For individuals embarking on a weight-loss journey using GLP-1 receptor agonist medications like Mounjaro and Wegovy, uncomfortable gastrointestinal symptoms such as bloating, nausea, and gas have become a common experience. Research indicates that approximately a third of patients using these drugs will encounter such side effects. While many are assured these discomforts are normal, clinicians suggest that a significant number of patients may be suffering from an undiagnosed condition known as Small Intestine Bacterial Overgrowth, or SIBO. Experts indicate that SIBO can often be managed through simple dietary adjustments, breathing exercises, and even changes in posture once identified. Professor Anthony Hobson of The Functional Gut Clinic states that anyone taking these drugs for an extended period has about a 60 percent chance of developing SIBO, advising testing if stomach pains persist.

To comprehend SIBO, it's essential to understand the digestive system. Food travels from the stomach into the small intestine for nutrient absorption, then moves to the large intestine where water and salts are extracted. The large intestine hosts a thriving community of beneficial bacteria, known as the gut microbiome, which aid in vitamin production and fibre breakdown. In contrast, the small intestine typically contains very few bacteria. SIBO occurs when bacteria accumulate abnormally in the small intestine. As food passes through, these misplaced bacteria ferment it, producing gas as a byproduct, which leads to bloating, pain, and diarrhea. The exact number of SIBO patients in the UK is unclear, as it is frequently misdiagnosed as other stomach problems, particularly irritable bowel syndrome (IBS), though some estimates suggest around seven million sufferers.

Several factors can contribute to SIBO, primarily linked to the body's digestive efficiency. Normally, stomach acid destroys most bacteria ingested with food. However, low stomach acid levels, often caused by heartburn medication or potentially by GLP-1 drugs, can allow bacteria to proliferate in the small intestine. Additionally, SIBO can arise when gut motility slows down, prolonging food transit to the large intestine and creating more opportunities for bacteria to escape into the small intestine. Patients who have undergone surgery are known to have slower guts, increasing their SIBO risk. Crucially, a growing body of expert opinion suggests that weight-loss drugs like GLP-1 injections can induce a similar slowing effect.

GLP-1 injections, now used by over 1.5 million people in the UK, were initially developed as diabetes treatments (e.g., Ozempic) and work by mimicking the GLP-1 hormone, the 'satiety hormone,' which signals fullness. This sensation reduces appetite and 'food noise,' leading to significant weight loss. For instance, obese patients on Mounjaro can lose up to a fifth of their body weight in the first year. Common side effects are stomach-related, including nausea, vomiting, bloating, and diarrhea. While these symptoms often subside within weeks for many, for some, they can be persistent and severe enough to necessitate discontinuing the medication. Experts now argue that these long-term issues could be SIBO-related, potentially treatable without stopping the injections.

Experts explain that GLP-1 injections trigger SIBO by slowing gastric emptying, meaning food remains in the stomach longer. This extended time allows for increased bacterial fermentation and gas buildup in the small intestine. Dr. Megan Rossi, a dietician and gut health scientist, confirms that GLP-1 naturally slows gastric emptying, increasing the risk of SIBO. Professor Anthony Hobson estimates that 40 to 60 percent of GLP-1 users may develop SIBO, noting that individuals with pre-existing belching or heartburn might be at higher risk. Therefore, anyone on GLP-1 medication experiencing persistent stomach issues for several weeks should be tested for SIBO.

SIBO diagnosis typically involves a breath test, which measures hydrogen and methane gas levels produced by excess bacteria. However, access to SIBO testing via NHS GPs is limited, often requiring patients to consult private specialists. Once diagnosed, the most common treatment is the antibiotic rifaximin, taken three times daily for two to eight weeks, which effectively eliminates bacterial overgrowth. Alongside medication, lifestyle adjustments are recommended. This includes temporarily cutting out foods known to fuel bacteria, such as garlic, onions, apples, asparagus, and oats, which are high in fermentable fibres. While high-fibre foods are generally healthy, temporary restriction during antibiotic treatment is advised, followed by gradual reintroduction. Prioritizing easy-to-digest meals like soups, stews, slow-cooked meats, or omelettes is also beneficial.

