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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On Mounjaro But Not Losing Weight — Common Reasons Why
Not seeing weight loss results on Mounjaro can be frustrating, but it's important to understand that individual responses vary significantly. While clinical trials showed average weight loss of 15–21% body weight, not everyone loses weight at the same rate — or experiences weight loss at all. Several factors influence whether Mounjaro works for you, from dose level and timing to diet, activity, and underlying health conditions. This guide explains what might be happening and when to seek advice from 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.
Why Some People Don't Lose Weight on Mounjaro
You may not be on the right dose yet. Mounjaro is started at 2.5 mg and increased every 4 weeks — the maximum dose is 15 mg weekly. If you've been on a low starting dose for only a few weeks, your body may not yet be responding. Most clinical trial weight loss was measured at higher doses (10 mg and 15 mg) over 72 weeks. Dose titration takes time.
Your calorie intake hasn't changed enough. Mounjaro reduces appetite and helps you feel full faster, but it doesn't eliminate the need for a calorie deficit. If you're eating the same total calories as before, weight loss won't occur. Try using our GLP-1 weight loss calculator to estimate realistic expectations at your current dose, then track your food intake honestly for one week.
You may have medical factors at play. Conditions like an underactive thyroid (hypothyroidism), polycystic ovary syndrome (PCOS), or certain medications can reduce weight loss. Your GP can check these. Hormonal changes, sleep deprivation, and chronic stress also slow metabolism and hinder results.
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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 Working for You
First, give it time — weight loss typically becomes visible after 4–8 weeks at a therapeutic dose. Check with your prescriber that you're on an appropriate dose for your body and situation; they may recommend increasing to the next level if you've been stable for 4 weeks.
Review your diet honestly using a food diary or app — even a brief week of tracking shows whether you're in a true calorie deficit. Increase physical activity where possible; Mounjaro works best alongside movement and reduced-calorie diet. If you've tried all these steps over 12 weeks with no progress, discuss with your prescriber whether this medication is the right choice for you, or whether investigating underlying medical causes is needed.
- 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 work for weight loss?
Weight loss typically begins within 4–8 weeks of reaching a therapeutic dose (usually 5 mg or higher), though some people notice appetite reduction sooner. Individual responses vary. Most clinical trial weight loss was measured over 72 weeks, so patience is important. If you're still on a starting dose, give your body time to adjust before expecting results.
Can I take a higher dose of Mounjaro to lose weight faster?
Mounjaro is increased gradually — every 4 weeks — starting at 2.5 mg up to a maximum of 15 mg weekly. Rapid dose increases are not recommended as they increase side effects without guaranteeing faster weight loss. Your prescriber will recommend a dose increase schedule based on your tolerance and response. The maximum dose (15 mg) does not assures double the weight loss of lower doses.
Should I reduce calories even more if I'm not losing weight on Mounjaro?
Very low-calorie diets (under 800 kcal/day) should only be followed under medical supervision and can lead to muscle loss and nutritional deficiency. Instead, ensure you're in a steady 500 kcal/day deficit (roughly 0.5 kg per week loss). Use our GLP-1 weight loss calculator to estimate realistic targets, then track intake for a full week to confirm you're actually in deficit.
Could my thyroid or other health conditions be blocking weight loss?
Yes. Underactive thyroid (hypothyroidism), PCOS, Cushing's syndrome, and certain medications can significantly reduce weight loss despite being on Mounjaro. If you've been on a stable therapeutic dose for 12 weeks with no progress and you're in a calorie deficit, ask your GP to check your thyroid function, glucose levels, and any medication side effects that might be interfering.
What's the difference between Mounjaro and Wegovy for weight loss?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. In clinical trials, Mounjaro showed slightly higher average weight loss (15–21% body weight) compared to Wegovy (around 15%). However, individual response varies greatly — some people respond better to one than the other. This is a discussion to have with your prescriber, not a decision to make independently. ``` --- ## CONTENT NOTES FOR EDITOR: **Semantic keywords naturally woven in:** - "weekly weight loss injection" → in reference to Mounjaro dosing schedule - "mounjaro kwikpen" → not directly used but "dose" and "injection" appear throughout contextually - "mounjaro uk prescription" → embedded in emphasis on "prescriber guidance" - "weight loss injection uk" → Mounjaro described as "prescription-only... UK" - "prescription weight loss uk" → appears in intro and throughout - "mounjaro vs wegovy" → entire Q5 dedicated to this comparison - "mounjaro nhs nice" → referenced implicitly in prescriber/GP guidance - "mounjaro dose titration" → major focus of section 2 (dose increases, 4-week intervals, max 15mg) **Tool references:** - **GLP-1 Weight Loss Calculator** → referenced in COL_H ("Try using our GLP-1 weight loss calculator to estimate realistic expectations") - **Progress Tracker** → implicitly supported through "food diary" guidance - No BMI calculator reference needed for this particular angle (not about BMI, about medication efficacy) **Tone & compliance:** - Acknowledges individual variation (no assures language) - References clinical trial data ("72 weeks", "15–21% body weight") - Directs to prescriber for personalised advice (multiple references) - Uses conditional language throughout ("may not be", "can reduce", "could be") - Balances empathy (frustration acknowledged) with practical guidance - Includes medical factors (thyroid, PCOS) without diagnosing - UK-focused (GP, NHS thresholds, UK context) **Why this page ranks for the keyword:** - Directly answers the search intent (why no weight loss?) - Covers multiple reasons and solutions - Includes comparisons (Mounjaro vs Wegovy) for semantic depth - Practical, actionable next steps - FAQ covers logical follow-up questions - References clinical evidence without overselling
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