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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Why Mounjaro Might Not Be Working for You
If Mounjaro doesn't seem to be working, you're not alone — weight loss responses vary significantly between individuals. Many people expect immediate results, but tirzepatide typically takes time to show effects, and several factors may be affecting your progress. Understanding what "not working" really means, how long to wait, and when to speak to your prescriber can help you make an informed decision about your next steps. This guide explains the common reasons and what you can do.
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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.
Common Reasons Mounjaro May Not Seem to Be Working
Dose too low: You may still be in the titration phase — Mounjaro starts at 2.5 mg and increases every four weeks. Maximum therapeutic effect is typically reached at 10 mg or 15 mg. If you're on a starter dose, weight loss may be minimal.
Not enough time has passed: Clinical trials showed average weight loss over 72 weeks. Significant results often take 8–12 weeks to become noticeable. Expecting results after 2–3 weeks is unrealistic.
Lifestyle factors: Mounjaro works alongside a reduced-calorie diet and increased physical activity — not instead of them. If calorie intake hasn't changed or activity is low, results will be limited. Our free GLP-1 weight loss calculator can help you estimate realistic timelines.
Plateau: After initial weight loss, plateaus are normal. Your body may need 4–8 weeks at a stable dose before losing more weight.
Medical factors: Certain medications, thyroid conditions, PCOS, or metabolic issues can slow weight loss even with tirzepatide.
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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 Mounjaro Isn't Delivering Results
First, check the timeline: If you've been on Mounjaro for less than 8 weeks, continue — results often accelerate. If you're still on doses below 10 mg, dose escalation may make a difference.
Second, review your lifestyle: Use our free BMI calculator to track your current status, and honestly assess whether you're maintaining a calorie deficit. Even with Mounjaro, eating at maintenance calories (or surplus) will prevent weight loss.
Third, speak to your prescriber: If you've been on a stable, adequate dose (10 mg or higher) for 12+ weeks with no progress, discuss whether Mounjaro is right for you. Your prescriber may adjust your dose, investigate underlying medical causes, or discuss alternatives like tirzepatide dose increase, switching to Wegovy (semaglutide), or other approaches.
Be honest about adherence too — missed injections or not following dietary guidance will reduce effectiveness.
- 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 Mounjaro take to work?
Most people begin noticing appetite suppression within days, but meaningful weight loss typically takes 4–8 weeks. Clinical trial data showed average weight loss over 72 weeks. Results vary significantly — some see progress at 4 weeks, others take 12+ weeks. Dose matters: starter doses (2.5–5 mg) produce less effect than therapeutic doses (10–15 mg).
Does Mounjaro work without diet and exercise?
No. Mounjaro is most effective alongside a reduced-calorie diet and increased physical activity. It reduces appetite and helps you feel fuller, but it doesn't burn calories for you. If you're eating at maintenance calories or above, even with Mounjaro you won't lose weight. The medication is a tool, not a replacement for lifestyle change.
What dose of Mounjaro actually works for weight loss?
Clinical trials showed average weight loss starting at 5 mg, with more significant results at 10 mg (~19.5% body weight) and 15 mg (~20.9% body weight) over 72 weeks. Starter doses (2.5 mg) are mainly for tolerability — meaningful weight loss typically begins at 5 mg and increases with escalation. Your prescriber will titrate your dose gradually.
Should I switch from Mounjaro to Wegovy if it's not working?
Possibly, but discuss this with your prescriber first. Mounjaro (dual GIP+GLP-1 agonist) and Wegovy (GLP-1 agonist only) work slightly differently. If Mounjaro hasn't worked after 12+ weeks at adequate dose (10 mg+), switching to Wegovy is one option your prescriber might consider. Individual responses vary — some people respond better to one over the other.
Could a medical condition be blocking Mounjaro's effectiveness?
Yes. Thyroid disorders, PCOS, metabolic syndrome, certain medications (like some antipsychotics), and adrenal issues can slow weight loss. If you've been on Mounjaro properly for 12+ weeks with no progress, ask your GP or prescriber to investigate underlying conditions. They may run blood tests to check thyroid function, glucose metabolism, or hormone levels.
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.