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 Aren't I Losing Weight on Mounjaro?
If you're taking Mounjaro and not seeing weight loss, you're not alone. Weight loss on this prescription weight loss injection varies significantly between individuals, and there are several practical reasons why progress might be slower than expected. This guide explains the most common factors and what you can discuss with your prescriber.
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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 Slower Weight Loss on Mounjaro
Insufficient calorie deficit: Mounjaro supports appetite control, but weight loss still requires a reduced-calorie diet. Clinical trials showed participants lost weight alongside structured dietary changes — the medication alone is not enough. If you're eating more calories than your body burns, weight loss will stall.
Still in dose titration phase: Mounjaro starts at 2.5 mg and increases every 4 weeks (2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg maximum). Significant weight loss typically happens at higher maintenance doses. If you're within the first 12–16 weeks, your dose may still be too low for optimal effect.
Medication interactions: Certain medications can interfere with weight loss. Talk to your prescriber if you're taking diabetes medications, blood pressure tablets, or hormonal contraceptives — some may need adjustment.
Medical conditions: Thyroid disorders, PCOS, depression, and medication side effects can slow metabolism or increase appetite, offsetting Mounjaro's effects.
- Under-reporting food intake (including oils, condiments, drinks)
- Insufficient physical activity alongside dietary changes
- Alcohol consumption — alcoholic drinks contain hidden calories
- Stress and poor sleep — both can increase cortisol and hunger hormones
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 You Can Do Right Now
Track your intake honestly for a week using a food diary or app — you may be surprised how many calories slip in unnoticed. Use our free progress tracker to log weight, measurements, and how you feel, not just the number on the scales (muscle weighs more than fat).
If you've been on Mounjaro for 16+ weeks and still on a lower dose, ask your prescriber about moving to the next dose level — this is a standard part of the weekly weight loss injection titration process. Check whether side effects like nausea are reducing your ability to eat enough, which paradoxically can slow metabolism.
Review your BMI using our free BMI calculator to understand where you sit relative to NHS guidelines, then discuss realistic timescales with your GP. Weight loss is not linear — many people plateau for weeks, then lose several kilos suddenly.
- 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.
How long should I wait before expecting weight loss on Mounjaro?
Most people see noticeable weight loss after 8–12 weeks, but this varies. Since Mounjaro requires dose titration (increasing every 4 weeks), significant loss often happens from week 12 onwards when you reach higher maintenance doses. Clinical trials showed the most weight loss occurred in the later stages. If you're only a few weeks in or still on the starting dose, it may be too early to assess effectiveness.
Does Mounjaro stop working over time?
No — Mounjaro's effectiveness doesn't wear off due to the medication itself. However, weight loss can plateau as your body adapts to dietary changes and physical activity. This is normal. Plateaus typically last 2–4 weeks before weight loss resumes. Increasing physical activity or slightly reducing calories can help break a plateau. Discuss this with your prescriber if it persists beyond 4 weeks.
Can I lose weight on Mounjaro without changing my diet?
Unlikely. Mounjaro helps reduce appetite and cravings, but clinical trials showed weight loss happened alongside reduced-calorie diets and increased physical activity. The medication amplifies these efforts — it doesn't replace them. Without dietary changes, you may lose some weight due to reduced food intake, but results will be minimal compared to combining the injection with a structured approach.
What if I'm on the maximum Mounjaro dose and still not losing weight?
If you're on 15 mg (the maximum weekly weight loss injection dose) for 4+ weeks with no progress, discuss this with your prescriber. They may: review your food intake and activity levels, check for underlying medical conditions (thyroid, hormonal), assess medication interactions, or explore whether this medication is the best option for you. Some people respond better to alternative approaches.
Is weight loss on Mounjaro faster than on Wegovy?
Clinical trials showed Mounjaro (tirzepatide, a dual GIP/GLP-1 agonist) resulted in slightly higher average weight loss (~20.9% at maximum dose) compared to Wegovy/semaglutide (~15% in STEP trials). However, individual variation is huge. Some people lose more on Wegovy; others on Mounjaro. Your prescriber can discuss which might suit you better based on your health profile and previous responses to medication.
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.