Ozempic’s most common side effects are gastrointestinal: nausea, vomiting, diarrhea, constipation, and stomach pain. These affect a significant portion of users, especially in the first weeks of treatment, and most improve as your body adjusts. But the side effect profile extends well beyond an upset stomach, and some risks are serious enough to carry an FDA boxed warning.
Why Ozempic Causes Stomach Problems
Ozempic (semaglutide) mimics a gut hormone called GLP-1 that your body naturally releases after eating. One of its key effects is slowing how quickly food leaves your stomach. This delay is actually part of how the drug works: it creates a prolonged feeling of fullness that reduces appetite and helps control blood sugar spikes after meals.
The tradeoff is that food sitting in your stomach longer than usual triggers nausea, bloating, and sometimes vomiting. These are the most frequently reported side effects across clinical trials, and they’re a direct consequence of the drug doing its job. Diarrhea and constipation are also common, as the slower digestive pace affects the entire GI tract.
When Side Effects Start and How Long They Last
GI side effects are most noticeable during two windows: the first few weeks after starting Ozempic and the days following each dose increase. Nausea is typically mild to moderate and tends to fade after the initial dose escalation phase. Most people find that symptoms decrease substantially within the first four weeks at any given dose.
Ozempic’s dosing schedule is specifically designed to reduce this discomfort. You start at 0.25 mg once a week for four weeks, a dose too low to be therapeutic but high enough for your body to begin adapting. At week five, the dose increases to 0.5 mg. Further increases to 1 mg or the maximum 2 mg happen in four-week steps. Eating smaller meals, avoiding high-fat or greasy foods, and eating slowly can also help. If side effects are persistent at a particular dose, your prescriber may hold off on the next increase until they settle down.
Gastroparesis and Severe GI Reactions
In January 2025, the FDA updated the Ozempic label with a new warning about severe gastrointestinal adverse reactions. The label now states that Ozempic is not recommended for patients with severe gastroparesis, a condition where the stomach empties extremely slowly or not at all. Patients are advised to tell their prescriber if they already have problems with slowed stomach emptying before starting the drug.
For most users, the slowdown in gastric emptying is temporary and mild. But in some cases, the effect can become more pronounced, leading to persistent vomiting, severe nausea, or abdominal pain that doesn’t resolve with time. These symptoms warrant a conversation with your prescriber, as they may indicate the drug isn’t a good fit.
Gallbladder Problems
Ozempic increases the risk of gallstones and other gallbladder events. In a meta-analysis of semaglutide trials involving people without diabetes, about 2.3% of participants on semaglutide developed gallstones compared to 0.9% on placebo. Broader gallbladder events (including inflammation and other complications) occurred in 3% of semaglutide users versus 2.2% on placebo.
Rapid weight loss from any cause raises gallstone risk, so it’s difficult to separate the drug’s direct effect from the effect of losing weight quickly. Either way, the practical concern is the same. Symptoms to watch for include sudden pain in the upper right abdomen, pain between the shoulder blades, and nausea or vomiting that feels different from your usual GI side effects.
Pancreatitis
Acute pancreatitis, or sudden inflammation of the pancreas, has been reported in people taking Ozempic. In one weight loss trial, 3 out of 1,306 participants on semaglutide developed acute pancreatitis while none of the 655 people on placebo did. Those numbers are small, and a larger meta-analysis across multiple semaglutide trials did not find a statistically significant increase in risk compared to placebo.
Still, pancreatitis is a serious condition, and the Ozempic label carries a warning about it. Severe abdominal pain that radiates to your back, especially with vomiting, is the hallmark symptom. If you’ve had pancreatitis before, your prescriber will weigh that history carefully before starting you on this drug.
Thyroid Tumor Risk
Ozempic carries the FDA’s most serious warning, a boxed warning, for the risk of thyroid C-cell tumors. In animal studies, semaglutide caused thyroid tumors at doses comparable to what humans take, and the risk increased with higher doses and longer treatment. Whether this translates to humans is still unknown.
Because of this uncertainty, Ozempic is completely off the table for anyone with a personal or family history of medullary thyroid carcinoma or a condition called Multiple Endocrine Neoplasia syndrome type 2. Signs to be aware of include a lump or swelling in the neck, difficulty swallowing, shortness of breath, and persistent hoarseness.
Worsening Diabetic Eye Disease
If you have type 2 diabetes with existing retinopathy (damage to blood vessels in the eye), Ozempic can worsen it. The SUSTAIN-6 trial found that among patients who already had diabetic retinopathy at the start of the study, 8.2% of those on Ozempic experienced complications like bleeding in the eye or worsening vision, compared to 5.2% on placebo. For patients without pre-existing eye disease, the difference was much smaller: 0.7% versus 0.4%.
The mechanism likely relates to how rapidly blood sugar improves. A sudden drop in blood sugar levels can paradoxically stress the small blood vessels in the retina. If you have diabetic retinopathy, more frequent eye exams during the first year of treatment can catch changes early.
Muscle Loss During Weight Loss
One side effect that doesn’t show up on the standard list but matters a great deal is the loss of lean muscle mass. When you lose weight on Ozempic, not all of it comes from fat. Research published in Circulation found that in the STEP 1 trial, participants lost an average of 15.2 kg (about 33.5 pounds), and roughly 45% of that weight came from lean mass rather than fat. A separate study in patients with type 2 diabetes found a similar ratio, with about 43% of weight lost coming from lean tissue.
This isn’t unique to Ozempic. Any significant weight loss, whether from medication, surgery, or dieting, involves some muscle loss. The concern is the scale of it: losing 40% to 45% of total weight from lean tissue can affect strength, metabolism, and bone health, particularly in older adults. Resistance training and adequate protein intake are the most effective ways to preserve muscle while on the medication. Some studies have shown the lean mass fraction dropping to 15% or less of total weight lost, suggesting that individual factors like exercise habits and protein consumption make a real difference.
Less Common but Notable Side Effects
Beyond the major categories, Ozempic can cause a handful of other effects worth knowing about:
- Kidney problems: Severe nausea, vomiting, or diarrhea can lead to dehydration, which in turn can damage the kidneys. This is an indirect effect, but it’s the reason staying hydrated matters, especially during the early adjustment period.
- Injection site reactions: Redness, itching, or mild pain at the injection site occur in some users. Rotating the injection location each week helps.
- Fatigue and dizziness: Reduced calorie intake combined with blood sugar changes can leave some people feeling tired or lightheaded, particularly in the first weeks.
- Hypoglycemia: Ozempic on its own rarely causes dangerously low blood sugar, but the risk goes up if you’re also taking insulin or certain other diabetes medications.
Most people tolerate Ozempic well enough to stay on it long term, and the GI side effects that dominate the first month tend to fade. The more serious risks, including pancreatitis, gallbladder disease, and the thyroid warning, are uncommon but important to recognize early.