Most GLP-1 side effects are digestive: nausea, constipation, reflux, and feeling full early. Slower stomach emptying explains much of the fullness and reflux and some of the nausea. The nausea also comes from the drug acting on the brain, and the weight loss itself isn't driven mainly by the stomach. In a 2026 analysis of a liraglutide trial, delayed stomach emptying explained only 4–6% of the differences in appetite measures (Friedman et al., Obesity, 2026, DOI (external link)).
Timing follows a pattern. In the STEP semaglutide trials, gastrointestinal side effects were most frequent during or shortly after dose increases, and most were mild to moderate and temporary (Wharton et al., Diabetes, Obesity and Metabolism, 2021, DOI (external link)). So the rough patch after a titration step is expected, and it usually passes. For how common each symptom is and how long it typically lasts, see the GI side effects overview.
Nausea
Nausea is the most common side effect, especially early. These habits help:
- Eat smaller portions and stop before you're full. Your stomach may be emptying more slowly, and overfilling it is the fastest route to feeling sick. The "I'm done" signal now arrives earlier, so listen for it.
- Go easy on fatty, fried, and very rich foods. They sit heaviest and take longest to clear.
- Eat slowly.
- Don't lie down right after eating.
- If your prescriber approves, anti-nausea medication (such as ondansetron/Zofran) can get you through the worst stretch after a dose increase.
If nausea is severe, or you're vomiting repeatedly and can't keep fluids down, don't try to push through it. Call your doctor.
Constipation
Slower digestion plus eating and drinking less adds up to constipation for many people. It lasts longer than nausea: in the STEP trials the median episode ran 47 days, against 8 days for nausea. It's manageable:
- Drink on purpose. When you're eating less, you may drink less too, so check your fluids first. See the hydration guide.
- Get fiber in from vegetables, fruit, legumes, and whole grains, or a supplement if food alone falls short on a small appetite.
- Move. Even a daily walk helps.
- Ask your prescriber about a gentle stool softener or laxative if it persists.
Other common ones
- Reflux or heartburn: smaller meals, and nothing heavy close to bedtime.
- Fatigue: often tied to eating too little, or too little protein and fluid. Make the calories you do eat count.
- Burping, bloating, early fullness: largely the slowed-stomach effect. Smaller, slower meals help.
Most of this fades as your body adjusts to each dose. If a side effect isn't improving once you've settled at a dose, raise it with your doctor. Our conversation guide covers how.
Red flags: call a doctor now
These symptoms aren't normal adjustment. Most match warnings in the prescribing information for these drugs (DailyMed, Zepbound label, revised 8/2026 (external link)), and they need prompt medical attention:
- Severe, persistent abdominal pain, especially pain that radiates to your back. It can signal pancreatitis.
- Persistent vomiting or being unable to keep any fluids down (risk of dehydration and kidney injury).
- Signs of a gallbladder problem: pain in the upper-right abdomen, fever, yellowing of the skin or eyes.
- Signs of a serious allergic reaction: swelling, trouble breathing.
- Symptoms of very low blood sugar if you also take insulin or a sulfonylurea: shakiness, sweating, confusion.
When in doubt, call. The routine side effects are annoying but expected; with these, speed matters.
Study findings above are attributed to their authors and were located via PubMed.
This is general education, not medical advice. Anything severe or persistent should go to your prescriber or urgent care.