Side Effects of Weight-Loss Injections: What the Research Shows

The most common side effects of GLP-1 weight-loss injections are gastrointestinal: nausea, vomiting, diarrhoea, constipation and stomach pain. Less common but serious warnings include pancreatitis, gallbladder problems and bowel obstruction. This page summarises what the Australian Consumer Medicine Information (CMI) and the major trials report. It is not a substitute for advice from your prescribing doctor or pharmacist.

This topic is also in our free guide, The Weight-Loss Medication Guide. Get it emailed to you

Common side effects

The CMI for the two injectable medicines approved for weight management in Australia lists similar common effects:

Wegovy (semaglutide): nausea, vomiting, diarrhoea, constipation, stomach pain, headache, dizziness and fatigue (Wegovy CMI, July 2026).
Mounjaro (tirzepatide): nausea, diarrhoea, vomiting, constipation, stomach pain, injection-site reactions and fatigue (Mounjaro CMI, September 2026).

healthdirect’s overview of weight-loss medicines also lists insomnia, dry mouth, kidney stones, glaucoma and possible mood changes among effects reported across the class (healthdirect). In the SURMOUNT-5 trial, gastrointestinal effects were the most common in both treatment groups and occurred mostly while the dose was being increased (ACC summary of Aronne et al., NEJM 2025).

How often people stopped because of side effects

Trials record how many participants stopped treatment because of adverse effects. The figures differ between trials because the participants, durations and doses differ:

STEP 1 (semaglutide, 68 weeks): 4.5% stopped because of gastrointestinal effects, compared with 0.8% on placebo. Serious adverse events were reported in 9.8% versus 6.4% (ACC summary of Wilding et al., NEJM 2021).
SURMOUNT-1 (tirzepatide, 72 weeks): between 4.3% and 7.1% stopped because of adverse events depending on dose, compared with 2.6% on placebo (HCPLive summary of Jastreboff et al., NEJM 2022).
SELECT (semaglutide, about 40 months, people with heart disease): 16.6% stopped because of adverse events, compared with 8.2% on placebo; for gastrointestinal effects specifically, 10.0% versus 2.0% (ACC summary of Lincoff et al., NEJM 2023).

Serious warnings in the Australian CMI

Both CMIs list the following as warnings or serious effects (Wegovy CMI; Mounjaro CMI):

Acute pancreatitis: inflammation of the pancreas. The Wegovy CMI describes the warning sign as severe, persistent abdominal pain.
Gallbladder problems: gallstones and an inflamed gallbladder. The Mounjaro CMI also lists jaundice (yellowing of the skin or eyes).
Bowel obstruction.
Allergic reactions, including severe allergy.
Diabetic eye disease and vision changes: diabetic retinopathy can worsen when blood glucose improves quickly.
Dehydration, which can lead to kidney problems, particularly with vomiting or diarrhoea.
Surgery and anaesthesia: tell your treating team you are taking the medicine. The Wegovy CMI warns of a risk of aspiration (stomach contents entering the lungs) under general anaesthesia or deep sedation. See weight-loss injections and surgery.

The Wegovy CMI also lists depression or suicidal thoughts (see your doctor if you have a history of these) and NAION, a rare cause of sudden vision loss (Wegovy CMI). Wegovy carries the Black Triangle, which means the TGA has asked for additional monitoring of side effects (healthdirect).

Pregnancy and contraception

Neither medicine should be used during pregnancy. The Wegovy CMI advises stopping at least 2 months before a planned pregnancy (Wegovy CMI).
The Mounjaro CMI warns that oral contraceptives may be less effective. It advises using a barrier method of contraception for 4 weeks after starting and for 4 weeks after each dose increase (Mounjaro CMI).

If you are planning a pregnancy, or planning skin surgery after having children, raise this with your prescribing doctor early.

When to seek medical help

Contact your doctor promptly, or go to an emergency department if symptoms are severe, if you have:

severe or persistent abdominal pain, with or without vomiting
pain in the upper right abdomen, fever, or yellowing of the skin or eyes
vomiting or diarrhoea that stops you keeping fluids down, or signs of dehydration such as very little urine
sudden loss of vision or a change in your eyesight
swelling of the face, lips or tongue, difficulty breathing, or a widespread rash
low mood, or thoughts of self-harm

These reflect the warnings in the CMIs above. If you are in crisis, call 000. For anything else, your pharmacist and prescribing doctor are the right first contacts. Do not stop, change or restart a medicine without talking to them.

Side effects that matter for skin surgery

Some of these effects bear directly on later surgery. Poor appetite and gastrointestinal upset can make it harder to eat enough protein, which affects muscle and wound healing; see nutrition before body contouring. If you are seeing Dr Kumar, tell his team at your first consultation if you are taking, or have recently taken, a weight-loss medicine, so the anaesthetic and surgical plan can account for it. General surgical risks are covered on risks and considerations, and in more detail on drkumar.com.au.

Where to next

If your weight has settled and loose skin is now the main issue, book a consultation with Dr Kumar or take the readiness assessment.