The most common GLP-1 side effects are gastrointestinal
Across the large placebo-controlled trials, the leading side effects of GLP-1 and dual GLP-1/GIP medications are stomach-related: nausea, vomiting, diarrhea, and constipation. The FDA labels report that gastrointestinal (GI) reactions occurred in about 73% of adults on semaglutide 2.4 mg (Wegovy) versus 47% on placebo, and in about 56% of adults on tirzepatide (Zepbound) versus 30% on placebo.
Importantly, the label language is consistent across trials: most of these GI events were mild or moderate, of short duration, and did not lead people to stop the medication. Severe GI reactions were uncommon — reported in about 4.1% of Wegovy-treated adults (vs 0.9% on placebo) and around 1.7-5% on tirzepatide depending on dose.
Because semaglutide (the molecule in Ozempic and Wegovy) and tirzepatide (in Mounjaro and Zepbound) are used at different doses for diabetes versus weight loss, exact percentages vary by product and dose. The ranges below come directly from the FDA prescribing information for the weight-loss doses, which are the highest and therefore a reasonable upper bound.
Side-effect frequency table (from the FDA labels)
The table below shows adverse-reaction rates reported in the pivotal weight-management trials for semaglutide 2.4 mg (Wegovy, N=2,116) and tirzepatide (Zepbound, pooled 5/10/15 mg, N=2,519), each compared with placebo. Percentages are rounded as published in the FDA labels. Tirzepatide figures are shown as a range across the three doses.
This is class-level education, not a head-to-head efficacy or safety ranking — trial populations and designs differ, so the two columns are not a direct apples-to-apples comparison.
How side effects change over titration
GLP-1 medications are started low and increased slowly on purpose — the escalation schedule exists largely to reduce GI side effects. Wegovy, for example, is initiated at 0.25 mg once weekly for 4 weeks and then stepped up roughly every 4 weeks toward a maintenance dose (FDA label). Tirzepatide follows a similar stepwise increase.
Both FDA labels state plainly that GI reactions were most frequent during dose escalation and decreased over time. The Wegovy Canadian product monograph quantifies typical durations in treated adults: median nausea about 8 days, vomiting about 2 days, and diarrhea about 3 days. Constipation is the outlier — it tended to last longer, with a median duration around 47 days.
This pattern is why the first days after each dose increase are usually when symptoms are worst, and why many people find the maintenance phase more tolerable than the ramp-up. How an individual should handle a rough titration — slowing the increase, staying at a dose, or stopping — is a decision for the prescribing clinician, not something to self-manage.
- Symptoms typically flare in the days after a dose step-up, then settle.
- Nausea, vomiting, and diarrhea are usually short-lived; constipation can linger.
- Discontinuation due to GI side effects was relatively low: ~4.3% on Wegovy and ~1.9-4.3% on Zepbound (by dose) vs ~0.5-0.7% on placebo.
- Dehydration from vomiting or diarrhea can, in rare cases, contribute to acute kidney injury — replacing fluids matters, and persistent vomiting is worth a call to a clinician.
Serious but uncommon risks: pancreatitis, gallbladder, and the thyroid boxed warning
Every FDA-approved GLP-1 product for weight or diabetes carries a boxed warning about thyroid C-cell tumors. In rodents, semaglutide and tirzepatide caused dose- and duration-dependent thyroid C-cell tumors at clinically relevant exposures. It is unknown whether these drugs cause such tumors, including medullary thyroid carcinoma (MTC), in humans — the human relevance has not been determined. Because of this, they are contraindicated in anyone with a personal or family history of MTC or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2). Warning signs the labels tell patients to report include a neck mass, trouble swallowing, shortness of breath, or persistent hoarseness.
Acute pancreatitis — including hemorrhagic or necrotizing forms, which can be fatal — has been observed with GLP-1 receptor agonists. The classic symptom the labels describe is persistent, severe abdominal pain, sometimes radiating to the back, which may or may not come with nausea or vomiting. If pancreatitis is suspected, the medication is stopped and the person is evaluated.
Gallbladder disease is more common than pancreatitis. In the Wegovy adult weight-loss trials, gallstones (cholelithiasis) were reported in 1.6% on the drug vs 0.7% on placebo, and gallbladder inflammation (cholecystitis) in 0.6% vs 0.2%. Rapid weight loss itself raises gallstone risk, but the labels note the excess occurred even after accounting for how much weight people lost.
