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
Gaining Weight On Mounjaro — What You Need to Know
It can be concerning if you're gaining weight on Mounjaro when you expected weight loss. While clinical trials showed an average weight loss of 15–21% body weight, individual results vary significantly—and some people experience weight gain or weight stability despite being on the medication. This doesn't mean Mounjaro isn't working for you; it means your prescriber needs to assess your individual situation. In the UK, whether you access Mounjaro through NHS tier 3 services or via private prescription, weight changes should always be discussed with your healthcare provider.
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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 Happen on Mounjaro
Weight gain or lack of weight loss on Mounjaro can happen for several reasons, and none necessarily mean the medication is failing. Dose too low: If you're still in the early titration phase (starting at 2.5 mg and escalating weekly), your dose may not yet be optimally effective. The NHS guidance and clinical practice suggest that most people require at least 7.5–10 mg to see meaningful effects. Adherence issues: If injections are missed or the medication isn't being used consistently, weight loss effects won't be achieved. Diet and activity unchanged: Mounjaro works alongside lifestyle changes, not instead of them. A reduced-calorie diet and increased physical activity are essential for weight loss. Medication interactions: Certain medications or health conditions can affect metabolism or weight regulation. Underlying health conditions: Type 2 diabetes, PCOS, thyroid dysfunction, or other metabolic conditions may slow weight loss even on medication. Body composition changes: Some people build muscle while losing fat, which may not register as weight loss on scales.
Clinically BackedWeight Loss
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
What to Do If You're Gaining Weight on Mounjaro
If you're gaining weight on Mounjaro, the first step is to contact your prescriber or the NHS tier 3 weight management service that prescribed it. Your prescriber will review your current dose, injection technique, diet adherence, and activity levels. They may recommend continuing to the next dose if you haven't reached your maintenance dose yet, as the trial dose response typically shows improvements at higher doses. They may ask about appetite suppression—if you're not feeling reduced appetite, the dose may need adjustment. Use our free BMI calculator to track your BMI alongside weight, as body composition changes matter more than the number on the scales. Keep a food and activity diary to share with your prescriber; this helps identify whether lifestyle factors are limiting results. Be honest about any missed doses or injection issues. If weight gain continues despite optimal dosing and lifestyle adherence, your prescriber may explore whether Mounjaro is the right choice for you or whether a weekly weight loss injection alternative might be more effective.
- 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 weight gain?
Mounjaro is not designed to cause weight gain, but weight gain can occur if the dose is too low, injections are missed, or lifestyle changes aren't in place. Clinical trials showed average weight loss, but individual responses vary. Weight gain may indicate your dose needs adjusting or that lifestyle factors need review. Always discuss this with your prescriber.
How long does it take to see weight loss on Mounjaro?
Most people see meaningful weight changes within 8–12 weeks when on an effective maintenance dose (typically 7.5 mg or higher) alongside dietary changes and physical activity. If you're still in the titration phase (first 16–20 weeks), you may not yet be on your optimal dose. Your prescriber will monitor your progress.
What if I've missed doses of Mounjaro?
Missed doses will delay or reduce weight loss effects. If you missed a dose within 4 days, take it as soon as remembered and continue your normal schedule. More than 4 days late? Skip that dose and take your next one on schedule. Consistency matters for results.
Can I switch from Mounjaro if it's not working for weight loss?
Yes. If you're not seeing results after reaching maintenance dose with good adherence, your prescriber can explore alternatives. In the UK, weekly weight loss injection alternatives include Wegovy (semaglutide). Any switch must be managed by your prescriber following a clinical assessment.
Should I change my diet if I'm not losing weight on Mounjaro?
Yes. Mounjaro reduces appetite and supports fullness, but it's not a replacement for a reduced-calorie diet and physical activity. If you're not losing weight, review your calorie intake with your prescriber or a dietitian. Our GLP-1 weight loss calculator can help you estimate results when lifestyle changes are optimised.
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.