What Are the Possible Side Effects of GLP-1s?

Quick answer: The most common GLP-1 side effects are gastrointestinal: nausea, diarrhea, constipation, and decreased appetite, especially in the first few weeks after starting or increasing a dose. How noticeable these are varies a lot by dose and by person. Rare but serious risks include pancreatitis, gallbladder issues, and severe digestive slowdown. GLP-1s aren't recommended for people with a personal or family history of certain thyroid cancers. Most mild side effects ease as your body adjusts, but severe or sudden symptoms should be reported to your clinician right away.

GLP-1 medications have become one of the most talked-about treatments in recent years, and for good reason. But like any medication, they come with a real side effect profile worth understanding before you start, not after. For a full overview of how these medications support weight loss, see our GLP-1 Weight Loss Main Guide.

What Are the Most Common Side Effects of GLP-1s?

The most frequently reported side effects are gastrointestinal, especially early in treatment or after a dose increase:

• Nausea, the most common of the group
• Diarrhea or constipation
• Vomiting
• Decreased appetite
• Bloating or stomach discomfort
• Fatigue

These effects are most common during the first few weeks after starting or increasing a dose, which is exactly why GLP-1s are typically started at a low dose and increased gradually. Giving your body time to adjust is one of the main reasons titration schedules exist.

It helps to think of the first two months as an adjustment window. This is when your body is getting used to the medication and when most side effects are most noticeable. By month three and beyond, side effects have typically eased for most people, and this is also around when most people start noticing meaningful results, since the body has adapted rather than still adjusting. How strongly any of this shows up depends heavily on your dose and your individual response, so two people on the same medication can have very different experiences.

Are There Less Common but More Serious Side Effects?

Yes, and they're worth knowing even though they're uncommon:

• Gallbladder issues, including gallstones, have been reported with GLP-1 use.
• Pancreatitis (inflammation of the pancreas) is a rare but serious risk that requires immediate medical attention if symptoms like severe abdominal pain occur.
• Severe gastroparesis or bowel blockage. Some mild, temporary slowing of digestion is a normal and expected effect of GLP-1s. In rare cases this slowing can become severe enough to require hospitalization. Working closely with your prescriber to find and stay at the right dose is the best way to avoid this. More is not better.
• Changes in kidney function can occur, particularly if dehydration develops from vomiting or diarrhea.
• Vision changes. A rare side effect, estimated at around 1 in 1,000,000 patients, has been reported. It appears most often connected to existing complications of type 2 diabetes rather than the medication itself, but it has been the subject of a small number of lawsuits and is worth being aware of.
• Low blood sugar (hypoglycemia) can occur when a GLP-1 is combined with certain other diabetes medications. Your care team accounts for this when reviewing your full medication list.
• Injection site reactions, such as redness or irritation, can occur with injectable formulations.

This isn't a complete list, and not everyone will experience any of these. It's a starting point for a conversation with your prescribing clinician, not a substitute for one.

Are There Reasons Someone Shouldn't Take a GLP-1?

Some conditions rule GLP-1s out entirely rather than showing up as a side effect once you're on one:

• Thyroid cancer history. GLP-1s aren't recommended for people with a personal or family history of certain thyroid cancers. This warning comes from findings in animal studies; the related tumor type hasn't been observed in humans, but it's treated as a hard stop for anyone with that history, out of caution.
• Pregnancy. GLP-1s aren't used during pregnancy, since effects on a developing fetus haven't been ruled out.
• Pancreatitis history, which your prescriber will also screen for.

Your prescriber reviews your health history for these and other contraindications before starting treatment.

Can GLP-1s Affect Muscle Mass?

Yes. Muscle mass loss is a common effect of weight loss on a GLP-1, and it's worth addressing on its own rather than lumping it in with digestive side effects. It can largely be minimized by:

• Losing weight at a measured pace, generally less than 10 pounds per month
• Doing regular resistance exercise
• Getting adequate protein intake

Ask your care team how to build these into your plan. For more detail, see our guide to protecting muscle during GLP-1 weight loss.

How Do I Know if a Side Effect Is Normal or Something to Call About?

A reasonable rule of thumb: mild nausea or a few days of digestive adjustment after starting or increasing a dose is common and typically expected. Severe abdominal pain, persistent vomiting that prevents you from keeping fluids down, signs of dehydration, sudden inability to pass gas or stool, or any symptom that feels sudden and severe is not something to wait out. When in doubt, contact your care team rather than guessing.

Can Side Effects Be Managed Without Stopping the Medication?

Often, yes. Common strategies include eating smaller meals, avoiding high-fat or very rich foods, eating more slowly, staying hydrated, and giving each dose increase more time before moving to the next step. None of these are a substitute for medical guidance, and any changes to your dose or schedule should go through your prescribing clinician rather than being adjusted on your own. Because side effects are so dose- and individual-dependent, quick and clear communication with your prescriber is the fastest way to personalize your dose and minimize what you experience.

Frequently Asked Questions

Do GLP-1 side effects get better over time?

For most people, yes. Gastrointestinal side effects are most noticeable in the first few weeks after starting or increasing a dose, and tend to ease as the body adjusts.

Should I stop taking my GLP-1 if I feel side effects?

Don't stop or adjust your dose without talking to your clinician first. Mild, expected side effects usually don't require stopping treatment, but that call needs to be made with your care team based on your specific situation.

Are the side effects different between GLP-1 medications?

Yes. Specific side effect profiles can vary between medications and individual patients, so what a friend experienced on one medication may not match what you'd experience on another. Dose and individual response matter just as much as which medication is involved.

Is nausea a sign the medication is working?

Not exactly. Side effects are tied to how the medication affects digestion and appetite signaling, not to whether it's producing the results you're looking for. Some people have significant side effects with modest results, and others have minimal side effects with strong results.

What's an emergency versus something I can mention at my next appointment?

Severe abdominal pain, inability to keep fluids down, or signs of dehydration warrant contacting your care team right away rather than waiting. Mild, expected digestive discomfort is usually fine to bring up at your next scheduled check-in.

References

1. Wharton S, Davies M, Dicker D, et al. Managing the gastrointestinal side effects of GLP-1 receptor agonists in obesity: recommendations for clinical practice. Postgrad Med. 2022;134(1):14-19.
2. U.S. Food and Drug Administration. Highlights of prescribing information: Ozempic (semaglutide) injection. Revised 2025.
3. U.S. Food and Drug Administration. Highlights of prescribing information: Wegovy (semaglutide) injection. Revised 2023.
4. Filippatos TD, Panagiotopoulou TV, Elisaf MS. Adverse effects of GLP-1 receptor agonists. Rev Diabet Stud. 2014;11(3-4):202-230.
5. Kim SY, Kim SH. Exploring the side effects of GLP-1 receptor agonist: to ensure its optimal positioning. Diabetes Metab J. 2025.

Medically reviewed by Dr. Casey Greene, PharmD, IFMCP, Peptide Subject Matter Expert, Evergreen Telehealth, on September 4, 2026.

This article is for general educational purposes and is not a substitute for individualized medical advice. Always talk with your prescribing clinician about any side effects you experience.

Information and statements on this material have not been evaluated by the Food and Drug Administration. Evergreen Telehealth content is for general education and should never replace professional medical advice that considers your individual health.