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 Mounjaro Injection Works for Weight Loss
Mounjaro (tirzepatide) is a prescription-only weekly weight loss injection licensed in the UK that works differently to other weight loss medications. It's a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways to help regulate appetite and support feeling fuller after meals. Clinical trials showed participants lost an average of 15–21% of their body weight — though individual results vary significantly. Mounjaro is not a structured approach; it's used alongside a reduced-calorie diet and increased physical activity as part of a comprehensive weight management approach. Our GLP-1 weight loss calculator can help you explore potential outcomes at different doses.
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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 Works and What Makes It Different
Mounjaro is unique because it activates both GIP and GLP-1 receptors — unlike other weight loss injections like Wegovy, which only target GLP-1. This dual mechanism may help with appetite regulation in two ways. When you take Mounjaro, it signals your brain that you're fuller, reduces cravings, and slows how quickly food leaves your stomach. This combination helps you eat smaller portions and feel satisfied for longer. It is not insulin and works completely differently from diabetes medications, though people with type 2 diabetes can use it. The medication comes as a pre-filled KwikPen device — you inject yourself once per week under the skin in your stomach, thigh, or upper arm. Starting at 2.5 mg, your prescriber will gradually increase your dose every 4 weeks to reach your maintenance level, typically between 5 mg and 15 mg per week.
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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.
Mounjaro Doses, Eligibility, and Clinical Evidence
In the UK, Mounjaro is available in doses from 2.5 mg to 15 mg per week. You'll start on the lowest dose and increase gradually — your prescriber decides the pace based on how you respond. In the SURMOUNT-1 clinical trial, participants on 15 mg lost an average of 20.9% of their body weight over 72 weeks, alongside diet and exercise. Eligibility requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea. Adults of South Asian, Black African, or Black Caribbean heritage may be eligible at lower BMI thresholds. Mounjaro is prescription-only and requires a clinical assessment — your GP or a private prescriber will check your medical history, current medications, and whether this injection is safe for you personally. Use our BMI calculator to check where you stand against NHS eligibility thresholds.
- 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 Ozempic or Wegovy?
No. All three are different medicines. Ozempic (semaglutide) treats type 2 diabetes — not licensed for weight loss in the UK. Wegovy (semaglutide) is a GLP-1-only injection for weight loss. Mounjaro (tirzepatide) is a newer dual GIP+GLP-1 injection, licensed for weight loss, with a different mechanism. Clinical trials show Mounjaro achieved slightly higher average weight loss than Wegovy, though individual results vary greatly.
How do I get Mounjaro in the UK?
Mounjaro is prescription-only. You can access it through NHS specialist weight management services (tier 3/4, though availability and waiting lists vary by region) or through a private online consultation with a UK-licensed prescriber. A clinical assessment is always required first — your prescriber checks your eligibility, medical history, and whether it's safe for you. You cannot buy it without a prescription.
What side effects does Mounjaro cause?
Common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain — often worse in the first weeks and improving over time. Less common: indigestion, fatigue, dizziness, hair loss, and injection site reactions. Serious side effects are rare but include severe stomach pain (possible pancreatitis), gallbladder problems, or serious allergic reactions. Always report persistent or severe symptoms to your prescriber immediately.
How much weight can I expect to lose with Mounjaro?
Clinical trials showed average weight loss of 15–21% of body weight over 72 weeks — but individual results vary significantly. Some people lose more, some less. Weight loss depends on your starting weight, diet adherence, physical activity, genetics, and how you respond to the medication. It's not a assures. Mounjaro works best alongside a reduced-calorie diet and regular exercise. Our weight loss calculator can estimate potential outcomes at different doses, but your actual results may differ.
Can I use Mounjaro if I have type 2 diabetes?
Yes. Mounjaro can be used by people with type 2 diabetes for weight management. In fact, it may help improve blood sugar control as a side benefit. However, if you're taking diabetes medications like insulin or sulphonylureas, your prescriber may need to adjust those doses to avoid low blood sugar. Always tell your prescriber about all medications you're taking — they'll manage any necessary adjustments before you start.
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.