Weight loss drugs such as Mounjaro and Wegovy can cause gastrointestinal side effects, which can interact with IBS symptoms and increase the risk of small intestinal bacterial overgrowth (SIBO).
In this guide we look at the impact of Mounjaro on gut health and digestive conditions such as IBS and SIBO.
Causes of IBS and the impact of Mounjaro
IBS (Irritable Bowel Syndrome) is an ongoing condition affecting the stomach and intestines.
The most common IBS symptoms are:
• Abdominal pain
• Diarrhoea
• Constipation
Whilst the cause of IBS is unknown, certain factors have been linked to diagnosis, including:
• Disordered gut-brain interactions resulting in gut sensitivity and changes to how the muscles in your bowels contract
• Unbalanced gut microbiome (dysbiosis) caused by gut infections, antibiotics or diet
• Gender, age and medical history
Can Mounjaro cause IBS?
At present there is no definitive research specifically examining whether Mounjaro can cause IBS.
However, studies have found that taking Mounjaro when you have IBS can:
• Interact with or worsen your symptoms, particularly if you have diarrhea-predominant IBS (IBD-D).
• Impair your gastrointestinal motility
Conversely, studies have found that Mounjaro and similar weight loss drugs can improve IBS symptoms by reducing visceral hypersensitivity, a common cause of IBS-related discomfort.
What is Mounjaro?
Mounjaro is a prescription-only injectable medication used to manage obesity and type 2 diabetes by:
• Lowering appetite and food-seeking behaviour
• Slowing movement of food through your digestive tract, so it stays in your stomach longer
It does this thanks to its active ingredient, tirzepatide, a drug that mimics two hormones, GLP-1 and GIP. Both hormones are produced in the gut after eating. They communicate between the gut, pancreas, liver, and brain to coordinate digestion, nutrient absorption, and energy use.
By mimicking GLP-1 and GIP, tirzepatide creates a sustained feeling of fullness to lose help users weight and/or manage type 2 diabetes.
However, many users of tirzepatide report gastrointestinal side effects, such as nausea, constipation and diarrhea. Recent studies have found it may impair gastrointestinal motility in both healthy people and those with IBS.
The rise of Mounjaro and other weight loss drugs
An estimated 1.6 million adults in England, Wales and Scotland used drugs such as Wegovy and Mounjaro to help with weight-loss between early 2024 and early 2025, according to a new study by UCL researchers.
The research, published in BMC Medicine and funded by Cancer Research UK, found that an additional 3.3 million people said they would be interested in using weight loss drugs over the next year.
These weight loss jabs are now incredibly popular and have been chosen by many because changing a lifetime of diet and lifestyle habits is a difficult task.
The link between Mounjaro and SIBO
Recent studies have shown that as weight -loss jabs like Mounjaro may impair gastrointestinal motility, this increases the risk of small intestinal bacterial overgrowth (SIBO).
A team of researchers in the USA have looked at a large population of weight loss jab users with type 2 diabetes from various prescribing centers across the world to assess the association between GLP-1 or dual GLP-1/glucose-dependent insulinotropic polypeptide (GIP) and if they are associated with incidence of SIBO.
Data from over 1 million patients was analysed. Short-term analysis demonstrated a higher incidence of diagnostically confirmed SIBO in patients treated with all types of weight loss jab.
Other studies show you do not necessarily need to have diabetes to increase your risk of SIBO when taking a weight loss jab.
What is SIBO?
SIBO (Small Intestinal Bacterial Overgrowth) is overgrowth of bacteria in the small intestine due to lack of movement down towards the large bowel resulting in a variety of troublesome gastrointestinal symptoms which can be particularly stubborn to treat.
SIBO is often misdiagnosed as IBS as it shares many of the symptoms.
Diagnosis of SIBO commonly involves breath testing and clinical assessment of symptoms.
Next steps if you have taken Mounjaro and experience gastrointestinal side effects
If you’ve been taking a weight loss jab and have had troublesome wind, bloating, diarrhoea or constipation, it may be worth considering a chat with a Gastroenterologist for SIBO breath-testing.
Alternatively, if you think your gut health symptoms could be SIBO, Altruvita can help you regain control of your digestive health and experience lasting relief.:
• Sign up for our free SIBO Unlocked guide
• Try our 2-Step SIBO Protocol
References
https://pmc.ncbi.nlm.nih.gov/articles/PMC9915969/
Jackson, S.E., Brown, J., Llewellyn, C. et al. Prevalence of use and interest in using glucagon-like peptide-1 receptor agonists for weight loss: a population study in Great Britain. BMC Med 24, 1 (2026). https://doi.org/10.1186/s12916-025-04528-7
Sun Y, Veccia D, Liu BDX, Tse W, Fass R, Song G. Diagnostic Evaluation of an Increased Risk of Developing Small Intestinal Bacterial Overgrowth Associated with Glucagon-like Peptide-1 (GLP-1) Receptor Agonists and Dual GLP-1/GIP Receptor Agonists: A Global Retrospective Multicenter Cohort Analysis. Diagnostics (Basel). 2025 Sep 7;15(17):2264. doi: 10.3390/diagnostics15172264.