20-30% of participants receiving the medication. For liraglutide 3 mg (Saxenda), the SCALE trials documented similar rates, with diarrhoea affecting roughly 20% of treated patients.

In the MHRA-approved product information, diarrhoea is categorised as a 'very common' side effect for both medications, meaning it affects more than 1 in 10 people taking the treatment.

The severity of diarrhoea varies considerably between individuals. Most cases are classified as mild to moderate, meaning they cause some discomfort but do not significantly interfere with daily activities or require treatment discontinuation. Severe diarrhoea occurs less frequently but represents an important safety consideration, particularly in vulnerable populations such as older adults or those with pre-existing renal impairment.

Temporal patterns are also relevant: diarrhoea most commonly occurs during the first 4-8 weeks of treatment or within days of dose escalation. For many patients, symptoms gradually resolve even whilst continuing the medication, suggesting physiological adaptation occurs. However, some individuals experience persistent gastrointestinal symptoms throughout treatment. Gastrointestinal side effects, including diarrhoea, are amongst the common reasons for dose adjustment or treatment discontinuation in clinical practice. Healthcare professionals should monitor symptom patterns and severity to guide individualised management decisions.

Managing and Reducing Diarrhoea During Treatment

Effective management of diarrhoea associated with weight loss injections involves both preventative strategies and symptomatic treatment. Dietary modification represents the first-line approach and can significantly reduce symptom severity. Patients should be advised to avoid high-fat meals, excessive dietary fibre during the initial treatment phase, artificial sweeteners (particularly sorbitol and mannitol), and caffeine, all of which can exacerbate loose stools. Eating smaller, more frequent meals rather than large portions may also help, as this reduces the burden on the digestive system.

Adequate hydration is essential, particularly if diarrhoea is frequent. Patients should maintain good fluid intake, including oral rehydration solutions if symptoms are moderate to severe. This helps prevent dehydration and maintains electrolyte balance. However, fluid intake targets should be individualised, particularly for people with heart failure or kidney disease who may have fluid restrictions. Probiotic supplementation has limited evidence specifically for GLP-1-associated diarrhoea and should only be considered after discussion with a healthcare professional.

From a medication management perspective, dose titration should follow approved schedules without rushing escalation. If diarrhoea becomes problematic at a particular dose level, maintaining that dose for an additional period before increasing may allow tolerance to develop. In consultation with a prescriber, temporary dose reduction can be considered for severe symptoms, with subsequent re-escalation once gastrointestinal function stabilises.

Symptomatic treatment with loperamide (available over-the-counter in the UK) may be appropriate for occasional use, but patients should consult a pharmacist or clinician if they need to use it repeatedly or for more than 48 hours. If patients require frequent antidiarrhoeal medication, this suggests the need for medical review and possible treatment adjustment. Healthcare professionals should provide clear written guidance about these management strategies at treatment initiation, empowering patients to implement appropriate self-care measures whilst recognising when professional input is required.

When to Seek Medical Advice About Diarrhoea

Whilst mild diarrhoea is an expected side effect of weight loss injections, certain warning signs indicate the need for prompt medical assessment. Patients should contact their GP or prescribing clinician if diarrhoea persists beyond 2 weeks without improvement, becomes progressively worse despite dietary modifications, or is accompanied by severe abdominal pain, fever, or blood in the stools. These features may suggest complications or alternative diagnoses requiring investigation.

Patients should stop taking the medication and seek urgent medical attention if they develop severe, persistent abdominal pain (possibly radiating to the back) with or without vomiting, as these could be symptoms of pancreatitis—a rare but serious side effect of GLP-1 receptor agonists. Similarly, right upper abdominal pain or yellowing of the skin or eyes may indicate gallbladder problems, which are also associated with these medications.

Signs of dehydration warrant urgent attention, particularly in older adults or those with diabetes. These include reduced urine output, dark-coloured urine, dizziness upon standing, persistent thirst, dry mucous membranes, and confusion. Severe or prolonged diarrhoea can lead to electrolyte disturbances, particularly hypokalaemia (low potassium), which may cause muscle weakness, cardiac arrhythmias, or exacerbate existing cardiovascular conditions. Patients taking concurrent medications that affect electrolyte balance (such as diuretics) require particularly careful monitoring.

Individuals should also seek medical review if diarrhoea significantly impacts their quality of life or ability to maintain nutrition. Unintentional weight loss beyond the expected therapeutic effect, inability to tolerate oral intake, or symptoms preventing normal daily activities all justify clinical reassessment. Healthcare professionals may need to arrange blood tests to check renal function, electrolytes, and hydration status, particularly if symptoms have been prolonged.

Immediate medical attention (via NHS 111 or emergency services) is appropriate if patients develop signs of severe dehydration, cannot keep down fluids, experience chest pain or palpitations, or have concerns about medication safety. Clear pathways for escalation when side effects become concerning are essential for patient safety.

Alternative Weight Loss Options and Considerations

For patients who cannot tolerate GLP-1 receptor agonists due to persistent diarrhoea or other gastrointestinal side effects, several alternative approaches to weight management exist within the UK healthcare system. NICE guidance (CG189) recommends a tiered approach, with lifestyle interventions forming the foundation of all obesity treatment. Structured weight management programmes delivered through primary care or specialist services provide dietary counselling, physical activity support, and behavioural modification strategies without pharmacological side effects.

When medication is appropriate, orlistat represents an alternative pharmacological option available on the NHS, though it also carries gastrointestinal side effects (including oily stools and faecal urgency) due to its mechanism of inhibiting fat absorption. Newer agents such as tirzepatide (a dual GIP/GLP-1 receptor agonist) may offer different tolerability profiles, though gastrointestinal effects remain common and availability on the NHS may be limited pending NICE appraisal and local commissioning decisions.

Bariatric surgery should be considered for patients meeting NICE criteria (BMI ≥40 kg/m², or ≥35 kg/m² with obesity-related comorbidities). For people with type 2 diabetes, lower BMI thresholds may apply (≥30 kg/m²) as outlined in NICE guidance. Referral to Tier 4 specialist weight management services is typically required. Procedures such as sleeve gastrectomy or gastric bypass offer substantial, sustained weight reduction, though they carry surgical risks and require lifelong dietary modifications and nutritional monitoring.

For individuals experiencing problematic side effects with weight loss injections, shared decision-making with healthcare professionals is essential. This involves weighing the benefits of continued treatment against the impact of side effects on quality of life, considering alternative options, and potentially accepting more modest weight loss goals through non-pharmacological means. No single approach suits all patients, and treatment plans should be individualised based on clinical circumstances, patient preferences, and tolerability. Regular review and adjustment of weight management strategies ensures optimal outcomes whilst maintaining patient safety and wellbeing.

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