GLP-1 Side Effects: How Common, How Long, What's Serious

The most common GLP-1 side effects are digestive: nausea, diarrhea, vomiting, constipation and stomach pain. In the Zepbound weight-loss trials, 56% of people on the drug had a gastrointestinal side effect, against 30% on placebo, and the label says most nausea, vomiting and diarrhea happened while the dose was being raised and decreased over time.

Most side effects are mild to moderate and fade as your body adjusts to each dose. A short list of rarer problems, such as pancreatitis, gallbladder disease and kidney injury from dehydration, needs a call to your doctor and is covered below. The pages linked above go deeper on each symptom.

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What are the most common GLP-1 side effects?

Each drug's US prescribing information has a table of common adverse reactions. Below are the figures for the four most-used weekly injections, copied from those tables, with the placebo rate under each number. Ranges cover the doses the label lists.

Side effect	Zepboundtirzepatide	Mounjarotirzepatide	Wegovysemaglutide	Ozempicsemaglutide
Nausea	25–29%placebo 8%	12–18%placebo 4%	44%placebo 16%	15.8–20.3%placebo 6.1%
Diarrhea	19–23%placebo 8%	12–17%placebo 9%	30%placebo 16%	8.5–8.8%placebo 1.9%
Vomiting	8–13%placebo 2%	5–9%placebo 2%	24%placebo 6%	5–9.2%placebo 2.3%
Constipation	11–17%placebo 5%	6–7%placebo 1%	24%placebo 11%	3.1–5%placebo 1.5%
Abdominal pain	9–10%placebo 5%	5–6%placebo 4%	20%placebo 10%	5.7–7.3%placebo 4.6%
Indigestion (dyspepsia)	9–10%placebo 4%	5–8%placebo 3%	9%placebo 3%	Not in table
Burping (eructation)	4–5%placebo 1%	Not in table	7%placebo <1%	Not in table
Fatigue	5–7%placebo 3%	Not in table	11%placebo 5%	Not in table
Headache	Not in table	Not in table	14%placebo 10%	Not in table
Dizziness	4–5%placebo 2%	Not in table	8%placebo 4%	Not in table
Hair loss	4–5%placebo 1%	Not in table	3%placebo 1%	Not in table
Injection site reactions	6–8%placebo 2%	Not in table	Not in table	Not in table

Three things to know when you read this table:

• The groups are different. Zepbound and Wegovy were tested in people with obesity or overweight, and Mounjaro and Ozempic in people with type 2 diabetes. Every label also states that rates from one drug's trials "cannot be directly compared" to rates from another drug's trials.
• "Not in table" does not mean "never happens." Some effects are described in the label's text instead of its table. For example, injection site reactions were reported in 3.2% of people on Mounjaro (0.4% on placebo), 1.4% on Wegovy (1% on placebo) and 0.2% on Ozempic.
• Side effects often come together. Many people get nausea and diarrhea in the same week, or swing between diarrhea and constipation.

Outside the gut, the labels list fatigue, dizziness, headache (Wegovy's table), hair loss and burping. The Zepbound label notes that hair loss was associated with weight reduction and was reported more often in women.

Why do GLP-1 drugs cause side effects?

Semaglutide (Wegovy, Ozempic) and tirzepatide (Zepbound, Mounjaro) slow how fast food leaves your stomach. Both labels say so. Food staying in the stomach longer is thought to explain much of the fullness, nausea and burping, especially after large or fatty meals.

The Zepbound label adds a useful detail: the delay in stomach emptying is largest after the first dose and becomes smaller over time. This fits the usual pattern in which side effects are worst early and after each dose increase.

Eating much less has effects of its own. Low fluid intake makes constipation and dizziness more likely, and losing weight fast is linked to hair shedding and gallstones. Low protein makes it harder to protect muscle.

GLP-1 side effects timeline: when do they start and how long do they last?

There is no exact clock, but the labels and trials show a clear pattern:

1. First weeks on the starting dose. Side effects often start here. The starting doses are low and are not meant as treatment doses. Their job is to let your gut adjust.
2. After each dose increase. Every step up can bring symptoms back for a while. The Zepbound and Mounjaro labels say most nausea, vomiting and diarrhea happened during dose escalation and decreased over time. The Wegovy label says digestive reactions were reported most often during dose escalation.
3. On a stable dose. For most people symptoms become milder or stop. Some people keep a smaller "shot-day" pattern, feeling worse for a day or two after each injection.

How long do GLP-1 side effects last? For most people, the bad stretch after a new dose lasts days to a few weeks, not months. Side effects that do not get better on a stable dose, or that get worse, are a reason to talk to your prescriber. Do not wait them out.

Few people stop because of side effects. In the Zepbound weight trials, 4.8% to 6.7% of people stopped because of an adverse reaction, against 3.4% on placebo, and most of them stopped during the first few months. In the Wegovy 2.4 mg trials the numbers were 6.8% and 3.2%.

