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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Why Does Mounjaro Cause Gallstones?
Mounjaro (tirzepatide) may increase the risk of gallstone formation, though gallstones are not assured to develop. This happens because Mounjaro slows how quickly food moves through your stomach and reduces bile flow, creating conditions where gallstones can form more easily. If you're considering Mounjaro as a prescription weight loss treatment in the UK, understanding this potential side effect and knowing what symptoms to watch for is important. Your GP or prescriber will assess this risk during your clinical assessment before starting treatment.
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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.
How Mounjaro Affects Gallbladder Function
Mounjaro is a dual GIP and GLP-1 receptor agonist that works primarily by reducing appetite and slowing gastric emptying — the rate at which food leaves your stomach. This slower digestion can reduce the frequency of gallbladder contractions, which normally release bile to help break down fats. When the gallbladder contracts less often, bile can become more concentrated in the gallbladder, and cholesterol can crystallise and form gallstones.
Additionally, gradual weight management itself — which occurs with Mounjaro treatment — increases gallstone risk. When you lose weight quickly, your liver releases more cholesterol into the bile, further raising the likelihood of stone formation. This is why gallstones are more common during significant weight loss, whether from medication, calorie restriction, or bariatric surgery. Clinical monitoring and awareness of symptoms are essential during Mounjaro treatment in the UK.
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How It Works
Three simple steps to understand your weight and take action.
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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Symptoms to Watch and When to Seek Help
Not everyone on Mounjaro develops gallstones, and some people have gallstones without any symptoms. However, watch for sudden, severe pain in the upper right abdomen, pain between the shoulder blades, or pain in your right shoulder — these are classic signs of gallstone-related problems. Other warning signs include nausea, vomiting, and pale-coloured stools. If you experience sudden, severe stomach pain radiating to your back while taking Mounjaro, seek urgent medical attention immediately, as this could indicate pancreatitis or acute cholecystitis (gallbladder inflammation).
Your GP may recommend an ultrasound scan to check your gallbladder before starting Mounjaro, especially if you have risk factors. During treatment, report any abdominal pain or digestive changes to your prescriber promptly. Early detection and management prevent serious complications.
- 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 gallstone risk higher with Mounjaro than with Wegovy?
Both GLP-1 medications and tirzepatide carry gallstone risk because they slow gastric emptying and gradual weight management increases risk. There's no established evidence that Mounjaro carries significantly higher gallstone risk than Wegovy (semaglutide), though individual risk varies based on genetics, diet, and weight loss rate.
Can I prevent gallstones while taking Mounjaro?
Eating regular meals with adequate fat content helps stimulate gallbladder contractions. Avoid very low-calorie diets alongside Mounjaro. Stay hydrated, maintain steady (not rapid) weight loss, and follow your prescriber's guidance on dose titration. However, some people develop gallstones regardless of prevention efforts.
What happens if I develop gallstones on Mounjaro?
Not all gallstones require treatment. If asymptomatic (no symptoms), your GP may monitor you. If symptomatic or complications develop, you may need dietary changes, medication, or in rare cases, surgical removal of the gallbladder. Your prescriber will advise whether to continue Mounjaro.
Does the NHS cover gallbladder ultrasounds before Mounjaro?
This depends on your tier 3 specialist weight management service and clinical need. If you have risk factors (age, family history, previous gallstones), your NHS service may recommend screening before starting. Private prescribers may also recommend baseline imaging.
How common are gallstones on Mounjaro in the UK?
Exact UK prevalence data is limited, but clinical experience suggests gallstone symptoms occur in a minority of users. Weight loss itself — not just the medication — contributes to risk. Your individual risk depends on genetics, diet, weight loss speed, and other factors your GP can discuss with you.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.