The short answer: That rotten-egg smell is hydrogen sulfide gas, and on a GLP-1 it's most likely coming from two things happening at once — food sitting longer in a stomach that's emptying more slowly than usual, and gut bacteria producing more sulfur gas as they work on it. No study has tested this specific symptom in GLP-1 patients directly, so what follows is the most likely explanation built from two things that are each well documented on their own, not a single proven cause.
Free tool: the GLP-1 Clearance Calculator shows how much medication is still active in your system based on your last dose.
Why they happen
GLP-1 medications slow gastric emptying on purpose — it's a documented part of how they work, and it's the same mechanism behind the constipation and bloating that are much more widely studied.1 When food sits in the stomach and upper gut longer than usual, gut bacteria have more time to work on it before it moves on.
Separately, when gut bacteria ferment certain compounds — particularly the sulfur-containing amino acids found in protein-rich foods like eggs, meat, and dairy — one of the byproducts is hydrogen sulfide gas, the same compound responsible for the smell of rotten eggs.2,3 That gas has to go somewhere, and some of it comes back up as a burp instead of continuing through the gut.
Put those two together — slower transit plus a normal bacterial process that produces sulfur gas — and you have a plausible explanation for why this shows up more on a GLP-1. What's missing is a study that actually measured this in people taking these medications, so treat the mechanism as the best current explanation, not an established fact.
Doesn't a fermentable fiber lower hydrogen sulfide? Here's why that's not the answer here
If you've read anything about hydrogen sulfide and gut bacteria, you may have come across research on fermentable fibers — things like inulin, FOS, or resistant starch — that show they can reduce hydrogen sulfide production. That research is real. One lab study measuring gas output from human stool samples found that fermentable fibers cut hydrogen sulfide production by 82–89%, while a minimally fermentable fiber like psyllium reduced it by less than 35% in the same model.3
It's tempting to read that as "switch to a more fermentable fiber." Two things argue against it for someone on a GLP-1:
- That study measured gas in the colon, not burping. Burps are an upper-GI event, tied to the stomach and esophagus. Lower-gut hydrogen sulfide production is a different compartment than what's likely driving this symptom, and nothing in that research measured burping itself.
- Fermentable fiber lowers hydrogen sulfide by increasing total gas, and that trade works against a slowed GLP-1 gut. A controlled trial using MRI found that inulin — a fermentable fiber — produced significantly more colonic gas than psyllium, while psyllium on its own produced very little.4 For someone whose gut is already moving slowly, more total gas usually means more bloating, not less discomfort. I go through that research in full on psyllium vs. inulin for GLP-1 users.
So while the hydrogen-sulfide-reduction data on fermentable fiber is real, it doesn't transfer cleanly to this symptom or this population. Trading a rotten-egg burp for more overall bloating isn't a trade most people on a GLP-1 want to make.
What actually helps
Since there's no dedicated research on this exact symptom, what follows is general practice, not a studied fix. A few things tend to help people manage it:
- Spread protein across the day instead of one large serving. A smaller, steadier supply gives bacteria less to ferment at once.
- Eat slowly and stop before you're overly full. A stomach that's already emptying slowly handles a smaller load better than a large one.
- Stay ahead of hydration. Steady fluid intake supports the whole digestive process, not just stool consistency.
- Support the three levers that are actually studied for GLP-1 digestive symptoms — bulk, water, and motility — rather than chasing this one symptom in isolation. I lay out the full approach in how I'd walk a patient through staying regular on a GLP-1.
When to talk to your doctor
Occasional sulfur burps are usually a nuisance, not a warning sign. Contact your prescriber if they come with severe abdominal pain, persistent vomiting, fever, or signs of dehydration, or if they're accompanied by a sudden change in your usual bowel pattern. Your care team can rule out something else going on and adjust your plan.
Frequently asked questions
Do GLP-1 medications actually cause sulfur burps? No trial has studied this symptom directly in GLP-1 patients. The connection is inferred from two separate, well-documented mechanisms — slowed gastric emptying and bacterial hydrogen sulfide production — not a single proven cause.
Should I switch to a fermentable fiber like inulin to reduce sulfur burps? The research points the other way for this population. Fermentable fibers can lower hydrogen sulfide in lab models, but they do it by increasing total gas output, which tends to mean more bloating in a gut that's already moving slowly.
Will they go away on their own? For many people digestive side effects ease somewhat as the body adjusts, especially between dose increases. For others they persist while on the medication. Both patterns are normal.
Is this the same thing as regular bloating on a GLP-1? Related but not identical — this is specifically about the smell, which points to hydrogen sulfide gas. See why digestive enzymes usually don't help GLP-1 bloating for the broader bloating picture.
Sources
- Gorgojo-Martínez JJ, et al. Gastrointestinal adverse effects of GLP-1 receptor agonists and their management. J Clin Med. 2022. PMID 36614945.
- Yao CK, Sarbagili-Shabat C. Gaseous metabolites as therapeutic targets in ulcerative colitis. World J Gastroenterol. 2023. PMID 36742165.
- Yao CK, et al. Modulation of colonic hydrogen sulfide production by diet and mesalazine utilizing a novel gas-profiling technology. Gut Microbes. 2018. PMID 29561196.
- Gunn D, et al. Psyllium reduces inulin-induced colonic gas production in IBS: MRI and in vitro fermentation studies. Gut. 2022;71(5):919-927. PMC8995815.
Dr. Kayle Martinsen
In clinical practice since 2008, functional-medicine based, working with patients on gut symptoms — gas, bloating, and irregularity.