Key takeaways
- Gastrointestinal side effects cluster in the two to three days after each injection and in the week after each dose increase; for most patients they are mild to moderate and fade within four to eight weeks of reaching a stable dose.
- In SURMOUNT-1, nausea affected about 25 to 33% of tirzepatide participants, diarrhea 19 to 23%, vomiting 8 to 12% and constipation 11 to 17%, versus much lower placebo rates; discontinuation for side effects was 4 to 7%.
- The escalation period (weeks 1 to 20 for tirzepatide, 1 to 16 for semaglutide) is when most side effects occur and most discontinuations happen; slowing escalation is the main tool for managing them.
- Constipation, reflux, fatigue and injection-site reactions often persist longer than nausea and need their own management.
- Severe or persistent abdominal pain, vomiting that prevents fluid intake, signs of dehydration, symptoms of hypoglycemia, a lump in the neck, or vision changes need same-day contact with a clinician.
The pattern that explains most of it
Both drugs are given once a week and have half-lives of five to seven days. Levels peak one to three days after the injection and decline slowly until the next one. Side effects track that curve: most patients feel the most nausea, fullness and fatigue in the two to three days after injecting and feel better toward the end of the week. Each dose increase raises the peak, so the week after an escalation is a repeat of week one at a higher level. Once a dose is held, the body adapts; gastric emptying, which is slowed most in the first weeks, partially normalizes with continued exposure, and nausea fades. The escalation period is therefore both the most productive period for weight loss and the hardest to get through, and most discontinuations happen in it.
The numbers
In SURMOUNT-1, across 72 weeks: nausea in 24.6% (5 mg), 33.3% (10 mg) and 31.0% (15 mg) of tirzepatide participants versus 9.5% on placebo; diarrhea 18.7% to 23.0% versus 7.3%; vomiting 8.3% to 12.2% versus 1.7%; constipation 11.7% to 16.8% versus 5.8%; dyspepsia 8.7% to 10.4% versus 4.4%; decreased appetite 8.8% to 10.4%. Most events were mild or moderate, occurred mainly during dose escalation, and resolved. Treatment was discontinued because of adverse events by 4.3%, 7.1% and 6.2% at the three doses versus 2.6% on placebo.
STEP 1 gave similar figures for semaglutide 2.4 mg: nausea 44.2% versus 17.4% on placebo, diarrhea 31.5% versus 15.9%, vomiting 24.8% versus 6.3%, constipation 23.4% versus 9.5%, with discontinuation for adverse events in 7.0% versus 3.1%. In the SURMOUNT-5 head-to-head, GI event rates were similar between the two drugs.
Week by week
Day 1 to 3 after the first injection. Many patients feel little. Some feel early fullness, mild nausea, burping or a metallic taste. Appetite falls within days, often before any nausea. Fatigue and light-headedness can accompany a sudden drop in intake.
Week 1 to 4 (tirzepatide 2.5 mg; semaglutide 0.25 mg). The starting doses are sub-therapeutic by design, and side effects are usually mild. This is the time to establish habits that prevent the worse weeks: small meals, protein first, stopping at the first sign of fullness, fluids through the day, and avoiding large fatty meals in the 48 hours after injecting.
Week 5 to 8 (tirzepatide 5 mg; semaglutide 0.5 mg). The first increase is where side effects become noticeable for most patients. Nausea peaks in the first week at the new dose, and for some it is accompanied by vomiting, particularly after larger meals. Diarrhea or constipation, or an alternation of both, begins here. Reflux appears in some patients as slowed gastric emptying keeps food in the stomach longer.
Week 9 to 20 (tirzepatide 7.5 to 15 mg; semaglutide 1 to 2.4 mg). Each further increase repeats the pattern. This is the period of highest side-effect burden and highest discontinuation, and the period when slowing the escalation is most useful: holding a dose for eight weeks instead of four, or asking for an intermediate strength, costs little in outcome and prevents many drop-outs. It is also the period of steepest weight loss, which brings its own effects: fatigue, cold intolerance, hair shedding (usually two to four months after rapid loss begins, and usually self-limiting), and occasionally dizziness on standing from low intake or dehydration.
Month 6 to 12 (stable maintenance dose). Nausea has usually faded to occasional. Constipation is the complaint most likely to persist and needs fibre, fluids, activity and, if necessary, an osmotic laxative. Reflux may persist and responds to smaller evening meals. Injection-site reactions (redness, itching, small lumps) are more common with tirzepatide and usually resolve within days. Some patients report low-level nausea indefinitely; a small dose reduction often resolves it with little effect on weight.
Beyond a year. Side effects at a stable dose are generally minimal. Symptoms that appear new after months of stability, particularly severe abdominal pain, should be evaluated rather than assumed to be the drug settling.
