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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How Mounjaro Works: The Pharmacology Explained
Mounjaro (tirzepatide) is a prescription-only weight management injection that works differently from other medications available in the UK. Unlike single-action weight loss treatments, Mounjaro activates two natural hormone pathways simultaneously — GIP and GLP-1 — which work together to regulate appetite, support feeling fuller after meals, and help manage food cravings. Understanding how this dual mechanism works can help you make informed decisions about whether this treatment may be suitable for you alongside lifestyle changes. This guide explains the pharmacology in plain language, aligned with NHS and MHRA guidance.
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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's Dual Mechanism Works
Mounjaro is a GIP and GLP-1 receptor agonist — this means it activates two separate hormone pathways in your body simultaneously, which is what sets it apart from other weight loss injections like Wegovy (semaglutide), which targets GLP-1 only.
- GLP-1 activation: Signals to the brain that you are full, reduces appetite and food cravings, slows how quickly food leaves your stomach, and helps regulate blood sugar levels
- GIP activation: Works alongside GLP-1 to enhance appetite suppression and support fat utilisation in the body
Clinical trials (SURMOUNT-1) showed that this dual action may help achieve greater average weight loss compared to single-action GLP-1 medications. However, individual results vary significantly, and weight loss is never assured. Mounjaro is always prescribed as part of a package that includes a reduced-calorie diet and increased physical activity — the injection alone is not sufficient.
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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.
NHS Eligibility and UK Prescribing Context
In the UK, Mounjaro is available through NHS tier 3 specialist weight management services and through private prescribing following online clinical assessment. NHS eligibility typically requires a BMI of 30 kg/m² or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, cardiovascular disease). For South Asian, Chinese, Black African, and Black Caribbean adults, lower BMI thresholds apply due to increased health risk at lower weights.
The MHRA approved Mounjaro for weight management with additional monitoring (Black Triangle ▼ status), meaning safety data continues to be closely reviewed. Your GP or prescriber will assess whether this treatment is appropriate for your individual health profile and discuss potential side effects before prescribing.
- 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 insulin?
No. Mounjaro is not insulin and does not work like insulin. It is a GIP/GLP-1 receptor agonist that mimics natural hormones your gut produces after eating. It works on appetite and fullness signals, not on glucose metabolism in the same way insulin does.
Why does Mounjaro cause nausea and stomach side effects?
Mounjaro slows gastric emptying (how quickly food leaves your stomach) as part of its mechanism. This can cause nausea, indigestion, and stomach discomfort, especially when doses are increased. These effects often reduce over time as your body adjusts. Eating smaller, lighter meals can help.
How quickly does Mounjaro start working?
Most people notice reduced appetite within the first 1–2 weeks of starting Mounjaro. However, weight loss typically becomes measurable after 4–8 weeks. The dose is increased gradually over 16–20 weeks to reach the maintenance dose of 15 mg, so effects build progressively over this time.
Can Mounjaro be used long-term?
Yes, Mounjaro is designed for ongoing use. Clinical trials followed participants for up to 72 weeks. Long-term safety data continues to be gathered (hence the Black Triangle monitoring status). Your GP or prescriber will review whether continued use remains appropriate for your health goals.
What happens if I stop taking Mounjaro?
Appetite and food cravings typically return to baseline within weeks of stopping Mounjaro. Weight loss may slow or reverse without the medication and continued lifestyle changes. Many people who stop find weight regain occurs, which is why sustained diet and activity changes remain important alongside 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.