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
What to Do If Mounjaro Isn't Working
If you're taking Mounjaro and not seeing the weight loss results you expected, you're not alone — and it doesn't necessarily mean the medication is failing. Weight loss with tirzepatide varies significantly between individuals, and several factors could be affecting your results, including your current dose, how long you've been taking it, lifestyle factors, and underlying health conditions. This guide explains what "not working" might mean and when to speak with your prescriber.
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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.
Understanding Mounjaro Results & Timeline
Mounjaro works differently for different people, and expectations matter. Clinical trials showed average weight loss of 15–21% of body weight over 72 weeks, but this was alongside a reduced-calorie diet and increased physical activity. Some people see noticeable results within 4–8 weeks; others take 12–16 weeks to see significant changes.
If you've been on a stable dose for less than 12 weeks, your body may still be adjusting. Dose titration is key — Mounjaro starts at 2.5 mg and increases gradually (minimum 4 weeks per dose). If you're still on a lower dose like 5 mg or 7.5 mg, you may not have reached the dose where you see optimal results. Many people require the 10 mg, 12.5 mg, or 15 mg dose to experience meaningful appetite suppression.
Track your progress using our free weight loss calculator to see whether you're on track, and remember that weight loss isn't linear — some weeks you may lose nothing, then lose 1–2 kg the following week.
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 Contact Your Prescriber
If you've been on a stable dose for 16+ weeks with no weight loss, or if you experience concerning side effects that prevent you from continuing, contact your prescriber. They may recommend dose escalation, review your diet and exercise approach, or assess whether tirzepatide is the right medication for you.
Your GP or private prescriber can also check for underlying factors — certain medications (like some antipsychotics or steroids), hormonal conditions (thyroid issues, polycystic ovary syndrome), or metabolic factors may reduce medication effectiveness. Be honest about your calorie intake and activity level, as weight loss requires a consistent deficit alongside the medication.
- 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 does it take for Mounjaro to start working?
Most people notice appetite changes within 2–4 weeks, but significant weight loss typically takes 8–12 weeks. You need to be on a stable dose for at least 4 weeks before increasing. If you're still titrating up your dose, results may not be apparent until you reach a therapeutic dose (usually 10 mg or higher).
Could I need a higher dose of Mounjaro?
Yes. The maximum dose is 15 mg weekly. If you're on 5 mg or 7.5 mg and seeing no results after 12 weeks, ask your prescriber about escalating. Many people don't experience strong appetite suppression until 10 mg or above. Dose increases should happen at 4-week intervals minimum.
What if Mounjaro makes me feel too sick to eat?
Severe nausea or vomiting may mean your current dose is too high for you, or you may have gastric side effects that need managing. Contact your prescriber — they can suggest anti-nausea strategies, reduce your dose, or consider a different medication. Don't stop injecting without medical advice.
Does diet and exercise still matter if I'm on Mounjaro?
Absolutely. Mounjaro reduces appetite but doesn't force weight loss. Clinical trials showed results only when combined with a reduced-calorie diet and physical activity. The medication helps you feel fuller and eat less, but you still need a calorie deficit. Use our GLP-1 weight loss calculator to estimate realistic results based on your actual intake.
Should I switch to Wegovy if Mounjaro isn't working?
Not necessarily immediately. Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is GLP-1 only. Some people respond better to one mechanism than the other, but switching requires clinical assessment. Discuss options with your prescriber — they'll decide whether escalating your current dose, adjusting lifestyle, or trying a different medication is most appropriate.
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.