GLP-1 side effects: how common is each one?
Made with AI tools and overseen by an Australian GP · Updated 8 November 2026
Every GLP-1 medicine lists the same gut side effects. What the leaflet doesn’t show is how often they happen compared with a dummy injection. Here are the numbers from the main weight-loss trials.
Quick answer: Gut side effects are the main ones. In the weight-loss trials, nausea affected 44% of people on semaglutide (16% on placebo) and 25–29% on tirzepatide (8% on placebo). Diarrhoea, vomiting and constipation were also common, mostly mild and early. Roughly 5–7% stopped treatment because of side effects.
Semaglutide 2.4 mg (Wegovy) vs placebo
From the pooled adult weight-loss trials in the US product information.
- Nausea: 44% vs 16%
- Diarrhoea: 30% vs 16%
- Vomiting: 24% vs 6%
- Constipation: 24% vs 11%
- Tummy pain: 20% vs 10%
- Headache: 14% vs 10%
- Tiredness: 11% vs 5%
- Indigestion: 9% vs 3%
- Dizziness: 8% vs 4%
- Hair loss: 3% vs 1%
Tirzepatide 5–15 mg vs placebo
From the pooled weight-loss trials in the US product information (Zepbound is the US weight-loss brand of tirzepatide; in Australia it’s Mounjaro). Ranges cover the three doses.
- Nausea: 25–29% vs 8%
- Diarrhoea: 19–23% vs 8%
- Vomiting: 8–13% vs 2%
- Constipation: 11–17% vs 5%
- Tummy pain: 9–10% vs 5%
- Indigestion: 9–10% vs 4%
- Injection-site reactions: 6–8% vs 2%
- Tiredness: 5–7% vs 3%
- Hair loss: 4–5% vs 1%
Reading the numbers
- People on placebo get these symptoms too. The gap between the two numbers is the medicine’s share.
- Most gut side effects were mild to moderate and happened mainly while the dose was being stepped up. In STEP 1 they usually faded with time.
- Stopping because of side effects: 6.8% vs 3.2% on semaglutide, and 4.8–6.7% vs 3.4% on tirzepatide.
- Don’t compare the two lists too literally: they come from different trials, and some of the gap reflects different placebo rates.
What helps
- Smaller, more frequent meals, especially in the first weeks.
- Going easy on large or high-fat meals.
- Fluids and fibre from food for constipation.
- Alcohol can make nausea and reflux worse.
- If side effects are hard going, talk to your doctor. Don’t change your dose on your own.
Less common but serious
Gallbladder problems, pancreatitis and a rare eye condition are covered in rare but serious risks.
Get urgent help (000 in Australia, or your local emergency number) for severe tummy pain that won’t settle, with or without vomiting, or swelling of the face or throat. See a doctor the same day if you can’t keep fluids down, have pain under the right ribs with fever or yellow skin, or notice sudden changes in your vision.
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Common questions
What is the most common side effect of GLP-1 medicines?
Nausea. In the weight-loss trials it affected 44% of people on semaglutide 2.4 mg versus 16% on placebo, and 25–29% on tirzepatide versus 8% on placebo.
How long does the nausea last?
It’s usually worst in the first weeks and after each dose increase. In the STEP 1 trial, gut side effects were typically mild to moderate and faded with time.
Does Mounjaro have fewer side effects than Ozempic?
Not clearly. The numbers come from different trials, and the head-to-head SURMOUNT-5 trial found gut side effects were the most common in both groups, mostly mild to moderate and during dose increases.
Do GLP-1 medicines cause hair loss?
Hair loss is a listed side effect, but uncommon: 3% on semaglutide 2.4 mg and 4–5% on tirzepatide, versus 1% on placebo in the trials.
Can GLP-1 medicines cause low blood sugar?
Not often on their own. The risk is mainly in people with diabetes, especially those also taking insulin or a sulfonylurea. Your doctor may adjust those medicines.
What should I do if the side effects are bad?
Talk to your doctor or pharmacist rather than changing your dose yourself. Severe tummy pain that won’t settle, or vomiting so bad you can’t keep fluids down, needs urgent help.
Evidence and more information
- US FDA prescribing information for Wegovy (semaglutide), February 2026: side-effect table, warnings
- US prescribing information for Zepbound (tirzepatide): side-effect table, gallbladder and pancreatitis rates
- Wilding et al, NEJM 2021: STEP 1, semaglutide 2.4 mg for weight management (weight loss, side-effect and gallbladder rates)
- Jastreboff et al, NEJM 2022: SURMOUNT-1, tirzepatide for obesity
- Aronne et al, NEJM 2025: SURMOUNT-5, tirzepatide versus semaglutide
- Mozaffarian et al, 2025: Nutritional priorities to support GLP-1 therapy for obesity (joint advisory of four US obesity and nutrition societies)
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