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
How Does Mounjaro Work in the UK?
Mounjaro is a prescription-only weight management injection that works differently from other weight loss medications available in the UK. It's a dual GLP-1 and GIP receptor agonist — meaning it activates two natural hormone pathways to help regulate appetite and support weight loss. Understanding how Mounjaro works, how it's prescribed through NHS services, and whether it's right for you requires understanding both the science and the UK prescribing context. This guide explains the mechanism, eligibility criteria, and how to access it through the NHS or privately.
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.
How Mounjaro Works: The Dual Mechanism Explained
Mounjaro (tirzepatide) activates two hormone receptors in the body — GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). This dual action sets it apart from other weight loss medications like Wegovy, which only targets GLP-1. Here's how the mechanism works:
- Appetite regulation: Both GLP-1 and GIP signal to your brain that you're full, reducing hunger and food cravings
- Slower digestion: Mounjaro slows how quickly food leaves your stomach, helping you feel satisfied after smaller meals
- Blood sugar support: GLP-1 stimulates insulin release when needed, supporting better blood sugar control
- Fat metabolism: The dual action may help your body better utilise fat stores alongside the calorie deficit created by reduced eating
Clinical trials (SURMOUNT-1) showed that at the maximum dose of 15 mg weekly, participants lost an average of 20.9% of their body weight — though individual results vary significantly and depend on lifestyle changes alongside the medication.
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.
NHS Eligibility and NICE Guidance for Mounjaro in the UK
NICE has approved tirzepatide (Mounjaro) for use within NHS specialist weight management services. In the UK, you don't access Mounjaro through your GP — instead, you're referred to a tier 3 or tier 4 specialist weight management service. Eligibility typically requires:
- BMI of 30 kg/m² or above, or BMI 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, sleep apnoea, cardiovascular disease)
- Lower thresholds for South Asian, Chinese, Black African, and Black Caribbean adults (BMI 27.5+) due to higher health risks at lower weights
- Evidence of previous weight loss attempts
However, NHS waiting times for specialist services can be lengthy in many regions. Private prescribing through UK-licensed online pharmacies offers faster access following a clinical assessment, though costs apply. Use our GLP-1 weight loss calculator to estimate potential weight loss outcomes 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.
Is Mounjaro the same as tirzepatide?
Yes. Mounjaro is the brand name for tirzepatide. In the UK, it's prescribed specifically for weight management (rather than diabetes). The medication is the same — tirzepatide — but marketed as Mounjaro for weight loss applications. It's a prescription-only medicine that requires a clinical assessment before dispensing.
How quickly does Mounjaro start working?
Most people notice appetite suppression within 2–3 days of their first injection, though this varies. Measurable weight loss typically appears within 4–8 weeks as the body adjusts. The dose is gradually increased over 16+ weeks (dose titration), so sustained weight loss usually builds over months rather than weeks. Patience and consistent lifestyle changes are essential.
What's the difference between Mounjaro and Wegovy in the UK?
Mounjaro (tirzepatide) is a dual GIP+GLP-1 agonist; Wegovy (semaglutide) targets only GLP-1. Clinical trials suggest Mounjaro may produce slightly greater average weight loss (~21% vs ~15%), though individual results vary. Both are prescription-only, once-weekly injections. Your prescriber will assess which is more suitable based on your medical history and preferences.
Can I get Mounjaro through the NHS in the UK?
Mounjaro is available through NHS tier 3 specialist weight management services following NICE guidance. Access varies significantly by region and waiting lists can be lengthy. Private prescribing through registered UK pharmacies offers quicker access without NHS referral, though you'll pay privately. Check your local ICB (Integrated Care Board) for NHS availability in your area.
What happens if I stop taking Mounjaro?
When you stop Mounjaro, appetite suppression effects typically fade within days to a week. Weight regain is common if lifestyle changes (diet and exercise) aren't maintained — this is why sustainable behaviour change is crucial during treatment. Your prescriber will discuss the long-term plan and how to support weight maintenance after stopping the medication.
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.