# GLP-1 Drugs and the Gut Microbiome: What 38 Studies Could Not Agree On

By Nora Bissett, Pricing Editor (https://glpwatchdog.org/authors/nora-bissett) | Published: 2026-08-21 | Updated: 2026-10-27

Canonical page: https://glpwatchdog.org/research/glp1-and-the-gut-microbiome

> A systematic review of 38 studies found no single direction of effect. For semaglutide a beneficial bacterium rose while microbial diversity fell, and in the one randomized human study the bacterial shifts were not associated with glucose control at all.

## Key points

- A PRISMA systematic review of 38 studies reported different directions of effect for liraglutide, exenatide, dulaglutide and semaglutide.
- For semaglutide, A. muciniphila increased while overall microbial diversity decreased, so the two markers point opposite ways.
- The one randomized human trial had 25 participants, 11 of them on liraglutide.
- In that trial, shifts in bacterial taxa were NOT associated with short-chain fatty acids, HbA1c or glucose.

GLP-1 drugs change the bacteria in your gut. That much is agreed. What is not agreed is *how*. A systematic review of 38 studies found the direction of the change differs by drug, differs between animals and people, and in the case of semaglutide points two ways at once: a bacterium associated with better metabolic function increases while overall microbial diversity falls.[1] There is no clinical action to take from any of this yet, though plenty of people will try to sell you one.

## What the systematic review found

The review followed PRISMA and gathered every study it could find on how GLP-1 analogs affect the composition, richness and diversity of gut microbiota, in animal and human models. Thirty-eight studies met the bar. The conclusions split by drug rather than converging.[1]

*Reported direction of effect, by drug, across 38 studies.[1]*

| Drug | What was reported |
| --- | --- |
| Liraglutide | Promotes growth of genera relevant to beneficial metabolic functions |
| Exenatide / exendin-4 | Animal studies: genera linked to improved metabolism. **Human studies: genera linked to better metabolism AND to escalated inflammation both increased** |
| Dulaglutide | Significant increases in *Bacteroides*, *Akkermansia* and *Ruminococcus* |
| Semaglutide | *A. muciniphila* increased — but microbial **diversity decreased** |

The semaglutide row shows the problem. [*Akkermansia muciniphila*](https://glpwatchdog.org/research/the-supplement-that-missed-its-endpoint) is the genus most often cited as a marker of metabolic health, and it went up. Diversity is the other measure most often cited as a marker of gut health, and it went down. A story that uses only the first half is available to anyone selling something.

> The review’s own explanation for the disagreement is mundane and important: results varied because the populations studied were dissimilar and the studies ran for different lengths of time.[1] That says more about how thin the evidence still is than about the bacteria.

## The randomized human data is very small

The MIGHTY study is a three-month parallel clinical trial in African American youth with type 2 diabetes, randomized to metformin alone (14 people) or metformin plus liraglutide (11 people). Twenty-five participants in total, eleven of them on the GLP-1. Any sentence drawn from it carries those numbers.[2]

- Metformin alone was associated with more *Eubacterium* and *Eubacterium rectale*, and less *Bacteroides ovatus*.
- Metformin plus liraglutide was associated with more *Bacteroides fragilis* and less *Streptococcus thermophilus*.
- Secondary bile acid metabolites rose in the metformin group, and one of them correlated with lower fasting glucose.
- **Shifts in microbiota taxa were not associated with plasma short-chain fatty acids, HbA1c or glucose.**

That last finding cuts against the popular claim. The bacteria moved, but the glucose control they are supposed to explain did not move with them. The authors suggest bile acid pathways may be important for blood sugar, but they drew that from the metformin group, where the bile acid rise was seen, not from liraglutide.[2]

## What you can actually do with this

Very little. No microbiome test sold to consumers can tell you whether your response to a GLP-1 is good or bad, because nobody has established what a good response looks like. No probiotic has been shown to improve outcomes on these drugs, and the products sold as [GLP-1 supplements](https://glpwatchdog.org/research/glp1-supplements-do-they-work) have not shown much on their own either; the [branded probiotic boosters](https://glpwatchdog.org/research/glp1-booster-supplements-by-brand) have no published trials of their own. The gastrointestinal side effects people do experience (nausea, constipation, reflux) have explanations in gastric emptying that do not require the microbiome at all.

If your interest here is symptoms rather than science, our pages on [constipation](https://glpwatchdog.org/research/glp1-constipation-what-works) and [burping and reflux](https://glpwatchdog.org/research/glp1-burping-reflux-and-gas) deal with what is treatable. If it is nutrition, [eating too little on a GLP-1](https://glpwatchdog.org/research/eating-too-little-on-a-glp1) covers what does run low.

## Frequently asked questions

### Do GLP-1 drugs change your gut bacteria?

Yes, but not in one consistent direction. A systematic review of 38 studies found liraglutide, exenatide, dulaglutide and semaglutide each associated with different shifts, and human results sometimes differed from animal ones. For semaglutide, a beneficial bacterium increased while overall microbial diversity decreased.

### Is that why the drug works?

There is no good evidence for that. In the one randomized human study here, shifts in bacterial taxa were not associated with short-chain fatty acids, HbA1c or glucose. Its authors suggested bile acid pathways may matter for blood sugar, but that suggestion was about the metformin group, not the GLP-1.

### Should I take a probiotic while on a GLP-1?

No probiotic has been shown to improve outcomes on these drugs. Since nobody has established what a beneficial microbiome shift on a GLP-1 even looks like, there is nothing for a probiotic to be aiming at.

### Is a microbiome test worth doing?

Not for this purpose. A test can describe which organisms are present; it cannot tell you whether that pattern means your treatment is working, because that link has not been established in this population.

## References

1. Gofron KK, Wasilewski A, Małgorzewicz S. Effects of GLP-1 Analogues and Agonists on the Gut Microbiota: A Systematic Review. Nutrients. 2025. PMID: 40284168. https://pubmed.ncbi.nlm.nih.gov/40284168/
2. Glaros SB, Mishra SP, Jain S, et al. Systemic and gut microbiome changes with metformin and liraglutide in youth-onset type 2 diabetes: the MIGHTY study. Gut Microbes. 2025. PMID: 41020378. https://pubmed.ncbi.nlm.nih.gov/41020378/

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Cite as: GLP Watchdog, Nora Bissett, "GLP-1 Drugs and the Gut Microbiome: What 38 Studies Could Not Agree On", published 2026-08-21, updated 2026-10-27. https://glpwatchdog.org/research/glp1-and-the-gut-microbiome

Canonical HTML page: https://glpwatchdog.org/research/glp1-and-the-gut-microbiome

Medical disclaimer: Nothing here is medical advice. We report on this subject; we do not treat anyone. Whether to begin a medication, supplement, or treatment, whether to adjust it, and whether to stop are questions for a clinician licensed to answer them, who knows your history.