What are the long-term side effects of GLP-1 drugs?

The trial data now covers several years:

• Up to 88 weeks of Zepbound (Study 4): the label says adverse reactions were similar to those in the 72-week trials.
• A median of 37 months of Wegovy in the cardiovascular outcomes trial: 16% of people on Wegovy and 8% on placebo stopped the study drug because of an adverse event.

Beyond the common effects, the long-term concerns in the labels are the warnings listed in the next section. Some, like gallbladder disease, are linked to weight loss itself. The thyroid tumor warning comes from rodent studies, and the labels say it is unknown whether this happens in humans. Keep long-term questions for your prescriber, who knows your history.

Which GLP-1 has the least side effects?

There is no simple answer, and anyone who gives one is guessing. The table above suggests that higher doses bring more digestive side effects. The labels do not support ranking the drugs against each other, because each drug was tested in different people.

The best evidence is a head-to-head trial. SURMOUNT-5 compared the highest tolerated doses of tirzepatide and semaglutide in 751 adults with obesity for 72 weeks. The most common adverse events in both groups were digestive, most were mild to moderate, and most occurred during dose escalation. The trial was open-label and funded by Eli Lilly, which makes Zepbound.

In practice, the drug with the fewest side effects is the one your body tolerates at the dose you need. Your prescriber can change the dose or the drug if side effects do not settle. That decision is theirs. Do not skip, delay or change a dose yourself.

Serious warnings in the GLP-1 labels

These are less common, but you should know them. The names below are the ones the labels use.

• Risk of thyroid C-cell tumors (boxed warning, all four labels). Do not use these drugs if you or your family has a history of medullary thyroid carcinoma, or if you have MEN 2. Report a lump in the neck, trouble swallowing, shortness of breath or hoarseness that does not go away.
• Acute pancreatitis. Persistent or severe abdominal pain, sometimes spreading to the back, with or without vomiting. Get care right away.
• Acute gallbladder disease. The Medication Guides list pain in the upper stomach, fever, yellow skin or eyes, and clay-colored stools.
• Acute kidney injury due to volume depletion. Most reported cases followed nausea, vomiting or diarrhea that led to dehydration. Tell your prescriber if you cannot keep fluids down.
• Severe gastrointestinal adverse reactions. The labels do not recommend these drugs for people with severe gastroparesis.
• Hypoglycemia. The risk goes up when a GLP-1 is used with insulin or a sulfonylurea. Your prescriber may need to change those doses.
• Diabetic retinopathy complications. All four labels tell people with diabetic retinopathy to have it monitored. Report any change in vision.
• Hypersensitivity reactions, including anaphylaxis and angioedema. Swelling of the face, lips or throat, or trouble breathing, is an emergency.
• Pulmonary aspiration during general anesthesia or deep sedation. Tell the care team before any planned surgery or procedure.

Earlier Zepbound and Wegovy labels carried a warning about suicidal behavior and ideation. The current labels list that warning as removed. Changes in mood, anxiety or depression are still worth telling your prescriber about, and thoughts of self-harm need help the same day.

What to track, and why

"I've felt sick a lot" is hard for a prescriber to act on. "Nausea at 6/10 on days 1 and 2 after every shot since I went up to 5 mg" is easy to act on. GLP AI lets you log each dose with its injection site, and log side effects from a catalog (nausea, constipation, diarrhea, fatigue, dizziness, hair loss and more) with a severity from 1 to 10. Symptoms that are not in the catalog, such as vomiting, can be added as a custom symptom.

The estimated medication level chart shows where each symptom falls in your dose week. Before an appointment, you can export a doctor report PDF with your medication history, side effects, weight and nutrition, and share it yourself. See the GLP-1 symptom tracker and the doctor visit prep guide for how that works.

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Sources verified 2026-10-26

1. Zepbound (tirzepatide) US prescribing information, revised 08/2026: Table 1 and section 6.1 (gastrointestinal reactions 56% vs 30%, discontinuation, Study 4, hair loss), section 5 warnings, section 12.2 (gastric emptying).
2. Mounjaro (tirzepatide) US prescribing information, revised 08/2026: Table 1 and section 6.1 (injection site reactions 3.2% vs 0.4%), section 5 warnings.
3. Wegovy (semaglutide) US prescribing information, revised 06/2026: Table 3 and section 6.1 (discontinuation, cardiovascular outcomes trial, injection site reactions), section 5 warnings, recent major changes.
4. Ozempic (semaglutide) US prescribing information, revised 05/2026: Table 1 and section 6.1 (injection site reactions 0.2%), section 5 warnings.
5. Aronne LJ, et al. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (SURMOUNT-5). N Engl J Med. 2025;393(1):26–36.