GLP-1 side effects: common, serious, and what helps

What GLP-1 medications can cause, from nausea to the serious warnings: why side effects happen, what helps, and when to call for help.

GLP-1 medications such as semaglutide and tirzepatide work partly by slowing how quickly the stomach empties and by acting on appetite signals in the brain. Those same effects explain most of their side effects. This guide sets out which effects are common, which are rare but serious, what tends to help, and when a symptom needs a clinician rather than patience.

What are the most common side effects of GLP-1 medications?

For both semaglutide and tirzepatide, the list of common side effects is led by the digestive system:

• Nausea, the most frequently reported
• Diarrhea and vomiting
• Constipation
• Stomach pain, bloating and gas
• Indigestion, heartburn and acid reflux
• Burping, sometimes with a sulfur smell

Beyond the gut, there are also reports of fatigue, headache, dizziness, hair loss and unusual skin sensations such as tingling or sensitivity (dysesthesia). Injections can cause redness, itching or swelling where the needle goes in. In people with type 2 diabetes who also take insulin or a sulfonylurea, low blood sugar is a recognized risk.

Why do GLP-1 medications cause side effects?

The digestive effects follow from how the medications work. GLP-1 slows the rate at which food leaves the stomach and signals fullness to the brain. A stomach that empties more slowly is more easily overfilled, which can show up as nausea, reflux, burping or vomiting, particularly after a large or fatty meal. Changes further along the gut account for diarrhea in some people and constipation in others.

The body adapts to a given dose over time. That is why side effects cluster in the early weeks and again after each dose increase, and why treatment starts low and steps up gradually instead of beginning at a maintenance dose.

When do side effects start, and how long do they last?

For most people, digestive side effects appear in the first weeks of treatment or shortly after a dose increase, and become less noticeable as the body adjusts. They are not a sign that the medication is working, and their absence is not a sign that it is not. If a step up brings symptoms that do not settle, a clinician can keep you at the current dose for longer before moving on. Our article on how long GLP-1 side effects last covers the pattern in more detail.

What are the serious side effects of GLP-1 medications?

Serious side effects are uncommon, but they are worth setting out plainly, and they are the reason a clinician reviews your history before prescribing:

• Thyroid C-cell tumors. Both molecules caused thyroid C-cell tumors in rodents, and it is not known whether they do in people. They must not be used by anyone with a personal or family history of medullary thyroid carcinoma, or with Multiple Endocrine Neoplasia syndrome type 2.
• Acute pancreatitis. Inflammation of the pancreas has been reported, including severe forms. The typical sign is severe abdominal pain that does not go away, sometimes reaching through to the back, with or without vomiting.
• Gallbladder problems. Gallstones and gallbladder inflammation have occurred. Substantial or rapid weight loss can itself increase the risk of gallstones.
• Kidney injury from dehydration. Vomiting and diarrhea can cause dehydration severe enough to affect the kidneys.
• Severe digestive reactions. Cases of blocked or stalled bowel (ileus and intestinal obstruction) and severe constipation have been reported. The medications are not recommended for people with severe gastroparesis.
• Severe allergic reactions, including anaphylaxis and swelling of the face, lips, tongue or throat (angioedema).
• Low blood sugar, mainly when a GLP-1 is taken with insulin or a sulfonylurea.
• Diabetic eye complications in people with type 2 diabetes who have diabetic retinopathy.
• Breathing in stomach contents during general anesthesia or deep sedation, because the stomach may not be empty. Tell anyone planning a procedure that you take a GLP-1.
• A faster resting heart rate, which clinicians monitor with semaglutide.

When should I contact a clinician, and when is it an emergency?

Message your care team promptly, rather than waiting for a scheduled check-in, if:

• Vomiting or diarrhea is stopping you from keeping fluids down, or you notice signs of dehydration: dark or very little urine, a dry mouth, or dizziness when you stand
• Constipation goes on for several days, or comes with bloating or pain
• A dose increase brings symptoms that are not settling
• You notice a lump or swelling in your neck, trouble swallowing, a hoarse voice that does not go away, or shortness of breath
• You take insulin or a sulfonylurea and feel shaky, sweaty, confused or unusually hungry
• You have diabetes and notice changes in your vision
• You have surgery or a procedure with anesthesia or sedation coming up

Call 911 or go to the nearest emergency room for severe abdominal pain that does not go away, with or without vomiting; pain in the upper right side of the abdomen with fever, or yellowing of the skin or eyes; swelling of the face, lips, tongue or throat, or difficulty breathing; or fainting. Telehealth is not the right route for urgent symptoms.

Who should not take a GLP-1?

There are two contraindications: a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2, and a previous serious allergic reaction to the medication or its ingredients. Weight-loss treatment is not recommended during pregnancy, and treatment is stopped when a pregnancy is recognized.

A history of pancreatitis, gallbladder disease, kidney disease, severe gastroparesis or diabetic retinopathy calls for a careful conversation with a clinician rather than an automatic yes. Our guide to who should not take a GLP-1 goes through each.

Are semaglutide and tirzepatide side effects different?

The profiles are similar. The same digestive effects lead both lists, and both have the same thyroid risk and largely the same serious risks. Two differences stand out. Semaglutide is associated with an increased resting heart rate. With tirzepatide, anyone taking oral birth control is advised to switch to a non-oral method, or add a barrier method, for four weeks after starting and for four weeks after each dose increase, because tirzepatide can reduce how well the pill is absorbed.

Do compounded GLP-1 medications have different side effects?

Everything above describes semaglutide and tirzepatide as molecules. Compounded medications are not FDA-approved and are not reviewed by the FDA for safety, effectiveness, or quality. A compounded preparation has its own added ingredients, its own concentration and its own way of measuring the dose. The FDA has warned about dosing errors with compounded injectable semaglutide, often from confusing units with milligrams. Read your pharmacy's directions, ask the pharmacy if anything on it is unclear, and keep to the dose your clinician prescribed.

What helps with GLP-1 side effects?

• Eat smaller meals, slowly, and stop at the first sign of fullness.
• Go easy on fatty, fried and very sweet foods, which sit longest in a slow stomach.
• Avoid lying down straight after eating if reflux or burping is a problem.
• Drink fluids steadily through the day, and more if you have vomiting or diarrhea.
• Build up fiber gradually, with plenty of water, if constipation is the issue.
• Ask before adding over-the-counter remedies, and tell your care team rather than skipping doses on your own.
• Do not move up to the next dose until the current one is tolerable. A slower schedule is a routine adjustment.

What about long-term side effects?

Many of the long-term questions people ask, about muscle, hair, the thyroid and what happens after stopping, remain open. Our articles on muscle loss, hair loss and stopping a GLP-1 cover what is known. Continuing treatment is a decision to revisit with your clinician over time, not a one-off.

This article is educational and is not a substitute for personalized medical advice. Whether any medication is appropriate for you is a decision for a licensed clinician who has reviewed your health history.