On this page
Most GLP-1 side effects are mild and digestive. Think nausea, constipation, diarrhea, and an upset stomach, worst when you start or step up the dose, and usually easing within a few weeks. The serious risks are rare, but a few warning signs (severe belly pain, a gallbladder attack, an allergic reaction) mean you should get care right away.
Almost everyone who starts a GLP-1 feels something in the first month, and it's almost always their stomach. The common side effects are predictable and usually fade as your body adjusts. The rarer risks matter more, so this guide gives the honest version of both.
The common side effects
Almost all of the frequent side effects are gastrointestinal. In the Wegovy prescribing information (FDA, 2024), the ones reported most often were nausea, diarrhea, vomiting, constipation, and abdominal pain, along with indigestion and burping. They're usually mild to moderate, they show up early, and they tend to flare again briefly each time you step up to a higher dose.
- Nausea. The most common, and usually the first thing people notice.
- Constipation or diarrhea, sometimes trading off over the same few weeks.
- Vomiting, abdominal pain, indigestion, and burping.
- Headache or fatigue in some people, mostly early on.
Tirzepatide (the ingredient in Zepbound and Mounjaro) has a similar digestive profile. If you're weighing the two drugs, our tirzepatide guide covers how its side effects stack up against semaglutide's.
Why they happen and how to manage them
The reason is built into how these drugs work. GLP-1 medications slow how fast your stomach empties, which is part of how they keep you full and quiet your appetite (StatPearls, NCBI, 2024). That same slowed digestion is what leaves you feeling queasy or overly full. It also explains why dose matters so much. Clinicians start low and raise the dose gradually, because a slow climb gives your gut time to adjust and keeps the nausea in check.
Most of the common effects respond well to small changes in how you eat:
- Eat smaller meals, and stop as soon as you feel full.
- Go easy on large, greasy, or very rich meals, which sit heaviest.
- Stay hydrated, especially if you've had vomiting or diarrhea.
- Give it time, and tell your clinician if a side effect is severe so the dose plan can change.
For the full picture of how the medication acts on your gut and brain, our how GLP-1s work guide goes deeper.
Rare but serious risks
The risks worth taking seriously are uncommon, but real. The Zepbound prescribing information (FDA, 2023), which mirrors the semaglutide label, points to a consistent short list:
- Gallbladder problems, including gallstones and inflammation (cholecystitis).
- Acute pancreatitis, which is rare but needs urgent care.
- Dehydration and kidney injury that can follow severe vomiting or diarrhea.
- Low blood sugar, mainly in people also taking insulin or a sulfonylurea.
- Allergic reactions and injection-site reactions.
People with type 2 diabetes may also see existing eye disease (diabetic retinopathy) worsen when blood sugar falls quickly, which is a reason for closer monitoring.
One warning sits at the very top of the label. In rodent studies, GLP-1 drugs caused thyroid C-cell tumors, including medullary thyroid carcinoma. Whether they do the same in people isn't known, so the FDA puts these medications under a boxed warning and rules them out for anyone with a personal or family history of that cancer.
The thyroid line scares people, but in practice it's a screening question. If there's no personal or family history of medullary thyroid cancer, it simply doesn't apply to you.
— John Venzor, DO
Get medical care right away for severe or persistent abdominal pain, especially if it spreads to your back and comes with vomiting, which can signal pancreatitis. The same goes for a likely gallbladder attack (sharp pain in the upper-right belly, sometimes with fever), signs of dehydration from heavy vomiting or diarrhea, or an allergic reaction such as swelling, a rash, or trouble breathing.
And do not start a GLP-1 at all if you or a close relative has had medullary thyroid carcinoma or MEN 2.
Who should not take a GLP-1
A clinician screens for this before prescribing, and a few histories are clear stop signs. According to the NIDDK (2024) and the FDA labels, you shouldn't take a GLP-1 if you or a close family member has had medullary thyroid carcinoma or MEN 2. A past episode of pancreatitis is a reason for real caution. These drugs also aren't used in pregnancy or while breastfeeding, and the labels advise stopping well before you try to conceive.
Surgery and anesthesia deserve a mention. Because GLP-1s keep food in the stomach longer, anesthesia specialists worry about the risk of aspiration during sedation. Current guidance from the American Society of Anesthesiologists (2023) is to tell your surgical and anesthesia team that you take a GLP-1 and follow their advice, which may include pausing the medication before a procedure.
For the average person who qualifies, none of this should be alarming. Most people tolerate a GLP-1 well, the common effects fade within weeks, and the serious risks stay rare when the right person is on the right dose with a clinician keeping an eye on things.