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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Gaining Weight On Mounjaro — What You Need to Know
If you're gaining weight on Mounjaro, you're not alone — and it doesn't necessarily mean the medication isn't working. Weight changes can happen for several reasons during treatment with this weekly weight loss injection, from side effects affecting appetite to dose adjustment periods. This guide explains what may cause weight gain whilst taking Mounjaro, when it's typical, and when to speak to your GP. Understanding these patterns helps you stay informed about your prescription weight loss journey.
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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 Happens on Mounjaro
Weight fluctuations during Mounjaro treatment are more common than many people realise. Several factors may contribute:
- Early treatment phase: During dose titration (the gradual increase from 2.5 mg to your maintenance dose), appetite suppression may not be consistent, and some people experience increased appetite during adjustment weeks as their body adapts.
- Nausea and constipation side effects: Ironically, severe nausea or constipation can reduce your ability to follow a reduced-calorie diet properly, affecting weight loss momentum.
- Fluid retention: Some people report temporary water retention, particularly in the first few weeks of treatment.
- Reduced activity levels: Fatigue and dizziness (listed as common side effects) may temporarily reduce physical activity, impacting overall energy expenditure.
- Dietary patterns: If side effects like nausea or reflux lead to eating very small amounts, your body may slow metabolism slightly. Consistency with your reduced-calorie diet remains essential alongside the medication.
It's important to remember that Mounjaro works best alongside sustained lifestyle changes. The weekly weight loss injection is a support tool, not a standalone solution.
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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.
When to Contact Your GP
Speak to your prescriber if:
- You've gained more than 2–3 kg despite being on a stable dose for 4+ weeks.
- Weight gain is accompanied by severe or worsening nausea, vomiting, or abdominal pain (which may indicate other issues requiring assessment).
- You're struggling to follow your reduced-calorie diet due to side effects.
- You suspect an underlying condition (thyroid changes, other medications, hormonal shifts) might be affecting your weight.
Your GP or specialist can review your dose, check your technique, assess side effects, and adjust your treatment plan. Use our free BMI calculator to track your current status, and our GLP-1 weight loss calculator to revisit projected outcomes at your current dose. Recording your weight regularly with our progress tracker helps you and your healthcare team identify real trends versus normal fluctuations.
- 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 common when starting Mounjaro?
Weight fluctuations during early treatment are relatively common, especially during dose titration (weeks 1–4 on each dose). Most people see weight loss once they reach a stable maintenance dose and their body adjusts to the medication. However, if weight gain persists beyond 4 weeks on a stable dose, discuss this with your GP.
Does Mounjaro stop working if I gain weight?
Not necessarily. Weight gain doesn't automatically mean Mounjaro is ineffective. It may indicate side effects are affecting your diet adherence, you're in an early adjustment phase, or there's a separate health factor at play. Your prescriber can assess your individual situation and make adjustments.
Can I increase my Mounjaro dose if I'm gaining weight?
Dose increases should only be made by your prescriber following their guidance on dose titration schedules (minimum 4 weeks between increases). Never increase your dose without clinical advice. If weight gain is a concern, discuss it with your GP rather than self-adjusting.
What's the difference between Mounjaro and Wegovy for weight loss?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. Both are prescription-only weekly weight loss injections available in the UK. Clinical trials showed Mounjaro produced slightly higher average weight loss (~21% vs ~15% body weight), but individual results vary significantly.
Should I stop taking Mounjaro if I'm gaining weight?
Never stop any prescription medication without speaking to your GP first. If you're experiencing weight gain, contact your prescriber to discuss the cause and explore solutions — whether that's adjusting your dose, changing your dietary approach, or investigating other health factors affecting your weight.
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.