Why These Medicines Can Cause Diarrhoea
Mounjaro and Wegovy both act on GLP-1 receptors involved in appetite regulation and digestion. One well-documented effect is a slowing of gastric emptying — food moves through the stomach more slowly than usual. The broader impact on gut motility can go in either direction for different people: some experience constipation, while others notice looser stools or diarrhoea, especially in the early weeks of treatment or after a dose adjustment.
When it tends to happen
Diarrhoea is most commonly reported in the first few days after starting the medicine or stepping up to a higher dose. For many people, it eases as the body adjusts — though this is not universal. Eating very rich, fatty, or sugary foods appears to worsen symptoms for some users, as does eating quickly or in larger portions than the reduced appetite might otherwise prompt.
The link to weight loss
It is worth knowing that diarrhoea itself is unlikely to be a meaningful driver of the weight changes people see on these medicines — those effects are primarily related to reduced appetite and calorie intake. However, frequent diarrhoea can affect hydration and how well nutrients are absorbed, which is worth monitoring. If you are also experiencing significant fatigue alongside digestive symptoms, you may find the page on mood and wellbeing during your weight-loss journey a useful read alongside this one.
When to be cautious
Diarrhoea that is severe, persistent beyond a week or two, contains blood, or is accompanied by significant abdominal pain should not be managed at home — contact your healthcare professional promptly. For a closer look at the full range of digestive symptoms specifically linked to Mounjaro, the page on diarrhoea on Mounjaro covers this in more detail. You can also read about the broader picture of Mounjaro side effects for wider context.
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 diarrhoea on a GLP-1 medicine is to record when it happens, how long it lasts, and what else was going on that day — what you ate, your dose timing, your hydration. That kind of pattern data is genuinely useful to bring to your prescriber, and it is much harder to reconstruct from memory after the fact.
GLP Trackr's side-effect tracker lets you log symptoms like diarrhoea with a timestamp, so over time you can see whether episodes cluster around injection days, particular foods, or dose changes. Pair that with the food log — which lets you scan a meal with your phone camera for an instant nutrition breakdown — and you can start to spot whether certain meals correlate with worse symptoms.
The app also includes a morning check-in where you can note energy levels and wellbeing alongside physical symptoms, giving you a fuller picture of how you are feeling day to day. If you want to bring a clear summary to your next appointment, the insights section can help you do exactly that.
GLP Trackr is a tracking tool, not a substitute for clinical advice. If your symptoms are severe, persistent, or worrying, please contact your healthcare professional — they are best placed to advise based on your individual history.
If logging your symptoms and spotting patterns sounds useful, GLP Trackr is free to try for 7 days — available on the App Store and Google Play. Start tracking today and go to your next prescriber appointment with real data in hand.
You can also explore the diarrhea and weight loss page for further reading on this topic.
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Frequently asked questions.
Yes, diarrhoea is among the commonly reported gastrointestinal side effects of both Mounjaro (tirzepatide) and Wegovy (semaglutide). It tends to be most noticeable in the early weeks of treatment or after a dose change, and often eases as the body adjusts.
No. Weight changes on Mounjaro and Wegovy are primarily driven by reduced appetite and lower calorie intake, not by diarrhoea. However, frequent loose stools can affect hydration and nutrient absorption, so it is worth monitoring and discussing with your prescriber.
For many people, digestive side effects are most noticeable in the first few days after starting or increasing the dose, and often settle within a week or two. Experience varies; if symptoms are persistent or severe, speak with your healthcare professional.
Contact your prescriber or doctor promptly if diarrhoea is severe, lasts longer than a week or two, contains blood, or comes with significant abdominal pain. These symptoms should be assessed clinically rather than managed at home.
GLP Trackr's side-effect tracker lets you log symptoms with timestamps so you can spot patterns — for example, whether episodes coincide with injection days or particular foods. You can share this record with your prescriber at your next appointment.