Other labeled risks include acute kidney injury (often from dehydration due to vomiting/diarrhea), serious hypersensitivity reactions (anaphylaxis, angioedema), and — from postmarketing reports — ileus and intestinal obstruction. The labels also warn about pulmonary aspiration during general anesthesia or deep sedation, which is why patients are told to inform their surgical and anesthesia teams before any procedure.
The vision (NAION) signal: what is and isn't known
Since 2024, researchers have been investigating a possible link between semaglutide and nonarteritic anterior ischemic optic neuropathy (NAION) — a sudden, usually painless loss of vision in one eye caused by reduced blood flow to the optic nerve. The evidence is mixed and still evolving.
A widely cited 2024 study from Massachusetts Eye and Ear (Hathaway et al., JAMA Ophthalmology) found higher NAION risk with semaglutide, but it drew from a specialized neuro-ophthalmology clinic, not the general population, so its cumulative-incidence figures (e.g., 8.9% vs 1.8% in diabetes over 36 months) are not population-wide rates. Larger analyses since then report a smaller, but still detectable, signal: a 37-million-patient OHDSI study (JAMA Ophthalmology, 2025) estimated a NAION incidence around 14.5 per 100,000 person-years among semaglutide users, and a 2026 U.S. veterans cohort (JAMA Ophthalmology) found roughly a 2-fold relative increase with a low absolute risk — about 0.3% over ~2 years vs 0.1% on a comparator, or roughly one extra case per several thousand treated patients.
The consistent takeaway across these studies: any absolute risk appears low, causality is not established, and experts advise weighing it against the medications' substantial cardiometabolic benefits. The practical guidance researchers offer is to seek prompt evaluation for any new vision change and to tell your eye doctor if you take one of these drugs.
Red flags: when to contact a clinician
Most GLP-1 side effects are the manageable GI kind. A smaller set of symptoms, drawn directly from the FDA warnings sections, are reasons to seek medical care rather than wait them out. This list is educational — it does not replace your clinician's instructions, and you should never start, stop, or change a dose on your own.
- Severe, persistent stomach pain, especially if it radiates to your back (possible pancreatitis).
- Right-upper-abdomen pain, fever, yellowing of the skin/eyes, or clay-colored stools (possible gallbladder disease).
- Sudden vision changes or loss in one eye (the NAION signal under study).
- A lump in the neck, trouble swallowing, shortness of breath, or a persistently hoarse voice (thyroid-tumor warning signs).
- Swelling of the face, lips, tongue, or throat; trouble breathing; or widespread hives (possible serious allergic reaction).
- Ongoing vomiting or diarrhea with signs of dehydration — dizziness, very dark or reduced urination (possible kidney injury).
- Symptoms of low blood sugar (shakiness, sweating, confusion) — relevant mainly if you also take insulin or a sulfonylurea.
- Any planned surgery or procedure requiring sedation — tell your care team you take a GLP-1, because of aspiration risk.
How compounded GLP-1s and supplements fit in
Compounded semaglutide and tirzepatide are not FDA-approved finished drugs. They are prepared by compounding pharmacies and do not carry the same FDA review of manufacturing, dosing, and labeling as brand products. The pharmacology is expected to be similar, so the same side-effect categories apply, but the exact dose, purity, and quality can vary — which is a meaningful consideration when weighing tolerability and safety. Compounded products should be discussed with a licensed clinician.
Supplements marketed as "natural Ozempic" or GLP-1 "boosters" (berberine, various fibers, herbal blends) are a separate category entirely. They are not FDA-approved to treat obesity, are not equivalent to prescription GLP-1 medications, and the FDA does not evaluate dietary supplements for efficacy before they are sold. A supplement is not a substitute for a prescribed GLP-1, and it can still cause its own side effects or interactions.
The bottom line
For most people, GLP-1 side effects are predictable and manageable: GI symptoms that peak during dose increases and ease over weeks. The rare, serious risks — pancreatitis, gallbladder disease, the thyroid boxed warning, and the still-uncertain vision signal — are worth understanding precisely so you know the red flags, not so you avoid an appropriate treatment out of fear.
Every number on this page comes from an FDA label or a peer-reviewed study, and every one describes populations, not you specifically. Whether a GLP-1 is right for you, how to titrate, and how to respond to a side effect are decisions for a licensed clinician who knows your history.