Sulfur Burps, Decoded: The Rotten-Egg Smell and the GLP-1 Question

Start typing “sulfur burps” into a search box and watch what the autocomplete does. A couple of years ago it finished the phrase with remedies and causes. Now a striking share of the suggestions are drug names — Ozempic, Wegovy, Zepbound, Mounjaro, tirzepatide. The question did not change. The people asking it did.

Which makes this a good moment to sort out what is actually known about a burp that smells like a boiled egg left in a car. Quite a lot, as it turns out — including one study where the smell itself was put to a panel of human judges. And one thing that is not known at all, despite being asserted confidently on roughly every page that ranks for this.

The chemistryOne gas is doing almost all of the work

The rotten-egg note is hydrogen sulfide. It is the same compound that gives that smell its name in every other context, and in the gut it turns up in trace amounts as a normal by-product of bacteria doing their job.

That it is the main culprit is not an assumption; it was tested, in a study that deserves more recognition than it gets. In 1998, researchers at the University of Minnesota fed sixteen healthy volunteers pinto beans and lactulose, collected the resulting flatus through a rectal tube, and had two human judges rate each sample for odour. Hydrogen sulfide was the dominant sulfur gas at around 1.06 micromoles per litre, ahead of methanethiol and dimethyl sulfide. And the judges’ ratings tracked the hydrogen sulfide concentration at a significance of p ≤ 0.001. Somebody really did sit down and score sixteen people’s gas on a scale, and the result is the reason we can say the smell is hydrogen sulfide rather than merely suspect it. It is a small study judged by two noses — but it asked the question directly, which almost nothing else on this topic does.

Burps and flatus are not the same event, and it is worth saying so: the gas in a burp mostly comes from the stomach and upper gut, the gas in flatus from the colon. But the chemistry of the smell is the same molecule, and it is the only part of this that has been measured properly.

The fork in the roadYour gut runs one of two exhaust systems

Here is the finding that reframes the whole topic, and it is nearly forty years old. Hydrogen sulfide in the gut has two main sources: sulfate-reducing bacteria, which take inorganic sulfate and reduce it to sulfide, and a much broader set of microbes that ferment the sulfur-containing amino acids cysteine and methionine out of protein. Reviews of the field put Desulfovibrio at roughly two-thirds of the colon’s sulfate-reducing population.

The interesting part is who carries them. When researchers cultured stool from 87 healthy volunteers, the population split almost cleanly in two. Fifty-seven people carried high counts of sulfate-reducing bacteria and produced no methane. Twenty-one were strong methane producers with no detectable sulfate reducers at all. Only nine had both. The two systems appear to compete for the same raw material, and in most people one of them wins.

That is a 1988 study using culture methods, on a British cohort, and later molecular work has found exceptions — so it is a strong pattern rather than a law. But it is the most satisfying answer anyone has to the question people actually mean when they ask this: why does mine smell like that when my roommate’s does not. Some of it is what you ate. A good deal of it may be which crowd took up residence years ago.

The GLP-1 questionWhat the labels say, and what they do not

Now the part everyone gets wrong, and it is worth getting right because the correction is more reassuring than the claim. Belching is a genuinely documented effect of GLP-1 medications, and the numbers are public.

So the belching is real, it is measured, and it is uncommon rather than universal — seven percent on the highest-dose semaglutide, against under one percent on placebo. That is a meaningful signal and a small absolute number, both at once.

What no label says is that the belches smell of sulfur. The words “sulfur” and “rotten” do not appear in the Wegovy, Ozempic or Zepbound prescribing information at all. The trials counted belches; nobody analysed them. The usual explanation offered online — that these drugs slow the stomach down, so food sits longer, so more fermentation happens, so more sulfide — is mechanistically plausible and lines up with how the drugs work on gastric emptying. It has also never been measured. Not one study has put a GLP-1 user’s burp through a gas analyser. Anyone telling you the mechanism is established is telling you about a hypothesis in a confident voice.

That distinction is the whole point of reading a label. “Belching happens in about seven percent of people on this dose” is a fact with a trial behind it. “This drug gives you sulfur burps” is a widely shared impression that may well be true and has never been checked. Both can be worth knowing. Only one of them is evidence.

The food sideSulfur has to come from somewhere

The raw material is dietary. Reviews of colonic sulfur metabolism identify two supply lines: sulfur-containing amino acids, concentrated in red meat, eggs and milk, and inorganic sulfate, which turns up in commercial breads, dried fruit, nuts, brassica vegetables and fermented drinks. Alliums — garlic, onions — carry their own sulfur compounds, which is why they announce themselves.

