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
Free tracker included with every download this week
Can You Gain Weight On Mounjaro?
Yes, it is possible to gain weight while taking Mounjaro, despite it being a prescription weight loss injection licensed in the UK. Weight changes on Mounjaro depend on several factors including diet, physical activity, dose, and individual metabolism. Most people lose weight on Mounjaro when used as directed with a reduced-calorie diet and increased exercise, but weight loss is not assured. Understanding how Mounjaro works and what influences weight changes can help you use it more effectively alongside lifestyle adjustments.
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Calculate Your BMI Instantly
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.
Why Weight Gain Can Occur On Mounjaro
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist that may help reduce appetite and support feeling fuller after eating. However, the medication itself does not force weight loss—it works by modulating hunger signals. Weight gain while taking Mounjaro typically occurs due to:
- Consuming more calories than your body burns—Mounjaro reduces appetite, but if you still eat beyond your maintenance needs, weight will increase
- Stopping or reducing physical activity—Exercise is essential for sustainable weight loss and metabolic health
- Side effect adaptation—Nausea and reduced appetite often ease after a few weeks, and some people may gradually increase food intake as symptoms subside
- Dose-related factors—Lower starting doses (2.5 mg) may provide less appetite suppression than higher maintenance doses (15 mg)
- Individual variation—People metabolise Mounjaro differently; some experience stronger appetite suppression, others less pronounced effects
Clinical trials showed an average weight loss of 15–21% body weight at the highest dose, but individual results vary significantly. Weight gain is not inevitable, but it requires consistent adherence to a reduced-calorie diet and lifestyle changes.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Managing Weight While On Mounjaro
To minimise the risk of weight gain on Mounjaro, maintain a structured approach to nutrition and activity. Track your food intake and calorie balance—use a food diary or app to ensure you are in a modest calorie deficit. The NHS recommends losing no more than 0.5–1 kg per week; rushing weight loss often leads to rebound weight gain. Stay hydrated, eat protein-rich foods to preserve muscle mass, and engage in regular physical activity—both cardiovascular exercise and strength training. If you are gaining weight despite taking Mounjaro as prescribed, discuss this with your prescriber. They may adjust your dose, review your diet, or assess whether the medication is right for you. Our free progress tracker can help you monitor your weight trends over time and identify patterns in your results.
- 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.
What should I do if I gain weight on Mounjaro?
Contact your prescriber immediately. Weight gain on Mounjaro usually indicates that calorie intake exceeds expenditure or there are underlying factors affecting the medication's effectiveness. Your prescriber may review your dose, adjust your treatment plan, or refer you to a dietitian. Ensure you are following a reduced-calorie diet and engaging in regular physical activity alongside the injection.
Does Mounjaro clinically studied weight loss?
No. Mounjaro may help support weight loss by reducing appetite, but clinical trials showed average weight loss of 15–21% body weight—individual results vary significantly. Weight loss requires consistent dietary changes and increased physical activity. Mounjaro is a tool to support these efforts, not a substitute for lifestyle changes.
How long before Mounjaro starts working for appetite?
Most people notice appetite suppression within the first 2–4 weeks of starting Mounjaro or increasing their dose. However, some experience side effects like nausea that may reduce appetite initially, while appetite can gradually return as your body adapts. This is why dose escalation is important—it helps maintain appetite suppression over time.
Can I gain weight back after stopping Mounjaro?
Yes, weight regain is common after stopping GLP-1 medications like Mounjaro if lifestyle changes are not maintained. Research suggests that without continued healthy eating and exercise habits, people may regain 50% or more of lost weight within a year of stopping. Sustainable weight management requires long-term commitment to diet and activity.
Is Mounjaro available on the NHS for weight loss?
Mounjaro is available through NHS specialist tier 3 weight management services in some areas, though availability and waiting times vary significantly by region. Private access is available through UK-licensed prescribers following an online clinical assessment. Your GP can advise you on local NHS options or private alternatives suitable for your circumstances.
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.