Authored on 8 April 2023 by Mr Adeel Arshad,
Medically reviewed 27 May 2026 by Dr Ruch Karunadasa.
Next review due: 27 May 2027
What are the side effects of Wegovy and Mounjaro?
Originally published 8 April 2023. Medically reviewed May 2026.
Wegovy (semaglutide) and Mounjaro (tirzepatide) are among the most prescribed weekly injections for weight management in the UK. They work well for many people, but like all prescription medicines they can cause side effects, especially in the first weeks of treatment or after a dose increase.
This guide covers what to expect, how Wegovy and Mounjaro compare, and when to contact your clinician. Everyone responds differently; do not assume another person's experience predicts yours.
What are Wegovy and Mounjaro?
Wegovy is once-weekly injectable semaglutide. It mimics GLP-1, a hormone involved in appetite and stomach emptying, helping you feel fuller on fewer calories when prescribed with diet and activity.
Mounjaro is once-weekly injectable tirzepatide. It activates GLP-1 and GIP receptors. UK licensing and dosing differ from Wegovy, but the practical patient experience often overlaps, especially around digestive symptoms.
Read what is Wegovy, what is tirzepatide (Mounjaro), and Mounjaro vs Wegovy for treatment comparisons.
Common side effects of GLP-1 weight loss injections
UK product information and clinical trials report similar very common gastrointestinal effects for Wegovy and Mounjaro:
- Nausea and vomiting
- Diarrhoea or constipation (some people get both at different times)
- Stomach (abdominal) pain or bloating
- Headache
- Tiredness or weakness (sometimes linked to eating less as well as the medicine)
In Wegovy trials, most participants completed treatment; digestive symptoms were usually mild to moderate and often improved with time. Constipation sometimes lasted longer than other gut symptoms but was mostly mild.
Why titration matters: both medicines step up dose gradually (typically every four weeks) to limit nausea and related effects. See Wegovy dose schedule and Mounjaro dose schedule. If you restart after a break, you may need to titrate again; read restarting treatment after a break.
Are Mounjaro side effects different from Wegovy?
On paper, both share a GLP-1-related gut side effect profile. Mounjaro's additional GIP activity does not remove the need to watch for nausea, vomiting, or constipation. Some patients tolerate one medicine better than the other, which is one reason people discuss switching with their prescriber.
Injection technique can influence comfort at the site; see Wegovy injection guide and Mounjaro injection guide.
Wegovy Pill, Nevolat, and other options
- Wegovy Pill (oral semaglutide, where licensed): similar class effects; timing and fasting around doses follow product instructions
- Nevolat (daily liraglutide): GLP-1 class gut effects; daily dosing with its own titration schedule
- Mysimba and orlistat: different side effect profiles (for example central nervous system or oily stools); see treatment pages
How to manage side effects
Track symptoms in the first weeks after starting or increasing dose. Practical tips that help many GLP-1 users:
- Nausea: smaller, plain, low-fat meals; eat slowly; stay hydrated; avoid very rich or greasy food
- Constipation: fluids, fibre-rich foods, and movement; ask PrivateDoc if symptoms persist
- Diarrhoea: hydration; oral rehydration salts if needed for fluid loss
- Headache or dizziness: rest and standard pain relief if appropriate; contact your prescriber if persistent or severe; mention blood pressure medicines you take
Diet structure helps: what to eat when taking Mounjaro, SIRTfood dietary guide, and 9 reasons GLP-1 results may stall (includes missed doses and lifestyle factors that worsen tolerability).
Uncommon and serious side effects
Product information documents less common but important risks for GLP-1 obesity medicines, including:
- Acute pancreatitis: severe, persistent central abdominal pain, often with vomiting; seek urgent care
- Allergic reactions: swelling of face, mouth, or throat, breathing difficulty, rapid heartbeat; seek urgent care
- Anaphylaxis or angioedema: rare; emergency treatment required
- Gallbladder disease and other warnings listed in the patient information leaflet for your specific pen
Wegovy and Mounjaro are not suitable for everyone (for example personal or family history of medullary thyroid cancer or MEN2). Your prescriber checks contraindications before treatment. Always read the patient information leaflet supplied with your medicine.
Related guides
- Mounjaro vs Wegovy
- Wegovy dose schedule
- Mounjaro dose schedule
- BMI for Wegovy or Mounjaro
- All weight loss treatments
PrivateDoc provides follow-up and side effect support as part of weight management prescribing. Report new or worsening symptoms so your plan can be adjusted safely.
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