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 You Need to Know
Some people taking Mounjaro (tirzepatide) experience weight gain rather than weight loss, which can be frustrating and confusing. This happens for several distinct reasons — from dose-related factors to lifestyle changes and underlying health conditions. Understanding why this occurs and knowing when to contact your GP is essential for getting the most from this prescription weight loss injection. This guide explains the most common causes and what you can do about it.
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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 gain on Mounjaro can occur for several reasons, even though clinical trials showed average weight loss of 15–21% of body weight. The most common causes include:
- Dose too low: If your starting dose (usually 2.5 mg) or current maintenance dose hasn't been increased adequately, appetite suppression may be insufficient. Mounjaro is titrated gradually — doses increase every 4 weeks. Some people need to reach 10 mg, 12.5 mg, or the maximum 15 mg dose to experience meaningful appetite reduction.
- Compensatory eating: Some people unconsciously increase portion sizes or snack frequency, offsetting the appetite-suppressing effects of the weekly weight loss injection. This is more common if lifestyle advice isn't followed alongside the medication.
- Fluid and muscle changes: Early weight fluctuations may reflect water retention or changes in muscle mass rather than fat gain. This is temporary but can mask fat loss on the scales.
- Underlying health conditions: Thyroid dysfunction, polycystic ovary syndrome (PCOS), and certain medications can interfere with weight loss despite Mounjaro.
- Medication interactions: If you take oral contraceptives, certain diabetes medications, or other prescriptions, absorption and efficacy may be affected. Your prescriber should review all medications.
Use our free GLP-1 weight loss calculator to understand realistic expectations at your current dose.
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.
When to Speak to Your GP or Prescriber
If you're gaining weight on Mounjaro, don't continue without addressing it — contact your GP or the prescriber who issued your prescription. Bring:
- A record of your weight over the past 4–8 weeks (use our free progress tracker to log changes)
- Details of your current dose and how long you've been on it
- A food and activity diary showing what you're eating and your exercise level
- Any other medications or health changes you've noticed
Your prescriber may increase your dose, review your diet and exercise approach, check for underlying health issues, or explore whether a different medication (such as Wegovy, the other licensed UK weight loss injection) might be more effective for you. Individual results vary, and finding the right dose and lifestyle combination takes time.
- 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 actually cause weight gain?
Mounjaro itself doesn't cause weight gain, but insufficient dose, dietary compensatory eating, fluid retention, or unaddressed health conditions can result in weight gain despite taking the injection. Clinical trials showed average weight loss, but individual results vary significantly. Your prescriber can review your dose and approach.
How long should I wait before expecting weight loss on Mounjaro?
Weight loss typically becomes noticeable after 4–8 weeks, but Mounjaro's doses increase every 4 weeks over 20 weeks to reach maximum effect. Some people don't see results until weeks 12–16 at higher doses. If you've been on your current dose for less than 4 weeks, it may be too early to judge effectiveness.
Is weight gain on Mounjaro a sign I should stop taking it?
No — stopping without medical advice is not recommended. Instead, contact your GP or prescriber. They can adjust your dose, review your lifestyle, check for underlying health issues, or discuss switching to an alternative weight loss injection. Don't change or stop your prescription independently.
What's the difference between Mounjaro and Wegovy if Mounjaro isn't working?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist; Wegovy (semaglutide) is a GLP-1 agonist only. Clinical trials showed Mounjaro produced slightly greater average weight loss, but individual responses differ. Your prescriber can discuss switching if needed — both are prescription-only UK weight loss injections requiring clinical assessment.
Should I change my diet and exercise if I'm on Mounjaro?
Yes — Mounjaro works best alongside reduced-calorie diet and increased physical activity. The medication reduces appetite but doesn't clinically studied weight loss without these lifestyle changes. Clinical trial participants combined the injection with behaviour change for best results. Your GP can refer you to a dietitian if helpful. ```
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.