Regarding eating habits while on GLP-1 jabs, experts strongly advise leaving a minimum of four hours between meals and avoiding snacking. Professor Hobson emphasizes this measure after antibiotics, explaining that the stomach needs two hours to digest food and another two hours to clear excess bacteria, a process interrupted by frequent eating. Deep-breathing exercises before meals, such as five to ten slow, deep breaths, can also expedite digestion and reduce SIBO symptoms, especially when gut motility is slowed by GLP-1 medication. Furthermore, maintaining good posture—eating upright and chewing slowly—can prevent gut compression and worsen bloating.

Despite the efficacy of lifestyle changes, research suggests that a course of rifaximin is the most effective SIBO treatment. Professor Hobson notes that while some patients and doctors are hesitant about antibiotics, evidence supports their role in alleviating SIBO symptoms.

Beyond SIBO, pharmacists warn against temporarily discontinuing GLP-1 weight loss jabs, for example, during holidays like Christmas. Ozempic, Mounjaro, and Wegovy, as GLP-1 agonists, regulate appetite, digestion, and glucose control. Abrupt cessation can lead to rebound high blood sugar, rapid weight regain, and stronger side effects upon restarting due to lost tolerance. Aaron Arman, Superintendent Pharmacist at CheqUp, advises against stopping treatment without a clear clinical reason, especially during periods of higher carbohydrate and sugar intake. A CheqUp survey revealed over a third of GLP-1 users plan to reduce their dosage over Christmas, with reasons ranging from wanting to 'enjoy themselves' to feeling they 'deserve a break,' often without consulting clinicians. Many users also expressed discomfort discussing their medication or appetite changes with family and friends, fearing judgment or embarrassment. However, some users are adapting their festive eating by opting for smaller portions, less alcohol, lighter dishes, or skipping certain courses.

The weight loss jab revolution presents multiple treatment options, necessitating a crucial understanding of when to change medication. Dr. Kathryn Basford of ZAVA explains that while injections like Mounjaro, Wegovy, and Saxenda all mimic appetite-controlling hormones, the ideal treatment is a personalized decision based on metabolism, health conditions, lifestyle, and medication tolerance. Key signs indicating a need to switch include persistent or worsening side effects such as nausea, vomiting, diarrhea, constipation, bloating, stomach pain, or unusual fatigue beyond the initial adjustment period. Severe symptoms like stomach pain radiating to the back, yellowing skin or eyes, very dark urine, difficulty breathing, or facial/throat swelling are red flags requiring immediate medical attention and cessation of treatment.

Other critical indicators for a medication change include new or worsening health issues such as gallbladder problems, pancreatitis, thyroid issues, digestive trouble, diabetes complications, or impaired liver/kidney function. Lack of expected weight loss is another common reason; patients should typically lose at least five percent of their starting weight within three months. If, after six months on the highest tolerated dose, this milestone is not met despite consistent healthy habits, alternative treatments are usually recommended. Issues with tolerance and adherence, such as inability to increase dosage due to side effects or repeatedly missing weekly injections, also suggest a need for a different approach or delivery format. Finally, life changes like pregnancy or breastfeeding, interactions with other medications (e.g., steroids, certain antidepressants causing weight gain), or negative impacts on mental health or eating habits (triggering disordered eating or anxiety around food) are strong reasons to consider a medication change or discontinuation.

Ultimately, a sustainable weight-loss journey should not be punishing or unsafe. If individuals experience intolerable side effects, insufficient weight loss, new medical issues, dosing difficulties, or a decline in mental well-being, consulting a healthcare professional is paramount. A doctor can assess progress, review options, and determine the best course of action, whether it's switching injections or adjusting the treatment plan. Regular check-ins and honest communication with medical professionals are crucial for ensuring the chosen treatment respects both health and long-term goals. Always consult a doctor before starting medically assisted weight loss, as these injections may not be suitable for everyone.