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 you're on Mounjaro (tirzepatide) and not seeing the weight loss results you expected, you're not alone — and there are several concrete reasons this might be happening. Mounjaro is a prescription-only GLP-1 and GIP receptor agonist that may help support weight management, but individual results vary significantly. This guide explains the most common reasons why tirzepatide may not be delivering the outcomes you hoped for, and what you can do about it.
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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 Might Not Be Working Yet
Several factors can affect how well Mounjaro works for you:
- You may still be on a low dose. Mounjaro starts at 2.5 mg and is titrated upwards every 4 weeks (2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg). Maximum dose is 15 mg once weekly. Clinical trial data showed the greatest average weight loss occurred at higher doses — the 15 mg dose resulted in approximately clinically studied weight loss in SURMOUNT trials, compared to around 15% at the 5 mg dose. If you've been on Mounjaro for fewer than 16 weeks, you may not yet be at your target dose.
- Timeline matters. Weight loss typically becomes noticeable after 4–8 weeks, but significant results often take 12–16 weeks or longer. Expecting rapid results in the first few weeks is a common source of disappointment.
- You may not be in a calorie deficit. Mounjaro is a tool — not a clinically supported solution. It may reduce appetite and support feeling fuller after smaller meals, but if total calorie intake remains high, weight loss will be limited. Track your food intake honestly for a week to assess this.
- Alcohol and high-calorie drinks. Liquid calories (alcohol, sugary drinks, high-calorie coffee drinks) are easy to overlook but can sabotage a calorie deficit. These pass through the stomach quickly and don't trigger the satiety signals that solid food does.
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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 Speak to Your Prescriber
If you've been on Mounjaro for 16+ weeks, are at or near the 15 mg maximum dose, are maintaining a genuine calorie deficit, and still haven't lost any weight, it's time to discuss this with your prescriber. Some people are genuinely non-responders to GLP-1 and GIP receptor agonists — this is rare but documented. Your prescriber may suggest:
- Confirming you're injecting correctly (technique matters)
- Checking for medication interactions or absorption issues
- Reviewing your diet and physical activity honestly
- Discussing whether Mounjaro remains the right choice for you
- Considering alternative weight management approaches
Never stop Mounjaro without guidance from your prescriber. Use our free BMI calculator to track your current status, and our GLP-1 weight loss calculator to review realistic projections at different doses.
- 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?
Weight loss typically becomes noticeable after 4–8 weeks, but significant results often take 12–16 weeks or longer. This depends on your dose (most people aren't at their target dose immediately), your starting weight, and how well you maintain a calorie deficit. The SURMOUNT clinical trials measured results at 72 weeks — not 8 weeks.
Could I be on the wrong dose of tirzepatide?
Yes — if you've been on Mounjaro for fewer than 16 weeks, you may still be titrating upwards. Each dose is held for 4 weeks minimum before increasing. Maximum dose is 15 mg weekly. Clinical trials showed greater weight loss at higher doses, so dose level definitely matters.
What if I'm doing everything right but Mounjaro still isn't working?
Some people are non-responders to GLP-1 and GIP receptor agonists — this is rare but real. If you've been at 15 mg for 8+ weeks with a genuine calorie deficit and no weight loss, discuss this with your prescriber. They can assess injection technique, check for absorption issues, or explore alternative approaches.
Does Mounjaro vs Wegovy make a difference if one isn't working?
Mounjaro and Wegovy work via different mechanisms — Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy is a GLP-1 agonist only. Some people respond better to one than the other. Your prescriber can advise if switching might be appropriate, though this isn't typically a first step.
Should I be eating more to help Mounjaro work?
No — the opposite. Mounjaro may help reduce appetite, but weight loss still requires a calorie deficit. You don't need to eat very little, but eating at or above maintenance calories will prevent weight loss regardless of the medication. Track your intake honestly for 1–2 weeks to establish your true intake.
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.