Introduction
Medicines such as Wegovy (semaglutide) and Mounjaro (tirzepatide) are powerful treatments that help with weight loss and blood sugar control.
They mimic natural gut hormones known as GLP-1 (and for Mounjaro, also GIP) that help regulate appetite, digestion, and metabolism.
Like all medicines, GLP-1 treatments can cause side effects, especially when first starting or when switching between them. Understanding what to expect and how to manage symptoms can make your treatment safer and more comfortable.
Common Side Effects
The most common side effects are related to digestion and may include:
- Nausea
- Vomiting
- Constipation
- Diarrhoea
- Bloating
- Reflux
- Stomach discomfort
These symptoms are most noticeable during the first few weeks or after a dose increase, and they usually settle as your body adjusts (1–3).
Less common effects include tiredness, dizziness, and mild injection-site reactions.
If you experience severe or persistent abdominal pain, contact your prescriber immediately, as this could be a sign of pancreatitis or gallbladder irritation.
Why Side Effects Happen
GLP-1 medicines work by slowing down digestion and helping you feel fuller for longer.
This is key to weight loss, but your digestive system needs time to adapt — which is why temporary nausea, bloating, or changes in bowel habits can occur early on.
Starting at a low dose and increasing gradually allows your gut to adjust.
Eating smaller meals, avoiding heavy or greasy foods, and staying hydrated can also help.
Switching Between Wegovy and Mounjaro
Switching between GLP-1 treatments is increasingly common, whether due to supply issues or a need for a different treatment plan. Because the two medicines work slightly differently in the body, your system may react as though you’re starting a new medication.
Switching from Mounjaro to Wegovy
If you’ve been using Mounjaro, your body is already used to GLP-1 therapy.
The starting dose of Wegovy will depend on how much Mounjaro you were using and how well you tolerated it.
- If you tolerated Mounjaro well and had no digestive side effects, your prescriber may decide to start you on a moderate or higher Wegovy dose straight away.
- If you experienced nausea, bloating, or constipation or if there’s been a gap between injections it’s usually safer to restart Wegovy at a lower dose and increase gradually.
It’s good practice to discuss your weight loss history with your prescriber to help decide the most suitable starting point. This personalised approach helps maintain effectiveness while reducing the risk of side effects.
Switching from Wegovy to Mounjaro
When switching the other way, from Wegovy to Mounjaro, most people find it easier to tolerate. Because Mounjaro acts on both GLP-1 and GIP receptors, it often causes slightly less nausea. Even so, your prescriber will likely start you on the lower Mounjaro dose and increase gradually to ensure your body adjusts comfortably.
Missed or Interrupted Doses
If you miss a dose:
Wegovy: Take it within 5 days; if more than 5 days have passed, skip it and take your next scheduled dose.
Mounjaro: Take it within 4 days; if more than 4 days have passed, skip until your next scheduled dose.
If you’ve missed several weeks, your body may lose some tolerance. Restarting at a lower dose helps reduce side effects (6).
Managing Common Side Effects
For nausea or vomiting:
- Eat smaller, more frequent meals.
- Avoid fatty or spicy foods.
- Sip water or sugar-free ginger tea throughout the day.
- If vomiting persists, your prescriber may recommend anti-sickness medicine.
For constipation:
- Increase fibre gradually (wholegrains, oats, vegetables).
- Drink 1.5–2 litres of water daily.
- Keep active with gentle exercise to help aid digestion.
- Mild laxatives such as Laxido or Lactulose can be used if needed.
For diarrhoea:
- Replace fluids with rehydration salts (e.g. Dioralyte).
- Eat plain foods (rice, toast, banana) until settled.
If any symptom is severe or persistent, pause dose escalation and discuss with your healthcare provider.
Practical Switching and Monitoring Advice
From clinical guidance and the ADA special report (6):
- Always restart at an appropriate lower dose after switching or long treatment gaps.
- Allow at least seven days between your last and first injection when changing medicines
- Keep a record of your blood sugar and side effects during the first month of any switch.
When to Seek Medical Advice
Contact your GP or pharmacist if you:
- Have vomiting, diarrhoea, or stomach pain lasting more than two days.
- Experience constipation not improving after a week.
- Feel faint, dizzy, or can’t keep fluids down.
- Notice yellowing of your skin or eyes (possible liver or gallbladder issue).
- Suspect pregnancy while using Wegovy or Mounjaro.
Call 111 for urgent advice or 999 in an emergency.
Summary
Wegovy and Mounjaro are powerful medicines for weight management and improving metabolic health.
When switching between them, your starting dose of the new medicine will depend on how well you tolerated the previous one and it is always best to consult your prescriber.
Most side effects are mild and short-term, and careful dose adjustments, hydration, and dietary awareness can make your treatment much easier to manage.
References
- Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med. 2021;384(11):989–1002.
- Frias JP, Nauck MA, Van J, et al. Efficacy and safety of tirzepatide versus semaglutide once weekly in type 2 diabetes (SURPASS-2). N Engl J Med. 2021;385(6):503–15.
- Drucker DJ. Mechanisms of action and therapeutic application of glucagon-like peptide-1. Cell Metab. 2022;35(1):1–23.
- Ghusn W, Hurtado MD. Glucagon-like receptor-1 agonists for obesity: weight-loss outcomes, tolerability, and side effects. Obesity Pillars. 2024;12:100127.
- Diabetes UK. What is Mounjaro? 2024. Available from: https://www.diabetes.org.uk/about-diabetes/looking-after-diabetes/treatments/tablets-and-medication/glp-1/mounjaro
- Whitley HP, et al. Special Report: Potential Strategies for Addressing GLP-1 and Dual GLP-1/GIP Receptor Agonist Shortages. Clin Diabetes. 2023;41(3):467–73.