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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Gallstones and Mounjaro — What UK Patients Need to Know
Gallstones are a known side effect concern with weight loss medications including Mounjaro (tirzepatide) and Wegovy (semaglutide). Both are GLP-1-based prescription medications used for weight management in the UK, and both can potentially increase gallstone risk — particularly when weight loss is rapid. This guide explains the mechanism, compares clinical evidence between the two medications, and outlines what your GP should discuss with you during a clinical assessment. Understanding this risk is essential before starting any weight loss injection, and your prescriber should assess your individual gallstone risk factors.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
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.
Gallstone Risk — Mounjaro vs Wegovy: Clinical Evidence
Gallstones form when bile in the gallbladder hardens into crystals, often triggered by gradual weight management. Both Mounjaro (tirzepatide) and Wegovy (semaglutide) slow gastric emptying and reduce appetite, which can lead to significant weight loss — a known risk factor for gallstone formation.
Clinical trial data: The SURMOUNT trials (Mounjaro) and STEP trials (Wegovy) both reported gallbladder-related events, though these were relatively uncommon. In SURMOUNT-1, gallbladder events occurred in approximately 1.5% of participants on Mounjaro 15mg versus 0.5% on placebo. Similar rates appeared in STEP trials for Wegovy. Both medications carry this risk — there is no clear evidence that one is significantly safer than the other regarding gallstones.
Key difference: Mounjaro uses a dual GIP+GLP-1 mechanism (tirzepatide), while Wegovy is a GLP-1-only agonist (semaglutide). This mechanistic difference does not substantially alter gallstone risk — the risk appears linked to gradual weight management itself, not the medication type. Your prescriber will assess your personal risk during your clinical assessment using a Mounjaro KwikPen or Wegovy pen, adjusting your dose titration schedule if needed to manage risk.
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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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Individual Suitability — Prescriber Assessment and Eligibility
Your GP or licensed prescriber must assess your personal gallstone risk before recommending either medication. This includes reviewing your UK prescription eligibility — BMI thresholds, weight-related health conditions, and your medical history.
Red flags your prescriber will check: Previous gallstones or gallbladder surgery, gradual weight management history, family history of gallstones, and obesity-related metabolic conditions. If you have existing gallbladder disease, Mounjaro UK prescription access or Wegovy may be contraindicated or require careful monitoring.
Neither medication is recommended if you have severe, untreated gallbladder disease. Both require NICE approval context and NHS specialist assessment, or private clinical evaluation. The decision between Mounjaro versus Wegovy should be individualised — your prescriber will consider your gallstone risk, BMI, eligibility criteria, and overall health. Always use our free BMI calculator to check your starting point, and discuss all risk factors openly with your 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.
Does Mounjaro cause gallstones more than Wegovy?
Clinical trials show both medications have similar gallstone risk rates around 1-1.5%, likely due to gradual weight management rather than the medication type. Mounjaro's dual mechanism (GIP+GLP-1) does not appear to increase gallstone risk compared to Wegovy's GLP-1-only action. Individual risk depends more on your personal factors than the medication choice.
What are the signs of gallstones while taking weight loss injections?
Seek urgent medical attention if you experience severe pain in the upper right abdomen, pain between the shoulder blades, right shoulder pain, or nausea and vomiting. These may indicate gallbladder problems. Contact your GP or NHS 111 immediately — do not wait for your next injection.
Can I take Mounjaro if I have a history of gallstones?
This requires careful prescriber assessment. If you have had gallstones previously, your GP or prescriber must weigh the benefits of weight loss medication against your individual risk. Some cases may be contraindicated; others may be manageable with close monitoring. Always disclose your full medical history during your clinical assessment.
How does gradual weight management from these injections trigger gallstones?
gradual weight management changes bile composition and gallbladder function, causing cholesterol to crystallise into stones. Both Mounjaro and Wegovy can produce significant weight loss — on average 15-21% body weight with Mounjaro and around 15% with Wegovy — which increases gallstone risk. Slower, more gradual weight loss reduces this risk.
Will my prescriber monitor my gallbladder while I'm on Mounjaro or Wegovy?
Your prescriber may recommend baseline assessment if you have risk factors, but routine gallbladder monitoring during treatment is not standard unless you develop symptoms. You should report any upper abdominal pain, nausea, or digestive changes immediately. Always inform your GP of new symptoms — do not assume they are normal injection side effects.
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