Short answer: GLP-1 medicines like semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound) slow down how fast your stomach empties. That is part of how they keep you full, and it is also why gut side effects are so common. In the Wegovy trials, 73% of adults on the drug reported a GI side effect, compared with 47% on placebo, and they increased during dose escalation.[1] The good news: most are mild to moderate,[2] and simple food and routine changes help.[3]
Started a GLP-1 and your gut feels like a different gut? It is. Here's the big picture, and where to go for each symptom.
What changes in your gut on a GLP-1
Your stomach empties more slowly. The Wegovy prescribing information says it plainly: semaglutide delays gastric emptying.[1] A normal meal now feels big, and it stays put longer.
Everything downstream can slow too. Less food, less fiber and often less fluid move through, so stools can get harder and less frequent.
It is worst when the dose goes up. On Wegovy, GI reactions increased during dose escalation.[1] In the main tirzepatide obesity trial, most GI side effects were mild to moderate and happened mainly during dose escalation.[2]
For some people it is enough to quit: 4.3% of adults on Wegovy stopped because of a GI side effect, versus 0.7% on placebo.[1] That is why it is worth getting ahead of.
One side effect at a time
- Constipation on a GLP-1: why it happens and what helps
- Bloating and gas on a GLP-1
- Nausea and diarrhea on a GLP-1
- Retatrutide and your gut: what the trials show
What helps, across the board
A 2022 multidisciplinary expert consensus on GLP-1 side effects starts with the basics:[3]
- Eat slowly, eat smaller portions, and stop at the first sign of full.
- Choose easy-to-digest, lower-fat food.
- Drink generously, especially with nausea, vomiting or diarrhea.
- Get enough fiber and move more if constipation is the problem.
- Don't lie down right after a meal, and don't eat close to bedtime.
Where probiotics fit, honestly
Some strains support regularity. A meta-analysis of 14 randomized trials found probiotics shortened whole-gut transit time by about 12 hours on average, with the clearest results for Bifidobacterium lactis.[4] For bloating, the research is mixed and strain-specific; the American Gastroenterological Association found the evidence too uncertain to recommend probiotics for IBS outside of clinical trials.[5] None of these studies were in people on GLP-1 medicines.
So a probiotic is support for digestive comfort next to fiber, fluids and meal changes, not a substitute for them. And no probiotic replaces, copies or boosts a GLP-1 medicine.*
More on why a bigger CFU number isn't the answer: when high CFU backfires.
See what your gut is doing on a GLP-1. Then get a formula built for it.
A Flore gut test maps your microbiome from a sample you take at home. We build a personalized formula from what we find, and every formula can carry up to two boosters. For people on GLP-1 medications, the boosters we reach for are Barrier Defense III (Bifidobacterium longum) and Digestion Aid I (Bacillus coagulans), to support regularity and digestive comfort.*
Not ready to test? Take the GoodOnes quiz to find a simple, single-job formula to start with.*
Start with your gut data → How the test worksBefore you add any supplement: if you take an oral semaglutide pill (Rybelsus or the Wegovy pill), don't take it within 30 minutes after your pill. If you have gastroparesis, a past bowel obstruction or bezoar, or trouble swallowing, talk to your clinician first.
When to call your prescriber
- Severe stomach pain that won't go away, with or without vomiting
- You can't keep fluids down, or you're dizzy or barely peeing
- No bowel movement for several days, or you can't pass gas
The prescribing information lists pancreatitis, gallbladder problems and ileus (a stalled bowel) among the serious risks to watch for.[1] Don't stop or change your dose on your own.
Frequently asked questions
What happens to your gut on a GLP-1?
GLP-1 medicines slow how fast your stomach empties, which is part of how they keep you full. That same slowdown is behind the most common side effects: nausea, fullness and bloating, constipation or diarrhea. In the Wegovy trials, 73% of adults on the drug reported a GI side effect, compared with 47% on placebo.
Can a probiotic help with GLP-1 side effects?
Some probiotic strains support regularity, with the best research on Bifidobacterium lactis, but none have been tested specifically in people on GLP-1 medicines. Use a probiotic alongside fiber, fluids and meal changes, never in place of them, and never in place of your medication.
Does a probiotic work with or boost a GLP-1 drug?
No. No probiotic replaces, copies or boosts a GLP-1 medicine. A probiotic is a supplement for everyday digestive comfort, and it should never change how you take your prescription.
Should I test my gut if I'm on a GLP-1?
You don't need a test to start with the basics. If you want to see what your gut is doing on a GLP-1 and get a formula built from that data, an at-home microbiome test is how Flore starts.
Keep reading: metabolic gut health hub · GLP-1, probiotics and weight.
Not sure where to start with your gut?
Ask the Flore Microbiome Concierge — a guided, personalized walkthrough of testing, your results, and a formula matched to your goals.
Talk to the Concierge →This article is educational and is not medical advice. It does not replace your prescriber. Never stop, skip or change a GLP-1 dose on your own.
*These statements have not been evaluated by the Food and Drug Administration. Flore products are not intended to diagnose, treat, cure, or prevent any disease.
Ozempic, Wegovy and Rybelsus are registered trademarks of Novo Nordisk A/S. Mounjaro and Zepbound are registered trademarks of Eli Lilly and Company. Flore is not affiliated with or endorsed by either company.
Sources
- WEGOVY (semaglutide) injection, Prescribing Information. U.S. Food and Drug Administration, 2024.
- Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity. N Engl J Med. 2022;387(3):205-216. PubMed 35658024
- Gorgojo-Martínez JJ, et al. Clinical recommendations to manage gastrointestinal adverse events in patients treated with GLP-1 receptor agonists: a multidisciplinary expert consensus. J Clin Med. 2022;12(1):145. PubMed 36614945
- Dimidi E, et al. The effect of probiotics on functional constipation in adults: a systematic review and meta-analysis of randomized controlled trials. Am J Clin Nutr. 2014;100(4):1075-1084. PubMed 25099542
- Su GL, et al. AGA clinical practice guidelines on the role of probiotics in the management of gastrointestinal disorders. Gastroenterology. 2020;159(2):697-705. PubMed 32531291