Managing the common effects
Nausea. Small frequent meals; protein and bland carbohydrate rather than fat; stop eating at the first sign of fullness; ginger, peppermint; avoid lying down after meals. Inject in the evening so the peak overlaps sleep. If nausea is persistent, ask the prescriber about slowing escalation or a short course of ondansetron.
Vomiting. Same measures, plus fluids in small sips. Vomiting more than twice in a day, or inability to keep fluids down, needs a call.
Diarrhea. Fluids and electrolytes; reduce fat and sugar alcohols; loperamide for short periods if a clinician agrees.
Constipation. Twenty-five to thirty grams of fibre, two to three litres of fluid, daily walking, magnesium or polyethylene glycol if needed. Start early; it is easier to prevent than to treat.
Reflux. Smaller meals, earlier dinner, head of bed raised; an over-the-counter acid reducer for short periods.
Fatigue. Usually intake-related. Check that you are eating enough (the calorie tool gives a floor) and enough protein.
Hair shedding. Telogen effluvium from rapid loss; protein, iron and time. It reverses.
The symptoms that are not routine
Severe, persistent abdominal pain, especially radiating to the back, with or without vomiting: possible pancreatitis. Stop the drug and seek care the same day.
Right upper abdominal pain, particularly after meals, with or without fever or jaundice: possible gallbladder disease, which is more common during rapid weight loss on any regimen and was more frequent than placebo in the trials.
Vomiting or diarrhea that prevents fluid intake, reduced urination, dizziness, confusion: dehydration with risk of acute kidney injury, which both labels warn about. Seek care.
Shakiness, sweating, confusion, palpitations, particularly if you take insulin or a sulfonylurea: hypoglycemia. Treat with glucose and contact the prescriber about dose adjustment.
A lump or swelling in the neck, hoarseness, difficulty swallowing, persistent shortness of breath: the label directs patients to report possible thyroid tumor symptoms.
Sudden vision changes in a patient with diabetes: possible retinopathy complication.
Rash, swelling of the face or throat, difficulty breathing: allergic reaction; emergency care.
Severe constipation with abdominal distension and no bowel movement for days, or persistent vomiting of undigested food: possible ileus or gastroparesis, both reported post-marketing.
Surgery and procedures
Slowed gastric emptying raises the risk of aspiration under anesthesia. Anesthesiology guidance recommends holding weekly GLP-1 medications for a week before elective procedures requiring sedation, and telling the surgical team you take one. Colonoscopy preparation may be less effective; tell the endoscopist.
Compounded product and side effects
The trial data apply to the molecules at their approved doses. Compounded product introduces two additional sources of side effects: dosing errors from multi-dose vials, which have produced overdoses with severe vomiting and hypoglycemia (the dosing-error article covers them), and formulation differences, including salt forms and added ingredients, whose effects are not characterized. A patient on compounded medication who develops side effects out of proportion to the dose should check the vial concentration and arithmetic first.
Frequently asked questions
How long do GLP-1 side effects last?
Nausea and other GI effects usually peak two to three days after each injection and after each dose increase, and settle within four to eight weeks of reaching a stable dose. Some patients have low-level nausea for months; constipation and reflux often persist longer than nausea.
Which week is the worst for side effects?
The week after each dose increase, and particularly the first week at each new dose. For tirzepatide that means weeks 1, 5, 9, 13, 17 and 21 if escalating on schedule; for semaglutide weeks 1, 5, 9, 13 and 17.
When should I stop taking a GLP-1 and call a doctor?
Stop and seek same-day care for severe persistent abdominal pain (possible pancreatitis or gallbladder disease), vomiting that prevents keeping fluids down, signs of dehydration or reduced urination, symptoms of low blood sugar, a lump or swelling in the neck, hoarseness or difficulty swallowing, sudden vision changes, or an allergic reaction.
Sources
- Jastreboff AM et al. SURMOUNT-1. N Engl J Med 2022;387:205-216 (supplementary appendix, adverse events by dose).
- Wilding JPH et al. STEP 1. N Engl J Med 2021;384:989-1002 (adverse events).
- Aronne LJ et al. SURMOUNT-5. N Engl J Med 2025 (adverse events by arm).
- Zepbound (tirzepatide) prescribing information, Eli Lilly, 2025.
- Wegovy (semaglutide) prescribing information, Novo Nordisk, 2025.
- 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;134:14-19.
- American Society of Anesthesiologists. Consensus-based guidance on preoperative management of patients on GLP-1 receptor agonists. 2023, updated 2024.
Citations are to primary sources (peer-reviewed trials, FDA labeling and announcements, and provider pricing pages). See our sources policy and corrections log.