Why diarrhoea can occur during GLP-1 weight loss
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) work partly by slowing gastric emptying and acting on appetite-regulating pathways in the gut. This change in how quickly food and fluid move through the digestive tract can lead to looser stools, particularly after a dose increase or in the first few weeks of treatment. According to NHS guidance, diarrhoea is among the commonly reported gastrointestinal side effects of GLP-1 receptor agonist medicines.
Beyond the medicine itself, the significant dietary changes that often accompany weight loss can also play a role. Eating more fibre, cutting processed foods, reducing fat intake, or simply eating very differently from before can all shift gut behaviour — sometimes causing loose stools while the digestive system adapts.
Other changes worth being aware of
Rapid weight loss can occasionally be associated with other changes in the body worth knowing about:
- Gallstones: Losing weight relatively quickly can increase the likelihood of gallstones forming, as changes in bile composition can occur. If you experience persistent upper abdominal pain, particularly after eating, speak to your healthcare professional promptly — this is a separate matter from typical GI side effects.
- Nutritional considerations: If diarrhoea is frequent or prolonged, it may affect how well nutrients are absorbed. Some people ask whether weight loss can cause anaemia; while the medicine itself is not a direct cause, poor nutritional intake or absorption over time is worth discussing with your prescriber or doctor.
- Mood and energy: Some people also wonder whether weight loss or the medicines can contribute to changes in mood or anxiety. Any significant mood changes are worth raising with your healthcare professional.
For more on the range of digestive changes people notice, see what causes diarrhoea during weight loss and the related question of whether diarrhoea itself can affect weight. Hair thinning is another change some people notice — you can read more about that at weight loss and hair loss.
average weekly weight change in the first 16 weeks of Mounjaro & Wegovy use*
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Tracking digestive symptoms with GLP Trackr
One of the most useful things you can do when experiencing side effects like diarrhoea is to log them consistently, so you can spot whether they follow a pattern — for example, appearing in the days after a dose, or linked to certain foods.
GLP Trackr's side-effect tracker lets you record symptoms as they happen, with a timeline view that makes it easy to see how they relate to your injection schedule. This kind of log is genuinely helpful to bring to a prescriber appointment — rather than trying to recall weeks of symptoms from memory, you can show a clear picture of what happened and when. The daily check-in feature also captures energy and mood alongside physical symptoms, useful if you want to track broader wellbeing changes at the same time.
If you want a joined-up view of your full journey — doses, symptoms, food, and progress — the complete guide to Mounjaro side effects is a good place to explore what others commonly experience and how to stay on top of it all.
Important: if diarrhoea is severe, persistent, or accompanied by signs of dehydration, contact your healthcare professional promptly. GLP Trackr helps you record what you are experiencing so you can discuss it with your prescriber — it is not a substitute for their advice, and any decisions about your treatment should involve them directly.
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Frequently asked questions.
Diarrhoea can occur during weight loss for several reasons. GLP-1 medicines like Mounjaro and Wegovy affect gut motility, and dietary changes can also shift bowel habits. For many people it is most noticeable early in treatment and tends to ease over time, though this varies individually.
Yes, diarrhoea is among the commonly reported gastrointestinal side effects of both medicines, particularly after dose increases. It often settles as the body adjusts. If it is severe or persistent, speak to your healthcare professional.
Losing weight relatively quickly can increase the likelihood of gallstones forming due to changes in bile composition. If you experience persistent upper abdominal pain, especially after eating, contact your healthcare professional promptly rather than managing it yourself.
Frequent or prolonged diarrhoea may affect nutrient absorption over time. While these medicines are not a direct cause of anaemia, any concerns about nutritional intake or persistent digestive symptoms are worth discussing with your prescriber or doctor.
GLP Trackr's side-effect tracker lets you log symptoms with dates so you can identify patterns — for example, whether diarrhoea follows a dose day. You can share this log with your prescriber to support informed conversations about your treatment.