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 Is Weight Loss Slow On Mounjaro?
If you're on Mounjaro and experiencing slower weight loss than expected, you're not alone—individual results vary significantly, and several factors can affect how quickly your body responds. Understanding the difference between Mounjaro and alternatives like Wegovy, knowing what the clinical trials actually showed, and recognising when to discuss adjustments with your prescriber can help you get the most from treatment. This guide compares the two main injectable weight loss options available in the UK and explores realistic expectations around weight loss timelines.
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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.
Mounjaro vs Wegovy: Clinical Trial Data & Mechanisms
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription weight loss injections licensed in the UK, but they work differently and achieved different results in clinical trials. Mounjaro is a dual GIP and GLP-1 receptor agonist—it activates two hormone pathways—whilst Wegovy is a GLP-1 receptor agonist only. In the SURMOUNT-1 trial, Mounjaro 15 mg achieved an average weight loss of approximately 20.9% of body weight over 72 weeks. The STEP-1 trial showed Wegovy 2.4 mg resulted in approximately 14.9% average weight loss over 68 weeks. However, these are population averages—your individual response depends on your starting weight, metabolism, adherence, diet, and physical activity. Some people lose weight steadily each week; others experience plateaus before breaking through to further loss.
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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.
Dose Titration, Timing, & When To Discuss Adjustments
Mounjaro uses a gradual dose titration schedule, starting at 2.5 mg and increasing by 2.5 mg every 4 weeks up to a maximum of 15 mg. Slower weight loss in the early weeks (2.5 mg to 5 mg) is normal—your body is adjusting to the medication. Many people see more significant weight loss after reaching their target maintenance dose. If you're still on a lower dose, ask your prescriber about continuing with scheduled increases. If you're at your target dose but weight loss has plateaued for 4+ weeks, discuss with your prescriber whether a dose increase is suitable. Weight loss injections work best alongside a reduced-calorie diet and increased physical activity—if dietary and exercise habits haven't changed, weight loss will naturally be slower. Use our free GLP-1 weight loss calculator to check your realistic projection based on your current dose.
- 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.
Is slow weight loss on Mounjaro normal?
Yes. Weight loss is rarely linear—you may lose weight steadily, then plateau, then lose again. Average losses in trials are around 20% of body weight over 18+ months. Week-to-week variation is normal and doesn't indicate failure. Factors like water retention, hormonal cycles, and food intake variations affect weekly scales.
How does Mounjaro's mechanism differ from Wegovy?
Mounjaro activates both GIP and GLP-1 receptors (dual agonist), whilst Wegovy activates GLP-1 only (single agonist). In theory, dual activation may produce greater weight loss, which SURMOUNT trial data suggests (20.9% vs 14.9% average). However, individual responses vary—some people respond better to one than the other.
Should I increase my Mounjaro dose if weight loss is slow?
Only on your prescriber's advice. Dose increases happen every 4 weeks as part of the standard titration schedule. If you're not yet at your target dose, continuing with scheduled increases is appropriate. If you're at maximum dose with plateau, discuss with your prescriber—a dose increase or other strategies may be considered.
How long should I stay on Mounjaro if weight loss is slow?
Discuss expectations with your prescriber before stopping. Many people experience delayed loss or plateaus—these don't mean the medication isn't working. Weight loss is typically assessed over months, not weeks. Your prescriber may suggest continuing, adjusting diet/exercise, or reconsidering suitability.
Can I switch from Mounjaro to Wegovy if weight loss is slow?
This is a decision for your prescriber. There is no official conversion chart between them. Some people do try alternatives, but switching requires clinical assessment and stopping one medication before starting another. Your prescriber will advise on whether a switch is appropriate for you. ```
Ready to take control of your weight loss journey?
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