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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Weight Loss Injections: What Is Mounjaro and How Does It Work?
Mounjaro (tirzepatide) is a prescription-only weekly weight loss injection licensed in the UK for adults with obesity or overweight with related health conditions. Unlike other weight loss injections, Mounjaro works through a dual mechanism — activating both GIP and GLP-1 hormone pathways to help regulate appetite, reduce cravings, and support feeling fuller after smaller meals. Clinical trials showed participants lost an average of 15–21% of their body weight, though individual results vary significantly. It's prescribed alongside reduced-calorie diet and increased physical activity. Use our GLP-1 weight loss calculator to explore potential outcomes at different doses.
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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.
How Mounjaro Weight Loss Injections Work
Mounjaro is a dual GIP and GLP-1 receptor agonist — meaning it activates two separate natural hormone pathways that regulate appetite and metabolism. This dual action is what distinguishes Mounjaro from other weekly weight loss injections like Wegovy, which only targets GLP-1.
When you inject Mounjaro, it:
- Signals to your brain that you're fuller, reducing hunger signals
- Helps regulate blood sugar and insulin response after eating
- Slows gastric emptying — how quickly food moves from your stomach
- May help your body utilise fat more effectively
The effect typically builds over the first 4–8 weeks as your body adjusts to each dose. Mounjaro comes as a pre-filled KwikPen — a simple once-weekly injection you can administer at home. Doses range from 2.5 mg up to 15 mg, with doses increased every 4 weeks based on tolerance and response. Your prescriber will guide your titration schedule.
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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.
UK Eligibility, Doses, and What to Expect
In the UK, Mounjaro is prescribed for adults with a BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. South Asian, Chinese, Black African, and Black Caribbean adults may be eligible at lower BMI thresholds (from BMI 27.5). Check your eligibility using our free BMI calculator.
Starting dose is 2.5 mg weekly, with increases to 5 mg, 7.5 mg, 10 mg, 12.5 mg, and up to 15 mg — each dose used for at least 4 weeks. Common side effects include nausea, diarrhoea, and constipation, particularly during dose escalation. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and gallbladder issues. You'll be monitored regularly by your prescriber. Track your weight loss progress with our free progress tracker.
- 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.
How do Mounjaro weight loss injections compare to Wegovy?
Both are once-weekly injections for weight management, but Mounjaro uses a dual GIP + GLP-1 mechanism while Wegovy (semaglutide) uses GLP-1 only. Clinical trials showed Mounjaro average weight loss of 15–21% versus Wegovy's ~15%. Both require prescription and clinical assessment. Results vary individually.
Can I get Mounjaro on the NHS in the UK?
NICE has approved tirzepatide for NHS specialist weight management services. Access is through tier 3 specialist services — not standard GP prescription. Availability and waiting times vary significantly by region. Private prescription is available through GPhC-registered online services following clinical assessment.
What is the starting dose of Mounjaro for weight loss?
The starting dose is 2.5 mg once weekly for the first 4 weeks. Your prescriber will increase to 5 mg in week 5, then escalate further based on your response and tolerance. Maximum dose is 15 mg weekly. Never increase your dose without guidance from your prescriber.
What are the most common side effects of Mounjaro injections?
Very common side effects include nausea, diarrhoea, constipation, vomiting, and stomach pain — most occur during dose escalation and improve over time. Less common: headache, fatigue, dizziness, and hair loss. Most people tolerate Mounjaro well once at maintenance dose. Inform your prescriber of persistent side effects.
Is Mounjaro better than diet and exercise alone for weight loss?
Mounjaro is most effective when combined with reduced-calorie diet and increased physical activity — not as a replacement for lifestyle changes. Research shows medication plus lifestyle changes achieve greater weight loss than either approach alone. Your prescriber will discuss whether Mounjaro is appropriate for your individual situation.
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.