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 Lost Any Weight On Mounjaro — Understanding Why
If you've been taking Mounjaro but haven't lost any weight yet, you're not alone — and this doesn't necessarily mean the medication won't work for you. Weight loss with Mounjaro varies significantly between individuals, and several factors can influence results, including dose level, how long you've been taking it, diet and activity levels, and individual metabolism. This guide explains what might be happening and what steps you can take with your NHS prescriber or GP.
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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.
Why You Might Not Be Losing Weight on Mounjaro
Mounjaro (tirzepatide) is a once-weekly weight loss injection that works by activating GIP and GLP-1 hormone pathways to reduce appetite and support feeling fuller. However, weight loss isn't automatic — it requires a calorie deficit, ideally alongside reduced-calorie diet and increased physical activity.
- You may still be in the titration phase: Mounjaro doses escalate every 4 weeks (2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg). Weight loss often accelerates at higher doses. If you're still in the first 4–8 weeks on a lower dose, your body may not yet be responding optimally.
- Individual variation is normal: Clinical trials showed average weight loss of 15–21% body weight, but some people lose more, some less, and some experience delayed response.
- Calorie intake may still be too high: Even with reduced appetite, eating above your maintenance calories will prevent weight loss. The medication helps with appetite — it doesn't assures a deficit.
- Underlying health factors: Certain medications, hormonal conditions (thyroid, PCOS), or metabolic factors can affect weight loss speed.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 Mounjaro Isn't Producing Weight Loss
If you haven't lost weight after 8–12 weeks on your current dose, contact your NHS prescriber or GP rather than stopping the medication. They may recommend:
- Continuing to the next dose: Your prescriber may increase your Mounjaro KwikPen dose if you're not yet at the maximum, as higher doses often produce better results.
- Reviewing diet and activity: A dietitian through NHS services can help identify whether calorie intake is the limiting factor.
- Checking for medication interactions: Some medications reduce GLP-1 effectiveness — your GP can review this.
- Extended timeframe: Some people respond after 12–16 weeks rather than 8–12. Weight loss isn't always linear.
- Switching medication: If Mounjaro truly isn't working after 16+ weeks at therapeutic dose, your prescriber might discuss Wegovy (semaglutide) as an alternative, though this is subject to NHS availability and your regional ICB decisions.
Use our free GLP-1 weight loss calculator above to track projected results at different doses, and our BMI calculator to monitor changes even if weight loss feels slow.
- 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 notice appetite reduction within 2–4 weeks, but meaningful weight loss typically appears after 8–12 weeks on therapeutic dose. Some people respond more slowly. Dose titration (increasing every 4 weeks) affects the timeline — higher doses produce stronger effects.
Could I be on the wrong dose of Mounjaro?
Yes — if you're still in the initial titration phase (weeks 1–12), you may not be on your full therapeutic dose yet. The maximum Mounjaro dose is 15 mg weekly. Ask your prescriber whether increasing your dose is appropriate after 4 weeks on your current level.
What if I'm doing everything right but still not losing weight?
Contact your NHS prescriber or GP. They can assess whether you're truly in a calorie deficit, check for medication interactions, review underlying health conditions, or discuss alternative medications like Wegovy if Mounjaro hasn't worked after 16+ weeks.
Does Mounjaro work for everyone?
No — not everyone responds equally. Clinical trials showed average weight loss of 15–21%, but individual results vary. Some people lose significantly more, others lose less, and a small percentage see minimal results. This doesn't mean failure — it may mean a different medication or approach suits you better.
Can I stop Mounjaro if it's not working?
Don't stop without speaking to your prescriber first. Stopping early prevents you from reaching your optimal dose, where the medication is most effective. Your GP can advise whether to continue, adjust, or switch medications based on your specific situation.
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.