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: Pancreatitis and Side Effects Explained
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections used in the UK, and both carry a potential risk of pancreatitis as a serious side effect. While both medications work via GLP-1 receptor activation, they differ in mechanism and clinical trial outcomes. Understanding the evidence on pancreatitis risk and other side effects can help you have an informed discussion with your prescriber about which medication might be appropriate for your individual health situation.
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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 Mechanism Differences
Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways. Wegovy is a GLP-1-only receptor agonist. In the SURMOUNT-1 clinical trial, tirzepatide showed average weight loss of approximately 20.9% of body weight at the 15 mg dose over 72 weeks. The STEP-1 trial for semaglutide showed average weight loss of approximately 14.9% at the 2.4 mg dose. Both trials were conducted alongside reduced-calorie diet and increased physical activity.
Regarding pancreatitis specifically: cases of acute pancreatitis have been reported in clinical trials and post-market use of both medications, though the exact incidence is rare. Both carry the same warning regarding pancreatitis risk, and both require immediate medical attention if severe, persistent stomach pain spreading to the back occurs—this may indicate pancreatitis.
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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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Side Effects Profile and Individual Assessment
Gastrointestinal side effects (nausea, vomiting, diarrhoea, constipation, stomach pain) are common with both medications, particularly during dose escalation. Most people find these effects decrease over time as the body adjusts. The serious side effect requiring urgent medical attention—acute pancreatitis—is rare but can occur with either medication.
A UK-licensed prescriber will assess your individual medical history, current medications, and any personal or family history of pancreatic disease before recommending either medication. Your GP or specialist can weigh the potential benefits and risks based on your specific health situation. Neither Mounjaro nor Wegovy is suitable for everyone, and the choice between them depends on individual factors your prescriber will evaluate.
- 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.
Can Mounjaro cause pancreatitis?
Yes, acute pancreatitis is a rare but serious potential side effect of Mounjaro. If you experience severe, persistent stomach pain that spreads to your back, seek immediate medical attention. This is one reason a prescriber will assess your full medical history before prescribing, including any history of pancreatic disease or gallbladder problems.
Does Wegovy carry the same pancreatitis risk as Mounjaro?
Yes, both Wegovy and Mounjaro carry the same warning about pancreatitis risk. Cases have been reported in clinical trials and post-market use of both medications. Both require immediate medical attention for severe abdominal pain. Your prescriber will assess your individual risk factors for either medication.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro is a dual GIP and GLP-1 receptor agonist—it activates two hormone pathways. Wegovy is a GLP-1-only agonist. Both slow appetite and gastric emptying, but Mounjaro's dual mechanism may produce different results in clinical trials. The SURMOUNT-1 trial showed tirzepatide achieved approximately 20.9% average weight loss, while STEP-1 showed semaglutide achieved approximately 14.9% average weight loss.
Which medication is better for pancreatitis risk—Mounjaro or Wegovy?
There is no clear evidence that one carries significantly lower pancreatitis risk than the other—both medications carry the same warning. The choice between them depends on your individual health profile, medical history, and what your prescriber assesses as most suitable. A UK-licensed prescriber will evaluate your specific circumstances and discuss both options with you.
How is pancreatitis risk assessed before prescribing these injections?
A prescriber will ask about your personal or family history of pancreatic disease, gallbladder problems, and other relevant medical conditions. They will also check your current medications. Use our free BMI calculator to understand your weight category, and discuss any concerns about pancreatitis risk with your GP or a licensed prescriber before starting either medication.
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