Almost everyone who starts a GLP-1 medicine hears about side effects, and most people get at least one. The good news is that the common ones are usually mild to moderate, tied to dose increases and temporary. This guide uses trial data to show how often each side effect happens, how long it typically lasts, what helps, and which rare symptoms need medical attention right away.
Key takeaways
- Stomach and bowel symptoms are by far the most common. In STEP 1, nausea affected 44% of people on semaglutide 2.4 mg versus 17% on placebo.
- Most GI side effects are mild to moderate, peak in the days after a dose increase and fade over weeks at a stable dose.
- Only a small share of trial participants stopped because of GI side effects: about 4.5% on semaglutide in STEP 1.
- Rare but serious risks include pancreatitis, gallbladder disease, kidney injury from dehydration and, during anesthesia, aspiration of stomach contents.
How common are GLP-1 side effects?
The table compares the main obesity trials for three drugs at their top tested dose. Each trial had its own placebo group, so compare each drug to its placebo rather than across columns.
| Side effect | Semaglutide 2.4 mg (STEP 1) | Tirzepatide 15 mg (SURMOUNT-1) | Retatrutide 12 mg (TRIUMPH-1, investigational) |
|---|---|---|---|
| Nausea | 44% (placebo 17%) | about 31% (placebo about 10%) | 42% (placebo 15%) |
| Diarrhea | 32% (placebo 16%) | about 23% (placebo about 7%) | 32% (placebo 14%) |
| Vomiting | 25% (placebo 7%) | about 12% (placebo about 2%) | 25% (placebo 5%) |
| Constipation | 23% (placebo 10%) | about 12% (placebo about 6%) | 26% (placebo 11%) |
Other frequently reported effects across the class include burping, heartburn, bloating, stomach pain, fatigue, headache, dizziness and injection-site reactions. Hair shedding is listed on the Wegovy and Zepbound labels and is usually linked to rapid weight loss rather than the drug itself.
How long do side effects last?
For most people, GI symptoms follow a predictable pattern:
- After each injection: symptoms, if any, tend to be strongest in the first one to three days and ease as the week goes on.
- After each dose increase: expect a flare that typically settles within one to two weeks.
- At a stable dose: most people find symptoms fade substantially within a few weeks to a couple of months.
In the trials, GI events were mostly reported during the dose-escalation period and were described as mild to moderate and transient. If symptoms don’t improve, the usual step is to slow the escalation or step back a dose, not to stop entirely. See the semaglutide titration schedule for how that works.
Practical ways to reduce side effects
Nausea and vomiting
- Eat smaller meals and stop eating as soon as you feel satisfied.
- Limit greasy, fried and very sugary foods, which empty from the stomach slowly.
- Avoid lying down right after eating.
- Bland foods (crackers, rice, toast) and cold foods are often easier on bad days.
Diarrhea
- Drink fluids steadily, and add an electrolyte drink if losses are heavy.
- Cut back on high-fat foods, sugar alcohols and large amounts of caffeine.
Constipation
- Increase water and fiber gradually, and stay active.
- Ask a pharmacist or prescriber about stool softeners or other over-the-counter options if needed.
Alcohol can intensify nausea and dehydration. See alcohol on peptides and GLP-1s.
Rare but serious risks
| Risk | Warning signs |
|---|---|
| Pancreatitis | Severe, persistent upper belly pain that may spread to the back, with or without vomiting |
| Gallbladder problems (gallstones, inflammation) | Pain in the upper right belly, fever, yellowing skin or eyes, clay-colored stool |
| Kidney injury | Very little urine, dizziness or confusion, usually after significant vomiting or diarrhea |
| Low blood sugar | Shakiness, sweating, confusion, fast heartbeat. Risk is higher when combined with insulin or sulfonylureas |
| Severe stomach slowing or blockage | Persistent vomiting, severe bloating, inability to pass gas or stool |
| Aspiration during anesthesia | Tell your surgical or endoscopy team you take a GLP-1. A 2024 multi-society guidance says most people can continue the drug, often with a liquid diet the day before |
| Allergic reaction | Swelling of the face, lips or throat, trouble breathing, severe rash |
| Sudden vision loss | European regulators list a rare optic nerve condition (NAION) as a very rare side effect of semaglutide. Seek urgent care for sudden vision changes |
GLP-1 drugs carry a boxed warning about thyroid C-cell tumors seen in rodents. They are not used by people with a personal or family history of medullary thyroid carcinoma or MEN 2. Report a lump in the neck, hoarseness or trouble swallowing.
Regulators have also reviewed reports of mood changes and suicidal thoughts. The FDA’s preliminary 2024 review did not find evidence that GLP-1 drugs cause these, but anyone with new or worsening depression should contact a clinician.
Muscle loss
Part of the weight lost on GLP-1 drugs is lean mass, not just fat. In SURMOUNT-1’s body-composition study, about a quarter of the weight lost on tirzepatide was lean mass, a similar share to placebo. Adequate protein and strength training help protect muscle.
When to call your prescriber
- You can’t keep fluids down for more than a day.
- Side effects stop you from eating enough or working normally.
- You notice any of the warning signs listed above.
- You’re planning surgery, a procedure with sedation, or pregnancy.
Unapproved “research” versions of these drugs add risks on top of these: wrong strength, contamination and no sterility guarantee. Read what research use only means.
Frequently asked questions
What is the most common side effect of GLP-1 drugs?
Nausea. In STEP 1 it affected 44% of people on semaglutide 2.4 mg, compared with 17% on placebo. Diarrhea, vomiting and constipation are also common.
How long does GLP-1 nausea last?
It is usually strongest for a few days after each injection and after each dose increase, and typically fades over several weeks once the dose is stable.
Do GLP-1 side effects go away?
For most people, yes. GI side effects in trials were mostly mild to moderate and occurred mainly during dose escalation.
Which GLP-1 has the fewest side effects?
Rates differ by dose and trial design, so direct comparisons are hard. Lower doses and slower escalation generally cause fewer side effects for every drug in the class.
When should I worry about GLP-1 side effects?
Get urgent care for severe ongoing belly pain, pain in the upper right abdomen with fever or yellow skin, inability to keep fluids down, signs of allergic reaction, or sudden vision changes.
Sources
- Wilding JPH et al., STEP 1, N Engl J Med, 2021
- Jastreboff AM et al., SURMOUNT-1, N Engl J Med, 2022
- Eli Lilly, TRIUMPH-1 topline results, May 2026
- Look M et al., Body composition changes with tirzepatide in SURMOUNT-1, Diabetes Obes Metab, 2025
- American Society of Anesthesiologists, multi-society guidance on GLP-1 receptor agonists before procedures, October 2024
- European Medicines Agency PRAC, NAION as a very rare side effect of semaglutide, 2025
- Wegovy, Ozempic, Zepbound and Mounjaro U.S. prescribing information
Medical disclaimer: Pep Guide is for educational purposes only. It is not medical advice, diagnosis or treatment. Research changes over time. Talk to a licensed healthcare provider before starting, stopping or changing any medication or compound.