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Common side effects of GLP-1-based weight-loss medicines are often gastrointestinal, but their labels also warn about less common, potentially serious problems. What a person may experience depends on the specific medicine, dose, treatment indication and health history; warnings and adverse-event reports do not, by themselves, predict an individual’s risk.
Which side effects are common?
U.S. prescribing information for Wegovy (semaglutide) and Zepbound (tirzepatide) lists gastrointestinal reactions among common adverse reactions. These can include nausea, diarrhea, vomiting, constipation and abdominal symptoms. Zepbound’s label also lists reactions such as abdominal pain and dyspepsia.
These lists describe reactions reported in the clinical programs for particular products. They do not establish one side-effect rate for all GLP-1 medicines, doses or patients, and the two products’ figures should not be treated as directly interchangeable. Both labels also discuss severe gastrointestinal reactions.
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What serious risks do labels and regulators flag?
Gallbladder disease and pancreatitis
U.S. Wegovy and Zepbound prescribing information identifies acute gallbladder disease and pancreatitis as safety considerations. The American Diabetes Association’s 2026 Standards of Care also discusses pancreatitis and biliary disease in relevant treatment contexts. These warnings mean the risks deserve attention; they do not show that every patient has the same likelihood of experiencing them.
On 29 January 2026, the UK Medicines and Healthcare products Regulatory Agency (MHRA) strengthened pancreatitis warnings across GLP-1 and dual GLP-1/GIP agonists. The agency cited rare reports of severe cases, including necrotising and fatal pancreatitis. It cautioned that early symptoms can be hard to distinguish from more common effects: “Pancreatitis may be challenging to recognise in its early stages, as initial symptoms such as abdominal pain, nausea or vomiting may be attributed to other causes such as common gastrointestinal side effects of GLP-1 and GLP-1/GIP treatment or infection.”
Because abdominal pain, nausea and vomiting can have many causes, this overlap is not a basis for self-diagnosis. If symptoms are concerning, contact a health professional for assessment.
Severe gastrointestinal effects and dehydration-related kidney problems
Product information also addresses severe gastrointestinal reactions. Significant vomiting or diarrhea can contribute to dehydration, which can be associated with kidney problems. These are safety concerns described in product information, not a prediction that a particular person will develop them.
Thyroid-tumor warning
FDA material in a 2025 warning letter discusses the boxed warning concerning medullary thyroid carcinoma for tirzepatide and says available data are insufficient to establish or exclude a causal relationship between tirzepatide and that cancer. That statement is a regulatory discussion in a warning letter, not a complete clinical evidence review, so it should not be read as settling the question.
What does a clinical trial tell us—and what doesn’t it?
In the STEP 1 randomized trial of semaglutide, serious adverse events were reported in 9.8% of participants receiving semaglutide and 6.4% receiving placebo. The New England Journal of Medicine’s 2021 report said the difference was mainly due to serious gastrointestinal and hepatobiliary disorders. The trial also reported a small number of mild acute pancreatitis cases.
Those figures apply to the participants and conditions of that particular trial. They are not a lifetime estimate, a rate for every semaglutide user, or a class-wide comparison across weight-loss medicines. Randomized trials can compare outcomes in their study populations, but a result from one trial cannot settle every rare-event or long-term safety question.
Why do reports and warnings not give a personal risk percentage?
Different evidence answers different questions. A product label summarizes information for a specific medicine and its clinical program; a randomized trial compares outcomes in a defined study; a regulator’s warning communicates a safety concern; and a spontaneous adverse-event report records an event reported after treatment. These sources should not be collapsed into a single estimate of how likely an individual is to have a side effect.
For example, the FDA reported 990 adverse-event reports associated with compounded semaglutide and more than 730 associated with compounded tirzepatide as of 31 May 2026. These are report counts, not incidence rates: they do not show how many people used each product or establish that the medicine caused every reported event. The FDA says causality is not always determinable and reports may be underreported.
The FDA has separately described reports that may be related to dosing errors involving compounded injectable semaglutide, including cases requiring hospitalization. Those reports concern compounded products and administration errors; they should not be presented as controlled-trial results for approved products.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Do safety findings differ between semaglutide and tirzepatide?
Some evidence does not point in exactly the same direction for every medicine, but that does not establish a simple safety ranking. A 2025 systematic review and meta-analysis reported increased gallbladder-related disorders, particularly cholelithiasis, in its semaglutide analysis. Its tirzepatide analysis did not find a statistically significant biliary risk. A result that is not statistically significant is not proof of no risk, and findings from a review apply to the studies it included—not automatically to every dose, indication or patient group.
Semaglutide and tirzepatide findings should therefore be interpreted alongside the product, dose, indication, population and type of evidence involved. The available evidence does not justify a universal ranking of their overall safety.
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What remains uncertain?
Rare events and longer-term safety questions are not fully resolved by the evidence summarized here. A warning identifies a concern that deserves attention, but is not itself a measure of personal risk. Trial results apply to the studied population and period, while spontaneous reports can signal possible problems without proving causation or showing how often they occur.
For individual treatment decisions, discuss your health history, symptoms and other medicines with a qualified clinician, and consult the current prescribing information for the specific product. The U.S. product labels and the UK MHRA update apply in their respective regulatory contexts; recommendations and approved product information can differ by country.
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