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
Why Am I Putting Weight On With Mounjaro?
If you're gaining weight while taking Mounjaro, you're not alone — some people experience unexpected weight gain despite using this prescription weight loss injection. This can happen for several reasons, from dose-related factors to lifestyle changes or underlying health conditions. Understanding why this happens, and knowing when to speak to your GP, is important for getting your treatment back on track.
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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.
Common Reasons for Weight Gain on Mounjaro
Weight gain while taking Mounjaro can occur for several reasons. Dose-related factors are common: if you're still in the early titration phase (weeks 1–12), your dose may not yet be high enough to produce significant appetite suppression. Mounjaro requires dose escalation every 4 weeks, starting from 2.5 mg up to the maximum 15 mg weekly injection. If you've recently stopped or paused your treatment, weight regain is typical as appetite hormones normalise.
Lifestyle changes matter significantly. Weight loss medications work alongside dietary changes and increased physical activity — if calorie intake has drifted upwards or exercise has reduced, weight gain can occur even on the medication. Medical factors also play a role: changes in thyroid function, certain medications, hormonal shifts (particularly in women), or undiagnosed conditions like polycystic ovary syndrome (PCOS) can interfere with weight loss efforts. Additionally, some people experience plateaus or rebound where their body adjusts to the medication's effects.
- Insufficient dose for your body weight or metabolism
- Early treatment phase — dose still titrating upwards
- Increased calorie intake or reduced physical activity
- Medication interactions or other health conditions
- Reduced appetite suppression as your body adapts
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.
What to Do — NHS & GP Guidance
If you're gaining weight on Mounjaro, contact your GP or the NHS specialist weight management service that prescribed it. Don't wait — regular monitoring is part of NHS tier 3 weight management care. Your prescriber can assess whether your current dose is appropriate for your body weight, whether the injection technique is correct (poor absorption can reduce effectiveness), and whether dose escalation to the next level is needed.
Keep a food diary and activity log for 1–2 weeks before your appointment — this helps identify whether lifestyle factors are contributing. Use our GLP-1 weight loss calculator to compare your expected results at different doses based on clinical trial data. Your GP may also recommend checking thyroid function or other blood tests to rule out underlying conditions. Never adjust or stop Mounjaro without medical guidance — always discuss your concerns with your prescriber first.
[SECTION 3 CONTENT ALREADY COMPLETE IN COL_J ABOVE — NO ADDITIONAL COL_J]- 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 itself doesn't typically cause weight gain, but weight gain can occur if your dose is insufficient, you're early in titration, or lifestyle factors have changed. If you're on a stable high dose (10–15 mg) and still gaining, underlying medical conditions may be involved — speak to your GP.
Should I stop Mounjaro if I'm gaining weight?
No — never stop Mounjaro without medical guidance. Contact your GP or specialist weight management service instead. They can adjust your dose, review your technique, check for medical issues, or discuss whether the medication is right for you.
How long does it take for Mounjaro to work?
Mounjaro requires dose escalation every 4 weeks, starting at 2.5 mg. Most people reach therapeutic doses (10–15 mg) by weeks 17–20. Weight loss typically becomes noticeable at higher doses. If you're still in early titration, weight loss may not be obvious yet.
What's the difference between Mounjaro and Wegovy for weight loss?
Both are prescription weight loss injections used in the UK. Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Mounjaro may have a slightly stronger appetite-suppressing effect, but individual results vary significantly.
Can my GP adjust my Mounjaro dose on the NHS?
If you're prescribed Mounjaro through an NHS specialist tier 3 weight management service, yes — your prescriber can adjust your dose at each 4-week review. If you're on a private prescription, you'll need to discuss dose changes with your private prescriber during follow-up appointments.
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.