What this page establishes
- Nausea was reported in about 44% of semaglutide 2.4 mg participants in STEP 1 versus 16% on placebo, and in 31 to 43% of tirzepatide participants in SURMOUNT-1 depending on dose.
- GI events are the most common adverse events in both trial programmes, generally mild to moderate, and concentrated during dose escalation.
- Delayed gastric emptying is largest after the first dose and after each escalation and diminishes over time.
- FDA has added or revised label subsections for Acute Pancreatitis and Severe Gastrointestinal Adverse Reactions across the class.
Quick answers
The nausea settled but the constipation didn't. Why?
They have different halves. The motility effect follows the escalation curve and diminishes over time, which is the part that eases.
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The other half is that fibre and fluid intake fell with portion size, and that does not reset at a stable dose, because the portions stay small.Can I just take a laxative or a fibre supplement?
Half of what is causing this is reduced fibre and fluid going in, so the input side is worth looking at before the pharmacy shelf.
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Anything ongoing is a conversation with a clinician rather than a standing habit. Fibre supplements have their own page at fibre supplements and glucomannan.Should I skip a dose while I feel this rough?
Dose changes belong to your prescriber, and slower titration is the standard lever when symptoms do not settle.
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The point where this stops being a tolerability question is when you cannot keep fluids down, because that is the front of the labelled dehydration pathway. Illness planning is covered at sick-day planning.
Your stomach is emptying more slowly, and that is the drug working rather than the drug going wrong. Delayed gastric emptying is what produces satiety, and it is also what produces the nausea, reflux and constipation. Nausea was reported in about 44% of semaglutide 2.4 mg participants in STEP 1 against 16% on placebo, and in 31 to 43% of tirzepatide participants in SURMOUNT-1 depending on dose. These events are generally mild to moderate and they cluster during dose escalation. A short list of GI symptoms is not part of that picture and needs urgent attention, and those are at the bottom of this page.
Rates in the trials
GI events are the most common adverse events in both trial programmes. Nausea led at about 44% on semaglutide 2.4 mg in STEP 1 versus 16% on placebo, with tirzepatide running 31 to 43% in SURMOUNT-1 by dose. A network meta-analysis of GLP-1 receptor agonists in non-diabetic people with overweight or obesity confirms GI adverse events as the dominant tolerability issue across the class.
Most people do not stop because of them. Discontinuation for adverse events ran roughly 4 to 6% on tirzepatide and about 7% on semaglutide in STEP 1, so the large majority of the people reporting nausea carried on.
| Measure | Semaglutide 2.4 mg (STEP 1) | Tirzepatide (SURMOUNT-1) |
|---|---|---|
| Nausea | about 44% (placebo 16%) | 31 to 43% by dose |
| Discontinuation for adverse events | about 7% | roughly 4 to 6% |
| Timing | concentrated in escalation | concentrated in escalation |
Why delayed gastric emptying causes all of it
One mechanism produces the whole set. GLP-1 receptor agonism slows gastric emptying and reduces gut motility, so food stays in the stomach longer and the fullness signal persists. That is a large part of why the drugs work at all.
Nausea follows when the stomach is fuller than the body expects. Reflux follows when residual gastric contents meet a relaxed lower oesophageal sphincter, which is why lying down after eating is when people notice it. Constipation follows when colonic transit slows, and it has a second independent cause: fibre and fluid intake both fall automatically when portions shrink, so the motility effect and the input reduction push in the same direction.
The escalation phase versus steady state
The labelling states that delayed gastric emptying is largest after the first dose and after each escalation, and that it diminishes over time. That single sentence explains most of what people experience as an unpredictable course.
Each step up resets you toward the start of that curve. Someone who tolerated 1 mg comfortably for two months can have a rough fortnight at 1.7 mg and then settle again. Slower titration is the standard lever when it does not settle, and it belongs to the prescriber. Nothing on this page is a reason to change your own schedule.
Constipation specifically
Constipation gets less attention than nausea and outlasts it more often. Two things cause it at once: slower colonic transit from the drug, and much less food, fibre and fluid going in.
The second half of that is worth sitting with, because it is easy to miss. Halving portion size halves fibre intake without any change in what you choose to eat. Fluid falls the same way, since a large share of daily water arrives in food. Both are inputs to stool volume and consistency, and both drop before anyone notices.
Reflux and lying flat
Reflux on this class has a timing signature. It shows up in the evening and overnight, because gastric contents that would normally have cleared are still there when you lie down.
