GLP-1 side effects

Most common: stomach and bowel effects

Stomach and bowel effects are the most common side effects across the GLP-1 class.³² The table shows the main weight-reduction trials for the two weekly injections approved for weight reduction, each against placebo.

In the two pooled Zepbound weight trials, 56% reported a stomach or bowel effect, against 30% on placebo.² Severe reactions of this kind affected 1.7% to 3.1%, against 1%.² The labels warn that vomiting and diarrhea can lead to dehydration.³ They do not recommend these drugs for people with severe gastroparesis, a condition of very slow stomach emptying.³

Retatrutide has no label because it is not approved.¹¹ TRIUMPH-2 was a phase 3 trial in 1,152 adults with obesity and type 2 diabetes.⁵ Nausea affected 14% to 28% across its 4, 9 and 12 mg doses, against 8% on placebo.⁵ Diarrhea affected 27% to 34%, against 13%.⁵

Other common effects

• Headache and fatigue: 14% and 11% with Wegovy 2.4 mg, against 10% and 5% on placebo.¹
• Injection site reactions: 6% to 8% with Zepbound, against 2% on placebo.²
• Hair loss: 3.3% with Wegovy 2.4 mg and 5.8% with 7.2 mg, against 1% on placebo.¹ On Zepbound it was reported by 7.1% of women and 0.5% of men.²
• Burping, reflux and dizziness: 4% to 5% each with Zepbound, against 1% to 2% on placebo.²
• Unusual skin sensations (dysesthesia): 22% with Wegovy 7.2 mg, against 6% with 2.4 mg and 0.3% on placebo.¹ In TRIUMPH-2 they affected 4% to 7% on retatrutide, against 1%.⁵
• Faster heart rate: an average rise of 1 to 4 beats per minute with Wegovy and 1 to 3 with Zepbound.¹²
• Low blood pressure: 1.6% with Zepbound, against 0.1% on placebo.² In TRIUMPH-2 it reached 6% on retatrutide 12 mg, against under 1%.⁵

Serious but less common

• Acute pancreatitis. Wegovy trials recorded 0.2 cases per 100 patient-years.¹ Reports after approval include necrotizing pancreatitis, sometimes fatal.¹ The labels say to stop the drug if pancreatitis is suspected.³²
• Gallbladder disease. Gallstones affected 1.6% on Wegovy against 0.7% on placebo, and gallbladder inflammation 0.6% against 0.2%.¹
• Kidney injury. Fluid loss from vomiting or diarrhea can cause acute kidney injury, and some cases reported after approval needed dialysis.⁷ In Zepbound weight trials it occurred in 0.5%, against 0.2% on placebo.²
• Low blood sugar. The risk rises when a GLP-1 drug is combined with insulin or a sulfonylurea.³ In a Zepbound trial in adults with type 2 diabetes, 4.2% had low blood sugar, against 1.3% on placebo.²
• Diabetic eye disease. In a 2-year Ozempic trial, retinopathy complications occurred in 3.0% against 1.8% on placebo.³ Rapid improvement in blood sugar can temporarily worsen diabetic retinopathy.³
• Serious allergic reactions. Anaphylaxis and angioedema have been reported after approval.³
• Bowel blockage. Reports after approval include ileus, intestinal obstruction and severe constipation with fecal impaction.¹
• Aspiration under anesthesia. Rare cases of stomach contents entering the lungs have been reported during general anesthesia or deep sedation, even after normal fasting.³ The labels tell patients to inform providers before any planned procedure.³
• NAION. This is an eye condition that may cause loss of vision.⁸ The European Medicines Agency lists it as a very rare side effect of semaglutide, affecting up to 1 in 10,000 people taking it.⁸

Warnings on the labels

The boxed warning on these labels covers thyroid C-cell tumors.³⁷ In rodents, semaglutide and tirzepatide caused these tumors at exposures relevant to human use.³⁷ Whether they cause them in people, including medullary thyroid carcinoma, is unknown.³⁷ The labels say not to use these drugs with a personal or family history of medullary thyroid carcinoma or with Multiple Endocrine Neoplasia syndrome type 2 (MEN 2).³ Signs to report include a lump in the neck, trouble swallowing, shortness of breath or lasting hoarseness.⁶ The orforglipron tablet carries the same warning.⁶ Its label notes, though, that the drug is not active in rats or mice and caused no tumors in them.⁶

In January 2026, FDA asked makers to remove the suicidal behavior and ideation warning from the Saxenda, Wegovy and Zepbound labels.⁴ Its review had found no increased risk.⁴

How many people stop because of side effects

• Wegovy trials: 6.8% stopped because of adverse reactions, against 3.2% on placebo.¹
• SELECT, a semaglutide heart trial followed for a mean of 39.8 months: 16.6% stopped for adverse events, against 8.2%.⁹
• Zepbound weight trials: 4.8% to 6.7%, against 3.4%.²
• TRIUMPH-2 (retatrutide): 4% to 12% across doses stopped for adverse events or death, against 5%.⁵

Pregnancy, birth control and combinations

• Semaglutide may cause fetal harm.¹ The Wegovy label says to stop it at least 2 months before a planned pregnancy, because of its long half-life.¹
• The Zepbound label says weight loss offers no benefit during pregnancy and that the drug should be stopped once pregnancy is recognized.²
• Tirzepatide may make birth control pills less effective.⁷ Its label advises a non-oral method, or an added barrier method, for 4 weeks after starting and after each dose increase.⁷
• The weight-reduction labels say not to combine the drug with another GLP-1 receptor agonist.¹²
• Pens must never be shared between people, even with a new needle.³

Problems reported with unapproved products

FDA has received reports of adverse events, some needing hospital care, after dosing errors with compounded semaglutide from multiple-dose vials.¹⁰ Most reports described people drawing up more than the prescribed dose, 5 to 20 times the intended amount.¹⁰ FDA says confusion between milliliters, milligrams and 'units' may have contributed.¹⁰ Reported effects included nausea, vomiting, abdominal pain, fainting, headache, dehydration, acute pancreatitis and gallstones.¹⁰ As of May 31, 2026, FDA had 990 adverse event reports linked to compounded semaglutide and more than 730 linked to compounded tirzepatide.¹¹ It says these are likely underreported.¹¹

Read next

Go back to What is GLP-1?, or read the class-wide companions GLP-1 benefits and uses and GLP-1 dosage. Compound and brand detail is on semaglutide side effects, tirzepatide side effects, retatrutide side effects, Wegovy side effects and Zepbound side effects.