The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Mounjaro vs Wegovy: Serious Side Effects Compared
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but they work differently and carry different risk profiles. Understanding the serious side effects of each — and how they compare — can help you and your GP make an informed decision about which medication may be more suitable for your individual health circumstances. This guide compares the serious adverse events from clinical trials and explains what you need to know before starting either treatment.
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What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
Clinical Trial Data and Serious Side Effect Profiles
Mounjaro and Wegovy differ in their mechanism of action, which influences their side effect patterns. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways simultaneously. Wegovy (semaglutide) is a GLP-1 receptor agonist only, affecting a single pathway. This dual mechanism in Mounjaro may explain some differences in how commonly certain side effects occur.
In the SURMOUNT clinical trials for Mounjaro, serious side effects were rare but included pancreatitis (inflammation of the pancreas), gallbladder complications, and severe allergic reactions. The STEP trials for Wegovy similarly reported pancreatitis and gallbladder issues as serious but uncommon events. Both medications carry a black triangle (▼) status in the UK, meaning they are under additional MHRA monitoring for safety.
- Pancreatitis: reported in both trials but at low frequency
- Gallbladder problems: cholelithiasis and cholecystitis documented in both drug groups
- Severe allergic reactions: rare but require immediate medical attention
- Diabetic retinopathy complications: possible in patients with existing diabetes
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Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
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Which Medication Is Right for You?
The choice between Mounjaro and Wegovy is not about which has "fewer" serious side effects — both are rare — but about which medication is clinically appropriate for your individual health profile. Your GP or prescriber will assess your medical history, current conditions, other medications, and specific risk factors before recommending one over the other. Using our free GLP-1 weight loss calculator, you can explore projected weight loss outcomes at different doses, which may inform your discussion with your prescriber about treatment goals.
Some patients may tolerate one medication better than the other based on gastrointestinal sensitivity, existing pancreatic or gallbladder conditions, or other contraindications. Prescribers follow strict eligibility criteria — including those from NHS NICE guidance — to ensure the chosen medication is safe for you specifically. Never start either medication without a clinical assessment from a UK-licensed prescriber.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Is Mounjaro safer than Wegovy?
Both Mounjaro and Wegovy have similar serious side effect rates in clinical trials. Safety depends on your individual health profile, not on which drug is inherently "safer". Your GP will assess which is more appropriate for you based on your medical history, existing conditions, and contraindications. Neither is universally safer.
What is pancreatitis and why is it a serious side effect?
Pancreatitis is inflammation of the pancreas, which can cause severe upper abdominal pain, nausea, and vomiting. It requires urgent medical attention. Both Mounjaro and Wegovy carry a small risk. Seek immediate medical help if you experience sudden, severe stomach pain radiating to your back while on either medication.
Can I switch from Wegovy to Mounjaro if I experience side effects?
Possibly, but only under prescriber guidance. Switching medications requires a clinical assessment and a washout period between doses. Do not stop one and start the other without medical supervision. Discuss any side effects with your GP or prescriber — they may adjust the dose before considering a switch.
Are serious side effects more common at higher doses?
Serious side effects like pancreatitis and gallbladder complications are rare at all licensed doses, though gastrointestinal side effects (nausea, diarrhoea) are more common at higher doses. Dose titration — starting low and increasing gradually — helps your body adjust and reduces early side effects. Most people tolerate these medications well.
What should I do if I think I'm having a serious side effect?
Stop the injection and contact your GP or NHS 111 immediately. Seek emergency care (999) if you experience severe abdominal pain, difficulty breathing, or signs of a serious allergic reaction. Do not wait for your next scheduled dose. Report all side effects to your prescriber — they monitor safety data and can provide guidance specific to your situation.
Ready to take control of your weight loss journey?
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