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 Work for Weight Loss
Mounjaro (tirzepatide) can be an effective weight management tool, but it doesn't work for everyone. Some people experience minimal weight loss or no change despite taking the medication correctly. This can happen for several reasons, ranging from dose-related factors to lifestyle habits, individual body chemistry, and underlying health conditions. Understanding why Mounjaro may not deliver expected results can help you and your GP or prescriber identify the best next steps for your weight management journey.
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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 Be Effective
Several factors can affect how well Mounjaro works for weight loss. Insufficient dose escalation is common — Mounjaro requires gradual dose increases starting at 2.5 mg, stepping up every 4 weeks through 5 mg, 7.5 mg, 10 mg, 12.5 mg, and reaching the maximum of 15 mg. Some people need the higher doses to experience meaningful appetite reduction, but this takes 20+ weeks to reach. Lifestyle factors matter significantly — Mounjaro works alongside a reduced-calorie diet and increased physical activity, not instead of them. If calorie intake remains high or activity levels are low, weight loss will be limited. Individual body chemistry varies — some people are simply less responsive to tirzepatide than others, particularly if they have insulin resistance or certain metabolic conditions. Additionally, medication interactions or contraindications with oral medications (which may reduce absorption) or certain medical conditions (like severe gastroparesis) can reduce effectiveness. Use our free GLP-1 weight loss calculator to understand potential outcomes at different doses.
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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.
Medical Factors That Can Reduce Mounjaro Effectiveness
Several health conditions and medications can interfere with Mounjaro's weight loss effects. Insulin resistance and polycystic ovary syndrome (PCOS) may require higher doses or longer treatment duration to see results. Type 2 diabetes or prediabetes can affect how quickly Mounjaro reduces appetite — blood sugar control sometimes takes priority in how your body responds. Thyroid disorders (particularly hypothyroidism) slow metabolism and can mask Mounjaro's effects if untreated. Certain medications — particularly steroids, some antipsychotics, and antidepressants — can increase appetite or promote weight gain, potentially offsetting Mounjaro's benefits. If you're taking multiple medications or have complex health conditions, your GP or prescriber can assess whether Mounjaro is genuinely suitable for you and whether adjustments to other treatments might help. Check your personal BMI using our free BMI calculator to understand your starting point and realistic weight loss targets.
- 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 start working?
Mounjaro may begin reducing appetite within the first week of the starting dose, but meaningful weight loss typically takes 4–8 weeks. Since the dose increases every 4 weeks, the most noticeable effects often occur around weeks 8–12 when you reach a therapeutic dose (5 mg or higher). Some people don't experience significant appetite reduction until reaching higher doses (10 mg+), which requires months of gradual titration.
What if I'm on the maximum dose of Mounjaro and still not losing weight?
If you're on 15 mg (maximum dose) for at least 4 weeks and experiencing no weight loss, discuss this with your prescriber immediately. Possible next steps include confirming adherence to a reduced-calorie diet, investigating underlying medical conditions (thyroid, insulin resistance, medication interactions), or considering whether Mounjaro is the right medication for you. Your prescriber might recommend returning to a specialist tier 3 weight management service for comprehensive assessment.
Can I take a higher dose of Mounjaro than 15 mg?
No. The maximum licensed dose in the UK is 15 mg once weekly. Doses beyond 15 mg have not been approved and are not recommended. If 15 mg is ineffective after adequate time, increasing the dose further is not an option — alternative approaches or medications would need to be considered instead.
Does Mounjaro stop working over time?
Some people experience a plateau in weight loss after months of use — this is called "tolerance". However, this is different from Mounjaro "not working". You may have lost weight already and your body has adapted. Continuing Mounjaro can help maintain weight loss. If you're experiencing appetite return, discuss with your prescriber whether adjusting other aspects of your lifestyle or exploring alternative medications might help.
What's the difference between Mounjaro and Wegovy if one isn't working?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1-only agonist. Clinical trials suggest Mounjaro may produce slightly greater average weight loss, but individual responses vary. Some people respond better to one medication than the other. A prescriber can assess whether switching to Wegovy might be worth considering, though this is a clinical decision requiring specialist guidance.
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.