GLP-1 Side Effects: How Common They Are and When to Call a Clinician

Last reviewed: 19 September 2026

What are the most common GLP-1 side effects?

Nausea, diarrhea, vomiting and constipation, in that order, on every label. In Wegovy’s adult weight trials the rates on the 2.4 mg injection were nausea 44%, diarrhea 30%, vomiting 24%, constipation 24% and abdominal pain 20%, against 16%, 16%, 6%, 11% and 10% on placebo. Headache (14%), fatigue (11%) and dyspepsia (9%) followed.

Zepbound at 15 mg showed nausea 28%, diarrhea 23%, vomiting 13%, constipation 11% and abdominal pain 10%, with injection site reactions in 8% and fatigue in 7%. Foundayo at 17.2 mg showed nausea 35%, diarrhea 25%, constipation 24% and vomiting 24%.

Read the placebo column too. Sixteen percent of people taking a placebo injection in Wegovy’s trials reported nausea, so the share attributable to the drug is the gap between the two numbers, not the headline figure.

How long do GLP-1 side effects last?

They cluster in the dose-escalation months and ease afterwards. Zepbound’s label says “the majority of nausea, vomiting, and/or diarrhea events occurred during dose escalation and decreased over time”, and Foundayo’s says the incidence “was higher during the FOUNDAYO dosage escalation period and decreased over time”.

That is why every label raises the dose in steps over three to five months, and why Wegovy’s says that if you do not tolerate a dose, your prescriber can delay the next increase by 4 weeks. How long it takes to reach the full dose lists each schedule.

Most reactions were not severe. Of Foundayo-treated patients who reported gastrointestinal reactions, 60% rated them mild, 36% moderate and 4% severe. Severe gastrointestinal reactions were reported by 4.1% of adults on Wegovy injection against 0.9% on placebo.

How many people stop a GLP-1 because of side effects?

Between about 5% and 10% in the label trials, roughly two to three times the placebo rate. Wegovy: 6.8% on the 2.4 mg injection against 3.2% on placebo. Zepbound: 4.8%, 6.3% and 6.7% at 5, 10 and 15 mg against 3.4%. Foundayo: 6%, 9% and 10% at 5.5, 9 and 17.2 mg against 3%.

Gastrointestinal reactions drove most of it: they alone caused 1.9% to 4.3% of Zepbound patients and 3% to 6% of Foundayo patients to stop, against 0.5% and 0.7% on placebo.

What is the boxed warning about thyroid tumors?

Every GLP-1 weight-loss label opens with it. Wegovy’s reads: “In rodents, semaglutide causes thyroid C-cell tumors at clinically relevant exposures. It is unknown whether WEGOVY causes thyroid C-cell tumors, including medullary thyroid carcinoma (MTC), in humans.”

The practical consequence is a contraindication: no one with a personal or family history of MTC, or with Multiple Endocrine Neoplasia syndrome type 2, should take these drugs. The labels tell prescribers to counsel patients to report “a lump in the neck, hoarseness, dysphagia, or dyspnea”. What medical history rules you out covers the other exclusions.

Can GLP-1s cause pancreatitis or gallbladder problems?

Both are listed warnings, and both were uncommon in the trials. Acute pancreatitis was confirmed in 4 Wegovy-treated adults, or 0.2 cases per 100 patient-years, against 1 on placebo. In Zepbound’s weight trials it was confirmed in 0.2% of treated patients and 0.2% of placebo patients. The symptom to know is “severe abdominal pain that sometimes radiates to the back”, with or without vomiting; the label says to stop the drug promptly and contact your physician.

Gallstones were reported by 1.6% of adults on Wegovy injection against 0.7% on placebo, and gallbladder inflammation by 0.6% against 0.2%. For Zepbound the figures were 1.1% against 1%, and 0.7% against 0.2%. The Wegovy label notes that “substantial or rapid weight loss can increase the risk of gallbladder disease”, but that it can also occur without it.

Can GLP-1s affect your kidneys?

Indirectly, through dehydration. Acute kidney injury occurred in 7 Wegovy-treated adults in the weight trials, 0.4 cases per 100 patient-years, against 4 on placebo at 0.2, and the label says some cases occurred “in association with gastrointestinal adverse reactions or dehydration”. Its patient information says: “It is important for you to drink fluids to help reduce your chance of dehydration.”

If vomiting or diarrhea stops you keeping fluids down, that is a reason to contact your clinician the same day, not to wait for it to pass.

Do GLP-1s cause low blood sugar?

Rarely on their own, more often alongside diabetes medication. In Zepbound’s trial of patients with type 2 diabetes, blood glucose below 54 mg/dL was reported in 4.2% of treated patients against 1.3% on placebo. In Foundayo’s, it was 2% against 0.2%, and 7% among those also taking a sulfonylurea against 0.5% of those who were not. In Foundayo’s trial of adults without diabetes it was 0.6%.

Tell the prescriber if you take insulin or a sulfonylurea. The Wegovy label says “reducing the dose of insulin or insulin secretagogue may be necessary”.

Do GLP-1s cause hair loss?

Hair loss appears in all three adverse reaction tables at low rates: 3% on Wegovy injection against 1% on placebo, 4% to 5% on Zepbound against 1%, and 4% to 5% on Foundayo against 2%. In Wegovy’s pediatric trial it was 4% against 0%. The labels do not state a cause.

When should you contact a clinician or get urgent help?

The labels’ patient counseling sections name the symptoms that should not wait:

• Severe abdominal pain, especially if it spreads to your back, with or without vomiting: stop the medication and contact your physician, because it can signal pancreatitis.
• Pain in your upper stomach, yellowing of the skin or eyes, fever, or clay-colored stools: call your healthcare provider. The Wegovy medication guide lists these as gallbladder symptoms and notes that some gallbladder problems need surgery.
• Severe or persistent vomiting, diarrhea or constipation: contact your provider, and sooner if you cannot keep fluids down.
• Swelling of your face, lips, tongue or throat, severe rash or itching, a very rapid heartbeat, problems breathing or swallowing, or fainting: stop the medication and get medical help right away. These are the label’s signs of a serious allergic reaction.
• Changes in vision, if you have type 2 diabetes: tell your healthcare provider. Diabetic retinopathy complications are a listed warning.
• A lump in the neck, hoarseness, or trouble swallowing or breathing: report it, per the boxed warning.
• Pregnancy: tell your prescriber. Both the Wegovy and Zepbound labels say to stop when pregnancy is recognized.

In an emergency, call 911. For everything else, use the channel your program gives you, and check before enrolling that it has one. Sunlight lists unlimited telehealth visits, MEDVi lists care coaching as included, and JRNYS prices its membership by coaching level. What happens in an online GLP-1 consultation explains how follow-up works.

These figures come from FDA-approved products. Compounded GLP-1s are not FDA-approved and have no adverse reaction table of their own; the FDA has reported overdoses of five to 20 times the intended dose with compounded injectable semaglutide. See FDA-reported problems with compounded GLP-1s and compounded vs brand-name GLP-1s. The DoIQualify quiz asks about the conditions above and shows whether you may meet common criteria; a result is not an approval. The compare table lists each program’s clinician access.

GLP-1 side effects FAQ

This article is informational and not medical advice. Contact your clinician or emergency services for urgent symptoms. Medical disclaimer