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 Injection Work?
Mounjaro (tirzepatide) is a prescription-only weekly injection that works differently from other weight loss medications. Rather than using a single hormone pathway, Mounjaro activates two natural systems in your body — GIP and GLP-1 — to help regulate appetite, support feeling fuller after meals, and manage blood sugar. This dual-action mechanism is what sets Mounjaro apart from other weekly weight loss injection options available in the UK. Understanding how it works can help you decide if this prescription medication might be right for you.
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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.
The Dual GIP and GLP-1 Mechanism
Mounjaro's effectiveness comes from its dual action on two hormone pathways. GLP-1 (glucagon-like peptide-1) is a natural hormone your gut produces after eating — it signals your brain that you're full, slows how quickly food leaves your stomach, and helps regulate blood sugar. GIP (glucose-dependent insulinotropic polypeptide) is another gut hormone that works alongside GLP-1 to influence appetite and energy use.
By activating both pathways simultaneously, Mounjaro may help you:
- Experience reduced appetite and fewer food cravings
- Feel fuller after smaller meals
- Have better control over hunger signals
- Support more sustainable eating patterns
This dual mechanism is why clinical trials showed Mounjaro helped participants lose an average of 15–21% of their body weight — though individual results vary significantly. The effect typically builds over weeks as your body adjusts to each dose.
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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 and How It Works
Mounjaro is given as a once-weekly KwikPen injection, but the dose increases gradually over several months in a process called titration. You start at 2.5 mg and increase every 4 weeks — moving through 5 mg, 7.5 mg, 10 mg, 12.5 mg, and up to the maximum 15 mg dose.
This step-by-step approach allows your body to adjust and helps your prescriber find the right dose for you. Most people experience the strongest appetite-suppressing effects at higher doses. The weekly injection schedule makes it easier to remember than daily medications, and you can inject into your stomach, thigh, or upper arm. If you miss a dose, take it within 3–4 days and continue your normal schedule.
- 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 a weekly weight loss injection?
Yes — Mounjaro is a once-weekly weight loss injection, meaning you inject it every 7 days. It comes in a pre-filled KwikPen device. Unlike some weight loss injections that require daily doses, Mounjaro's weekly schedule makes it more convenient for most people managing their weight.
How is Mounjaro different from Wegovy or other weight loss injections?
Mounjaro activates two hormone pathways (GIP and GLP-1), whilst Wegovy activates only GLP-1. This dual mechanism may explain why clinical trials showed slightly higher average weight loss with Mounjaro. Both are prescription-only and require a clinical assessment before prescription.
How long does it take for Mounjaro to start working?
Some people notice appetite changes within days, but most experience stronger effects after 2–4 weeks as their body adjusts. The full effect typically builds as your dose increases over the first 4–5 months. Individual timelines vary — discuss expectations with your prescriber.
Can I use MyWeightCheck's GLP-1 weight loss calculator with Mounjaro?
Yes — our GLP-1 weight loss calculator can help you estimate potential weight loss at different Mounjaro doses based on clinical trial data. It's a useful tool for discussing realistic expectations with your prescriber.
Does Mounjaro work without diet and exercise changes?
Mounjaro is most effective when combined with a reduced-calorie diet and increased physical activity. Clinical trials showed weight loss happened alongside lifestyle changes — the injection supports your efforts, not replaces them. Your GP or prescriber can advise on a complete approach.
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.