Semaglutide Side Effects: What Passes and What Needs a Call

The best figures come from the branded product. In the weight-management trials summarized in the Wegovy prescribing information, gastrointestinal effects were reported by 73% of people on Wegovy injection against 47% on placebo. Nausea was reported by 44% versus 16%, diarrhea by 30% versus 16%, vomiting by 25% versus 6%. Severe gastrointestinal reactions were reported by 4.1% versus 0.9%. Those are Wegovy trial figures, not a measurement of compounded semaglutide, which has not been studied in trials like these; they are the best published picture of what branded semaglutide does.

Two things those numbers hide. First, placebo groups report a lot of stomach trouble too, so not every symptom is the drug. Second, the rate is front-loaded: for most people the early weeks are the hardest, and a symptom that is getting steadily better is behaving the way it should.

Semaglutide slows how quickly the stomach empties. That is part of how it works, and it is also why a meal that would have been comfortable last month now sits heavy.

What helps is mostly mechanical. Smaller meals, eaten slowly, stopping at the first sign of fullness rather than the plate's end. Rich, fried and very fatty food tends to make it worse. So does lying down soon after eating. Sipping fluids through the day rather than in large amounts at once is easier on a slow stomach.

Nausea that is improving week by week is ordinary. Nausea that is getting worse, that comes with vomiting you cannot control, or that stops you keeping fluids down is not, and needs a call that day, partly for the reason in the kidney section below.

Constipation was reported by 24% of people on Wegovy injection against 11% on placebo in the label's pooled trials. Eating less means less bulk, and a slower gut moves it more slowly.

Fluids, fiber and walking are the first three levers, in roughly that order. Tell your clinician early rather than late: constipation is easier to manage before it settles in, and the Wegovy label's postmarketing section lists severe constipation including fecal impaction, and intestinal obstruction, among reported events. Abdominal pain with a swollen belly, vomiting and no bowel movement or wind is a same-day problem.

Burping is listed in the Wegovy label as more common on the drug than placebo, along with indigestion, reflux and bloating. The rotten-egg taste people describe as "sulfur burps" is widely reported rather than formally measured; the usual explanation is food fermenting in a stomach that is emptying more slowly. People often find it tracks with larger, richer or very high-protein meals. It is unpleasant rather than dangerous, unless it arrives with severe abdominal pain or persistent vomiting.

Pancreatitis. The prescribing information for every semaglutide product warns about acute pancreatitis, including rare severe forms. The sign to act on is persistent, severe abdominal pain, sometimes reaching through to the back, with or without vomiting. It is different from ordinary queasiness in intensity and in that it does not ease. If that describes you, stop and seek urgent care. Tell us before you start if you have ever had pancreatitis.

Gallbladder. Gallstones and gallbladder inflammation were more common on Wegovy than placebo in the trials, and rapid weight loss by any means raises the risk on its own. The warning signs are pain in the upper right abdomen, often after a fatty meal and sometimes reaching the right shoulder blade, fever, and yellowing of the skin or the whites of the eyes. Jaundice, or pain with fever, is an emergency.

A worry that has followed this drug class for years is pancreatic cancer. In September 2026, a registry study from Denmark, Sweden and Norway was presented at the European Association for the Study of Diabetes (EASD) annual meeting. As reported, it matched 97,464 people with type 2 diabetes starting semaglutide against people starting other diabetes medicines, and found no increase in pancreatic cancer (pooled hazard ratio 0.91, confidence interval 0.71 to 1.16).

That is reassuring, and it should be read with its caveats. It is a conference abstract and has not been through peer review. It was funded by Novo Nordisk, the manufacturer, as a regulator-mandated safety study, and several authors work for the company. Average follow-up was under two years, which is short for a cancer outcome. And it studied people with diabetes, not people using semaglutide for weight. We could not find the abstract itself published online, so the figures here are as reported in press coverage of the meeting.

Every semaglutide label carries a boxed warning about thyroid C-cell tumors. In mice and rats, semaglutide caused these tumors. Whether it does so in humans is not known; the labels say the human relevance has not been determined.

What that means in practice: semaglutide is not prescribed to anyone with a personal or family history of medullary thyroid carcinoma, or with Multiple Endocrine Neoplasia syndrome type 2. Tell your clinician about any thyroid cancer in the family. A lump in the neck, trouble swallowing, shortness of breath or hoarseness that does not go away should be reported. Routine calcitonin tests or thyroid ultrasounds are not recommended for screening, because the label notes they can lead to unnecessary procedures.

Semaglutide is not usually a direct cause of kidney injury. The route is dehydration. The labels describe postmarketing reports of acute kidney injury, some needing dialysis, mostly in people who had nausea, vomiting or diarrhea that led to fluid loss.

Eating less often means drinking less too. Keep fluids up deliberately, especially in hot weather or during a stomach bug. If you already have reduced kidney function, or take a diuretic or another medicine that affects the kidneys, your clinician needs to know, because kidney function may need checking if you have significant stomach side effects. Passing much less urine is a same-day call.

Two separate issues come up.

NAION. In June 2025 the European Medicines Agency's safety committee concluded that non-arteritic anterior ischemic optic neuropathy, a condition that damages the optic nerve and can cause vision loss, is a very rare side effect of semaglutide medicines, and that treatment should be stopped if it occurs. The absolute risk is low. Sudden loss of vision or a sudden blurred area in one eye needs same-day medical care, and tell us too.

Diabetic retinopathy. In people with type 2 diabetes, a rapid improvement in blood sugar can temporarily worsen existing diabetic retinopathy. In the Ozempic label's two-year trial, retinopathy complications were more common on Ozempic than placebo, with most of the difference in people who already had retinopathy. If you have diabetes and retinopathy, an eye examination belongs in the plan before you start.

The side effects are the same; what they cost is different.

Dizziness and constipation matter more at 70 than at 50, dizziness because of what a fall costs and constipation because of what it does to comfort and appetite. Dehydration arrives faster in an older adult and is more dangerous alongside a blood-pressure medicine or a diuretic. And anyone taking a sulfonylurea or insulin for diabetes has a higher risk of low blood sugar on semaglutide, which may mean the other medicine needs adjusting by whoever prescribes it.

The Wegovy label also notes that in its cardiovascular trial, people 75 and older reported more hip and pelvis fractures on Wegovy than placebo, and more serious adverse reactions overall. None of this rules semaglutide out after 60. It is why the medication list gets read properly at the start. Semaglutide after 60 goes further.