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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Putting On Weight With Mounjaro — What This Means
Weight gain while taking Mounjaro is unusual but can happen for several reasons — including dose timing, lifestyle factors, medication interactions, or underlying health changes. If you're putting on weight with Mounjaro through the NHS, it's important to understand why this might be occurring and what steps you can take with your GP or specialist weight management service. This guide covers the causes, what NHS guidance suggests, and how to move forward.
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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.
Why Weight Gain Occurs on Mounjaro — NHS Context
Mounjaro is a GIP and GLP-1 receptor agonist — a weekly weight loss injection licensed by the MHRA for UK adults with a BMI of 30 or above, or 27+ with weight-related health conditions. Clinical trials showed average weight loss of 15–21% body weight, but individual results vary significantly.
Weight gain despite Mounjaro may occur due to:
- Dose too low: If you're on an earlier dose (2.5 mg, 5 mg, 7.5 mg), your body may not yet be responding optimally. Doses increase every 4 weeks — weight gain sometimes reverses at higher doses.
- Lifestyle changes: Increased calorie intake, reduced physical activity, or stress-related eating can offset the medication's effects.
- Medication interactions: Other prescribed medications can affect appetite or metabolism — discuss all medications with your GP.
- Underlying health changes: New thyroid problems, hormonal changes, or other conditions may need investigation.
- Injection technique or storage: Improper storage (freezing, heat exposure) or injection timing can affect medication effectiveness.
Your NHS tier 3 or 4 weight management specialist should assess these factors during your next review appointment.
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.
Next Steps — What Your NHS Specialist Will Do
If you're putting on weight with Mounjaro on the NHS, contact your specialist weight management service immediately — do not wait for your next scheduled appointment. Your prescriber will:
- Review your current dose and timeline — most people require 8–12 weeks to see weight loss effects.
- Assess your diet and physical activity using the same free BMI calculator and progress tracker available to track trends over time.
- Check for medication interactions or health changes.
- Consider dose escalation, switching to an alternative GLP-1 medication (such as Wegovy), or reviewing your suitability for continuing treatment.
Weight loss medications work best alongside a reduced-calorie diet and increased physical activity — your NHS service will provide dietitian and behaviour change support.
- 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 weight gain on Mounjaro common?
No. Clinical trials showed most people lose weight, but individual responses vary. Some experience slower weight loss or plateau. True weight gain while on Mounjaro is uncommon and usually signals a need to review your dose, lifestyle, or underlying health — discuss this urgently with your NHS specialist.
Could my dose be too low?
Yes. Mounjaro starts at 2.5 mg and increases every 4 weeks up to 15 mg maximum. Early doses (2.5–5 mg) are adjustment phases. If you're still in week 1–12, weight gain may reverse as your dose escalates. Your specialist will monitor this and adjust accordingly.
Should I stop taking Mounjaro if I'm gaining weight?
No — do not stop without guidance from your NHS specialist or GP. Stopping abruptly can trigger rebound weight gain. Instead, contact your weight management service to review your treatment plan. There may be simple explanations or alternative options.
What if Mounjaro isn't working for me?
If after 12+ weeks at therapeutic dose (10–15 mg) you've gained weight or seen no change, your NHS specialist may recommend switching to an alternative weekly weight loss injection such as Wegovy, or reviewing your overall suitability for pharmacological treatment.
Can diet alone explain weight gain on Mounjaro?
Absolutely. Mounjaro supports appetite reduction but does not clinically studied weight loss if calorie intake increases significantly. Use our free progress tracker and BMI calculator to log your weight and food intake — this data helps your NHS team identify dietary factors contributing to weight gain.
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.