Eating the last meal of the day earlier is what most clinicians suggest first, on straightforward mechanical reasoning rather than trial evidence. New or worsening reflux that persists is worth reporting, partly because it is treatable and partly because persistent upper GI symptoms need a look rather than an assumption.
The serious ones
FDA has added or revised label subsections for Acute Pancreatitis and Severe Gastrointestinal Adverse Reactions across this class. Those are the events that separate from ordinary tolerability.
Pancreatitis presents as severe abdominal pain radiating to the back, often with vomiting, and it is urgent. Bowel obstruction and ileus present as abdominal distension with no bowel movement and no flatus. Gallbladder disease presents as right upper abdominal pain, sometimes with fever or jaundice, and gallstone formation is one of the effects linked to the rate and magnitude of weight loss rather than to the molecule directly. None of these are common. All of them need assessment rather than patience.
Dehydration and the kidney warning
The GI symptoms connect to the one labelled harm pathway that recurs everywhere on this class. FDA labelling warns of serious kidney injury resulting from dehydration, in some cases requiring haemodialysis, and states that most reported cases followed nausea, vomiting or diarrhoea. Labelling advises monitoring renal function in patients with reactions that could cause severe dehydration.
That reframes a two-day vomiting episode. It is the front end of the chain the regulator is warning about.
When to contact a clinician
The first three are urgent. The rest warrant contact rather than an emergency department.
- Severe abdominal pain radiating to the back, with or without vomiting, which is the pancreatitis pattern.
- Abdominal distension with no bowel movement and no flatus, which suggests obstruction or ileus.
- Blood in vomit or stool.
- Right upper abdominal pain, fever or jaundice, which points at the gallbladder.
- Persistent vomiting or an inability to keep fluids down, which is the labelled dehydration and kidney-injury pathway.
The evidence, one row per claim
| Claim | Tier | Source |
|---|---|---|
| Nausea was reported in about 44% of semaglutide 2.4 mg participants in STEP 1 versus 16% on placebo, and in 31 to 43% of tirzepatide participants in SURMOUNT-1 depending on dose. | established | Rubino et al., GI tolerability in SURMOUNT-1 to -4 (Diabetes Obes Metab) |
| GI events are the most common adverse events in both trial programmes, generally mild to moderate, and concentrated during dose escalation. | established | Rubino et al., SURMOUNT GI tolerability |
| Discontinuation for adverse events ran roughly 4 to 6% on tirzepatide and about 7% on semaglutide in STEP 1. | emerging | Cross-trial GI tolerability data |
| Delayed gastric emptying is largest after the first dose and after each escalation and diminishes over time. | established | MOUNJARO US Prescribing Information, Eli Lilly |
| FDA has added or revised label subsections for Acute Pancreatitis and Severe Gastrointestinal Adverse Reactions across the class. | established | OZEMPIC US Prescribing Information, FDA |
| A network meta-analysis of GLP-1 RAs in non-diabetic people with overweight or obesity confirms GI adverse events as the dominant tolerability issue across the class. | established | GI adverse events with GLP-1 RA in non-diabetic overweight/obesity: systematic review and network meta-analysis (PMC) |
Sources
- Rubino et al., GI tolerability in SURMOUNT-1 to -4 (Diabetes Obes Metab)
- MOUNJARO US Prescribing Information, Eli Lilly
- OZEMPIC US Prescribing Information, FDA
- GI adverse events with GLP-1 RA in non-diabetic overweight/obesity: systematic review and network meta-analysis (PMC)
Keep reading
- GLP-1 and Hydration: Why Fluid Intake Drops and Why It MattersA soft wellness topic with a hard regulatory warning behind it: dehydration following GI symptoms is the docum
- GLP-1 and the Gallbladder: Gallstones and Biliary RiskThe relative risk is real and quantified. The absolute risk is small. Rapid weight loss is a large part of the
- GLP-1 and Kidney Health: Protection, and the Dehydration WarningChronic protection and acute vulnerability at the same time. FLOW's benefit is specific to type 2 diabetes wit
- GLP-1 and Surgery: What to Tell Your AnaesthetistTell the anaesthetic team you are on this drug. Perioperative guidance changed materially in October 2024, and
- GLP-1 and Sleep, Including Sleep ApnoeaSleep apnoea is the one sleep outcome with a trial and an approval behind it. Everything else here is much wea