How much difference does that make? One controlled-feeding study puts a number on it. Researchers took five healthy men and moved their meat intake from none to 600 grams a day under metabolic-ward conditions, measuring sulfide in stool as they went. It rose from 0.22 to 3.38 millimoles per kilogram — better than a fifteen-fold change — and tracked meat intake significantly. Five men is five men, and 600 grams of meat a day is not a diet so much as a demonstration. But the direction is clear and the mechanism is not mysterious: more sulfur in, more sulfide out.

This is the least glamorous section of the article and probably the most useful one. If the smell is new, the thing that changed is more often the grocery list than the gut.

The newer ideaHydrogen sulfide and the SIBO conversation

There is an emerging line of research worth knowing about, with the caveat that it is emerging rather than settled. A 2022 study of 171 people with IBS found that those with the diarrhoea-predominant type had more hydrogen sulfide on breath testing, more sulfate-reduction activity in their stool, and more of the organisms that produce it — Fusobacterium and Desulfovibrio by name. That work sits behind the commercial breath tests that now report hydrogen sulfide alongside hydrogen and methane.

The limits belong in the same breath. The study had no healthy control group. Its own authors note that hydrogen sulfide “is not discussed in the consensus or guideline” and that thresholds depend on symptom severity — meaning the cutoff a test result is judged against comes from the people who built the test, not from a professional society. The wider framework of small-intestinal bacterial overgrowth and breath testing got a critical appraisal from European and American neurogastroenterology groups in 2024. This is a live research question with a commercial product already attached to it, which is a combination worth reading carefully.

The one people worry aboutWhere giardia actually fits

Somewhere in every thread on this topic, someone says rotten-egg burps mean a parasite. Here is what the source actually says. The CDC’s list of giardiasis symptoms reads: diarrhea; gas; smelly, greasy poop that can float; stomach cramps or pain; upset stomach or nausea; dehydration. Symptoms typically begin one to two weeks after exposure and usually last two to six weeks. The list says “gas.” It does not say sulfur burps. The Merck Manual mentions belching; it does not describe the smell either. The rotten-egg-equals-giardia line is a clinical association passed along through hospital patient pages and GI clinic blogs, not something the CDC has written down.

For scale: 13,829 giardiasis cases were reported to US surveillance in 2022, about 5.2 per 100,000, with the CDC estimating more than 1.1 million actual illnesses a year once unreported cases are accounted for. Cases peak in August, which is a fact about swimming and camping rather than about burps. It is a real and treatable infection, and it comes as a package — the diarrhoea and the greasy floating stool are the parts clinicians recognise it by, not a smell on its own. A burp is not a diagnosis in either direction, and anything persistent belongs with a clinician who can test rather than with a search box.

What actually helpsTwo products, one of which has the evidence

The honest answer here is thinner than the supplement aisle suggests, but it is not empty. Bismuth subsalicylate — the pink stuff — binds sulfide directly, and this has been measured. In the same Minnesota group’s work, combining fecal incubations with ten human subjects, bismuth subsalicylate cut hydrogen sulfide release by more than 95 percent. That is chemistry, not a wellness claim: the bismuth reacts with sulfide and takes it out of circulation. It is also a salicylate, which is why it is a short-term product rather than a daily habit, and worth a word with a pharmacist rather than a shopping decision made at speed.

Simethicone, the other thing usually reached for, works on a different problem entirely. It changes the surface tension of gas bubbles so they merge and pass more easily, and its trial evidence sits in bloating and bowel-preparation studies. No trial shows it reduces odour, and there is no mechanism by which it would — it moves gas around, it does not neutralise sulfide. It may still make you more comfortable. It will not change the smell.

The rest of it is arithmetic. The sulfur comes from food, so a week of paying attention to which meals precede the worst mornings will tell you more than any test will — the same tracking logic behind our smell decoder and the underlying chemistry on the other end of the system. If a GLP-1 medication is in the picture, the belching itself is on the label and your prescriber has seen it before; nothing here is a reason to change a dose or stop a treatment, and that decision belongs with them. And if the smell arrives alongside persistent diarrhoea, weight loss you did not intend, or symptoms that simply do not settle, that combination is worth a clinician’s attention regardless of what it smells like. This article is educational, describes what the cited research measured, and is not medical advice.