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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Mounjaro Weight Loss Injection — What You Need to Know in the UK
Mounjaro (tirzepatide) is a prescription-only weekly weight loss injection licensed in the UK for adults with obesity or overweight with health complications. It works differently to other weight loss injections because it activates two natural hormone pathways—GIP and GLP-1—which help regulate appetite and support feeling fuller after meals. In clinical trials, people using Mounjaro lost an average of 15–21% of their body weight, though individual results vary significantly. Access is available through NHS specialist weight management services or private prescription following a clinical assessment. This guide explains how Mounjaro works, eligibility criteria, doses, side effects, and what to expect.
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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 Works and UK Eligibility
Mounjaro is a dual GIP and GLP-1 receptor agonist—this dual mechanism sets it apart from other weight loss injections like Wegovy (semaglutide). GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are natural hormones that regulate hunger and blood sugar. Mounjaro activates both pathways, which may help reduce appetite, decrease food cravings, and support feeling fuller after smaller meals. It also slows how quickly food leaves the stomach, though this effect typically reduces over time.
UK eligibility for Mounjaro:
- BMI of 30 kg/m² or above (obesity), or
- BMI of 27–29.9 kg/m² with at least one weight-related health condition such as type 2 diabetes, prediabetes, high blood pressure, abnormal cholesterol, sleep apnoea, or cardiovascular disease
- For South Asian, Chinese, Black African, or Black Caribbean adults, eligibility may start at a lower BMI threshold (27.5 kg/m² or above) due to higher health risks at lower weights
You cannot take Mounjaro if you are under 18, pregnant, breastfeeding, or have a known allergy to tirzepatide. Use our free BMI calculator above to check your personal BMI instantly and understand where you fall.
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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, Weight Loss Results, and Side Effects
Mounjaro is injected once weekly using a pre-filled KwikPen device. Doses start at 2.5 mg and increase gradually over weeks, with a maximum dose of 15 mg per week. Most people remain on doses between 5 mg and 10 mg. Based on the SURMOUNT-1 clinical trial, people using Mounjaro lost approximately 15% of body weight on 5 mg, 19.5% on 10 mg, and 20.9% on 15 mg over 72 weeks—alongside a reduced-calorie diet and increased physical activity.
Common side effects include nausea, diarrhoea, constipation, and stomach pain, particularly when starting or increasing doses. Most improve within a few weeks. Rare but serious side effects include severe stomach pain (possible pancreatitis) and serious allergic reactions—seek immediate medical attention if these occur. Try our free weight loss projection calculator to estimate potential results with different Mounjaro doses.
Mounjaro access in the UK:
- NHS: Through specialist tier 3/4 weight management services. Referral from your GP is required, and availability varies significantly by region. Waiting lists can be several months to years in many areas.
- Private: Through GPhC-registered online pharmacies following an online clinical assessment with a UK-licensed prescriber. Available without NHS referral. Costs range from approximately £129–£320 per month depending on dose.
Mounjaro is a weekly weight loss injection, not a daily one like some alternatives. Storage requires refrigeration between 2–8°C before first use; after opening, pens can be kept at room temperature for up to 30 days.
- 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 does Mounjaro compare to Wegovy (semaglutide)?
Both are prescription weight loss injections given once weekly. Mounjaro uses a dual GIP + GLP-1 mechanism, while Wegovy is a single GLP-1 agonist. Clinical trials showed Mounjaro may produce slightly greater average weight loss (15–21% vs ~15% for Wegovy), but individual results vary. Your prescriber can advise which is more suitable for you based on your health profile.
How quickly will I lose weight on Mounjaro?
Weight loss is gradual and varies between individuals. Clinical trials showed average weight loss of about 3–4 kg per month at higher doses, but some people lose weight faster or slower. The NHS recommends sustainable weight loss of 0.5–1 kg per week. Results depend on dose, diet, physical activity, and individual metabolism.
Can I get Mounjaro through my NHS GP?
Mounjaro is prescribed through NHS specialist weight management services (tier 3/4), not standard GP practice. Your GP can refer you to these services if you meet eligibility criteria, but availability and waiting times vary significantly across different NHS regions in the UK.
What happens if I stop taking Mounjaro?
When you stop Mounjaro, appetite regulation returns to baseline over time, and weight may be regained if diet and activity levels don't change. Mounjaro is typically used long-term alongside lifestyle changes. Your prescriber will discuss whether continued treatment is appropriate for you.
Is Mounjaro the same as Ozempic?
No. Ozempic (also semaglutide) is approved for type 2 diabetes at doses up to 2 mg and is not licensed for weight loss in the UK. Mounjaro (tirzepatide) is specifically licensed for weight management. They are different medications with different mechanisms and uses. Never use Ozempic for weight loss without explicit medical guidance.
Ready to take control of your weight loss journey?
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