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
Why Is Mounjaro Not Working For Me?
If Mounjaro isn't delivering the weight loss results you expected, you're not alone — many people on tirzepatide experience slower progress or plateaus than anticipated. Weight loss medications work differently for everyone, and sometimes the underlying reason relates to dose, lifestyle factors, or individual response to the medication. Understanding why your results have stalled, and whether an alternative like Wegovy might suit you better, requires looking at the clinical evidence alongside your personal circumstances. This guide explains the most common reasons Mounjaro may not be working as expected and how it compares to other GLP-1 options available in the UK.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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.
Mounjaro vs Wegovy: Key Differences in How They Work
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously. Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates a single pathway. In clinical trials, both showed weight loss benefits, but the mechanisms differ.
SURMOUNT-1 trial (Mounjaro): Average weight loss was approximately 15% body weight at 5mg, 19.5% at 10mg, and 20.9% at 15mg over 72 weeks alongside reduced-calorie diet and exercise.
STEP-1 trial (Wegovy): Average weight loss was approximately 14.9% body weight at 2.4mg (maximum dose) over 68 weeks with diet and activity changes.
The dual mechanism of Mounjaro suggests a theoretical advantage, but individual response varies significantly. Some people respond better to GLP-1 alone, whilst others benefit from the dual approach. Your prescriber assesses which is more suitable based on your personal health profile.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Why Mounjaro May Not Be Working: Common Reasons and Solutions
Several factors can explain slower-than-expected weight loss on Mounjaro. Insufficient dose: If you're on 5mg or 7.5mg, you may not yet be at your optimal therapeutic dose — most meaningful results appear at 10mg or above. Dose titration is gradual (minimum 4 weeks between increases), so give your current dose time to work.
Lifestyle factors: Mounjaro works alongside reduced-calorie diet and physical activity — it's not a standalone solution. If calorie intake remains high or exercise is minimal, weight loss will plateau. Track your intake using our free progress tracker to ensure you're in a calorie deficit.
Gastric adaptation: The appetite-suppressing effect can diminish over weeks as your stomach adapts. Your GP or prescriber may recommend a dose increase or, in some cases, a trial of a different medication like Wegovy. Individual suitability varies — a prescriber assessment is essential before any switch.
When to Consider Switching: Mounjaro to WegovyIf you've been on Mounjaro at 10mg or above for at least 12 weeks with minimal progress, discussing a switch with your prescriber is reasonable. Wegovy's single GLP-1 mechanism may suit some people better than Mounjaro's dual mechanism — the evidence suggests no clear superiority, only individual variation.
Approximate dose equivalence: Mounjaro 5mg ≈ Wegovy 0.5mg, Mounjaro 10mg ≈ Wegovy 1.7mg, and Mounjaro 15mg ≈ Wegovy 2.4mg. However, these are clinical practice estimates only — any medication switch must be managed by your prescriber, not self-initiated. Your GP can refer you to a specialist weight management service for reassessment, or if you're on private treatment, your prescriber can discuss alternatives. Use our free GLP-1 weight loss calculator to compare projected outcomes at different doses of either 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 Mounjaro take to start working?
Most people notice appetite reduction within the first 1–2 weeks at any given dose. Measurable weight loss typically appears within 4 weeks. However, the full effect of each dose level takes about 4 weeks to stabilise, which is why prescribers wait at least 4 weeks before increasing. If you're still within your first month, continue as instructed and reassess after 4 weeks on your current dose.
Could my Mounjaro be fake or poorly stored?
Unopened Mounjaro KwikPens must be stored in a refrigerator at 2–8°C. If your pen was left at room temperature above 30°C before use, or if you're unsure of its storage history, mention this to your prescriber — they can advise whether your supply is safe to use. Always source medication from a registered UK pharmacy.
Is Wegovy definitely better than Mounjaro if mine isn't working?
No — Wegovy is not universally "better". Clinical trials show comparable average weight loss (14.9% vs 15–21%). Some people respond better to GLP-1 alone, others to the dual GIP+GLP-1 approach. Only a prescriber can assess which is more suitable for you based on your health profile and previous response. Individual results vary significantly.
Should I increase my Mounjaro dose myself if it's not working?
No. Never adjust your dose without prescriber guidance. Increasing too quickly risks severe side effects (nausea, pancreatitis risk). Your prescriber follows a structured titration schedule: minimum 4 weeks at each dose before escalating. If progress has stalled, contact your prescriber — they may recommend waiting longer, increasing to the next dose, or exploring alternatives like Wegovy.
Does Mounjaro work differently if I'm on other medications?
Yes — some medications can interact. Oral contraceptives may have reduced absorption (use barrier contraception during Mounjaro and for 4 weeks after each dose increase). Diabetes medications may need adjustment. Inform your prescriber of all medications you take. They assess potential interactions before prescribing and monitor for effectiveness.
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.