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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Not Losing Weight on Mounjaro — What You Need to Know
If you're on Mounjaro and not seeing weight loss, you're not alone — and there are several reasons why this can happen. Mounjaro (tirzepatide) is a prescription weekly weight loss injection licensed in the UK, but individual results vary significantly. Some people experience slower weight loss than expected, and this doesn't mean the medication isn't working for you. This guide explains common reasons for slower progress and what steps you can take next 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
Several factors can affect how much weight you lose on Mounjaro, even if you're taking the medication correctly:
- Dose may be too low. Starting doses of 2.5 mg or 5 mg are intentionally conservative to assess tolerability. Many people don't reach meaningful weight loss until doses of 10 mg or higher. Your prescriber may recommend dose escalation if you've been on the same dose for at least 4 weeks.
- Diet hasn't changed enough. Mounjaro may help reduce appetite and cravings, but it's not a substitute for a reduced-calorie diet. Clinical trials showed weight loss alongside both dietary changes and increased physical activity. If calorie intake remains high, weight loss will be minimal.
- Portion sizes are still large. Even if you feel fuller on Mounjaro, you may still be eating larger portions than you realise. Using smaller plates, eating more slowly, and logging food intake can reveal where adjustments are needed.
- Hidden calories. Liquid calories (drinks, oils, sauces) and frequent snacking are easy to underestimate. These add up quickly without creating the sense of fullness that solid food provides.
- Physical activity is low. Weight loss happens primarily through diet, but increasing movement supports better results and cardiovascular health. Even 150 minutes of moderate activity per week can make a measurable difference.
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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 to Do if Weight Loss Has Stalled
If you've been on Mounjaro for 8+ weeks and weight loss has plateaued, contact your prescriber rather than stopping the medication. Options include dose escalation (if you're not already at 15 mg, the maximum), review of your diet and activity, or discussion about whether Mounjaro is the right choice for your situation. Your GP or private prescriber can also check for underlying health issues like thyroid problems or metabolic factors that might be slowing progress. Track your weight regularly using our free progress tracker to identify genuine plateaus versus normal weekly fluctuations — weight can vary by 2–3 kg day-to-day due to water retention, food volume, and hormones.
- 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 before Mounjaro starts working for weight loss?
Most people begin to notice appetite suppression within the first 2–4 weeks. Measurable weight loss typically becomes apparent after 4–8 weeks, though individual timelines vary. If you haven't seen any weight loss after 12 weeks, discuss this with your prescriber — dose adjustment or lifestyle review may help.
Does Mounjaro stop working after a few months?
Mounjaro doesn't "stop working" in the same way, but weight loss can plateau after initial loss. This is normal and happens with all weight loss methods. Continuing the medication and adjusting diet or activity can help break through plateaus. Stopping the medication often leads to weight regain.
Can I increase my Mounjaro dose myself to lose weight faster?
No. Mounjaro is prescription-only, and doses must be adjusted only by your prescriber following a clinical assessment. Doses increase gradually (minimum 4 weeks at each level) to manage side effects. Faster escalation increases nausea and other adverse effects without improving results.
What if I'm on 15 mg Mounjaro and still not losing weight?
At the maximum dose, if weight loss remains minimal after 12+ weeks, discuss alternative approaches with your prescriber. This might include referral to a specialist weight management service, review of medications affecting weight (some antidepressants or steroids worsen weight loss), or consideration of other options like bariatric surgery.
Does Mounjaro work better than Wegovy for weight loss?
Clinical trials showed Mounjaro (tirzepatide, a dual GIP+GLP-1 agonist) achieved slightly higher average weight loss (~21%) than Wegovy (semaglutide, a GLP-1 agonist only, ~15%), but results vary widely between individuals. Your prescriber will recommend based on your health profile and eligibility, not just theoretical efficacy differences.
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.