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: Severe Side Effects Compared
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections available in the UK, but they work differently and carry different side effect profiles. Understanding how their severe side effects compare can help you have an informed conversation with your prescriber about which medication might be more suitable for your individual health situation. This guide compares the clinical evidence on serious adverse events from both SURMOUNT and STEP trial data.
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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.
Severe Side Effects: SURMOUNT vs STEP Trial Data
Mounjaro (tirzepatide) — a dual GIP and GLP-1 receptor agonist — was evaluated in the SURMOUNT-1 clinical trial. Severe gastrointestinal events (pancreatitis, severe nausea requiring hospitalisation, acute gallbladder disease) were reported in a small percentage of participants. The most serious adverse event monitored was acute pancreatitis, which occurred rarely but required medical attention.
Wegovy (semaglutide) — a GLP-1 receptor agonist only — was evaluated in the STEP-1 trial. Serious gastrointestinal complications were similarly rare. Both medications showed comparable rates of severe adverse events requiring withdrawal from the study, though individual tolerability varied significantly across participants.
- Mounjaro KwikPen: dose titration over 20 weeks may help the body adapt to GIP+GLP-1 signalling
- Wegovy: longer dose escalation (17+ weeks) with a single mechanism of action
- Both: gastrointestinal symptoms most common in early weeks, often improving with continued use
Severe pancreatitis, whilst rare, is the most serious possible complication with either medication and requires immediate medical attention.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Suitability and Individual Assessment
Which medication carries fewer severe side effects for you depends entirely on your individual health profile, medical history, and how your body responds to GLP-1 and GIP receptor stimulation. Your prescriber will assess your personal risk factors — including whether you have a history of pancreatitis, gallbladder disease, or diabetic retinopathy complications — before recommending one medication over the other.
Key factors your prescriber will consider:
- Whether you tolerated GLP-1 medications previously (if applicable)
- Your kidney function and metabolic health
- Any history of medullary thyroid cancer or multiple endocrine neoplasia type 2 (MEN2) in your family
- Whether you take oral medications that might be affected by slowed gastric emptying
- Your ability to manage dose titration and report side effects
Use our free GLP-1 weight loss calculator to explore potential outcomes, but your prescriber's clinical judgment is essential for personalised safety assessment.
- 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.
Are Mounjaro's severe side effects worse than Wegovy's?
Clinical trial data shows comparable rates of serious adverse events between Mounjaro and Wegovy. Severe side effects are rare with both medications. Individual responses vary significantly — some people tolerate one medication better than the other. Your prescriber will assess which is safer for your specific health profile.
What is the most serious side effect of Mounjaro?
Acute pancreatitis is the most serious potential side effect, though it is rare. Signs include severe stomach pain radiating to the back, nausea, and vomiting. This requires immediate medical attention. Other serious risks include gallbladder complications and diabetic retinopathy changes in people with existing diabetes.
Can you switch from Mounjaro to Wegovy if side effects are severe?
Yes, switching is possible under prescriber guidance. However, abrupt switching is not recommended — your prescriber will typically advise stopping one medication and waiting before starting another, or using a gradual transition. Any medication change requires clinical assessment and monitoring.
Does Mounjaro's dual mechanism (GIP+GLP-1) mean it causes more side effects?
Not necessarily. The dual mechanism may actually help some people tolerate the medication better because GIP activation can reduce some GLP-1-related gastrointestinal effects. Clinical trials show similar overall side effect rates. Individual responses depend on genetics, metabolism, and dose tolerance.
How do I know if a side effect is "severe" enough to stop Mounjaro?
Severe side effects requiring immediate medical attention include severe pancreatitis symptoms, serious allergic reactions, or vision changes. Contact your GP or NHS 111 immediately if you experience severe persistent stomach pain, difficulty breathing, or fainting. Your prescriber can advise whether to pause or stop treatment based on your specific symptoms.
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