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 Much Weight On Mounjaro — What You Should Know
If you're on Mounjaro but not seeing the weight loss results you expected, you're not alone — individual responses to GLP-1 and GIP receptor agonists vary significantly. Weight loss on prescription weight loss injections depends on many factors: dose, how long you've been taking it, diet adherence, physical activity, metabolism, and individual biological response. This guide explains why weight loss may be slower than expected, how Mounjaro compares to Wegovy, and what to discuss with your prescriber. Remember that even modest weight loss brings real health benefits.
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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.
How Mounjaro And Wegovy Work Differently — Trial Data Comparison
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two separate hormone pathways, which is why clinical trials showed an average weight loss of approximately 15–21% of body weight over 72 weeks when used alongside a reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates a single hormone pathway. STEP trial data showed an average weight loss of approximately 14.9% of body weight at 68 weeks under similar lifestyle conditions.
The difference in average weight loss between the two is modest — around 5–7 percentage points — but individual results vary considerably. Some people respond better to Mounjaro's dual mechanism, whilst others achieve excellent results with Wegovy alone. Neither medication is assured to produce significant weight loss, and both require clinical assessment before being prescribed. Your GP or prescriber can help determine which is more suitable based on your individual health profile.
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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.
Why Weight Loss May Plateau — Common Reasons And Next Steps
Weight loss plateaus on any weekly weight loss injection are common and usually temporary. Possible reasons include: not yet on your optimal maintenance dose (Mounjaro typically requires 16+ weeks to reach the 15 mg maximum dose), inadequate calorie deficit despite reduced appetite, insufficient physical activity, rapid initial weight loss followed by natural stabilisation, or individual variation in medication response.
What to do: Track your food intake honestly using a food diary or app — appetite suppression from Mounjaro can mask portion sizes. Check you're moving enough; increasing activity often restarts weight loss. Give your current dose at least 4 weeks before escalating. Use our free GLP-1 weight loss calculator to review realistic projections at your current dose. If you're at maximum dose and plateau persists beyond 12 weeks, discuss alternatives with your prescriber — they may suggest switching to Wegovy, increasing other medication doses, or reviewing your diet and exercise plan.
- 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.
Should I switch from Mounjaro to Wegovy if I'm not losing weight?
Not necessarily immediately. First, ensure you're on your optimal dose (usually 15 mg for Mounjaro, which takes 16+ weeks to reach), maintaining a genuine calorie deficit, and exercising regularly. If you've been on maximum dose for 12+ weeks with minimal loss, discuss switching with your prescriber. Wegovy may suit you better, though there's no assures it will produce different results — individual response varies. Your prescriber can assess whether a switch is appropriate based on your health profile.
How long does it take to see weight loss results on Mounjaro?
Most people notice appetite reduction within 1–2 weeks, but measurable weight loss typically appears after 4–8 weeks once you're on an effective dose. Since Mounjaro requires dose escalation (increasing every 4 weeks from 2.5 mg to 15 mg), the full weight loss benefit may not be visible until weeks 16–20. Patience and consistent diet adherence are essential. Use our progress tracker to log weekly changes — sometimes the scale moves slower than expected but other metrics improve first.
Is Mounjaro vs Wegovy just about which one causes more weight loss?
No. While trial data shows Mounjaro averages slightly higher percentage weight loss, individual responses are highly variable — some people lose more on Wegovy, others on Mounjaro. The choice depends on: your personal response, side effect tolerance, injection frequency preference (both are weekly, but pens differ), cost, NHS or private access, and prescriber recommendation. Both are prescription-only and require clinical assessment. Your prescriber will help identify which is more suitable for you.
What if I'm doing everything right but still not losing weight?
This can happen — medication response varies biologically. Ensure you're genuinely in a calorie deficit (track honestly for 2–3 weeks), exercising, and sleeping well. Rule out medical causes: thyroid problems, PCOS, medication side effects, or metabolic adaptation can slow loss. Discuss with your GP if loss is completely absent after 12 weeks on maximum dose. You may need blood tests, a dose adjustment, a switch to alternative medication, or a different weight management approach — your prescriber can advise based on your full health picture.
Can I use the BMI calculator to track progress on Mounjaro?
Yes — our free BMI calculator helps you monitor your BMI change as you lose weight, which is useful because BMI alone doesn't reflect the full picture. Track your BMI alongside waist circumference, how clothes fit, energy levels, and blood pressure — these often improve before significant scale weight loss. Remember that BMI has limitations and doesn't account for muscle gain from exercise, so use it as one of several progress measures alongside our free progress tracker for ongoing accountability.
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.