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
What Is the Mounjaro Jab?
The Mounjaro jab is a once-weekly prescription injection containing tirzepatide, a dual GIP and GLP-1 receptor agonist licensed in the UK for weight management. It's administered as a pre-filled KwikPen device and works by helping regulate appetite and supporting weight loss when combined with lifestyle changes. Clinical trials showed participants lost an average of 15–21% of their body weight, though individual results vary significantly. Your GP or a licensed prescriber will assess whether this medication is suitable for you based on your personal health profile.
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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 the Mounjaro Jab Works
Unlike many weight loss injections, the Mounjaro jab activates two hormone pathways in your body: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism sets it apart from single-action medications. When you inject Mounjaro, it signals to your brain that you feel fuller after smaller meals, reduces food cravings, and slows how quickly food leaves your stomach—meaning you stay satisfied longer. It also helps your body better regulate blood sugar and fat utilisation. The jab doesn't work like insulin; instead, it mimics natural hormones your gut already produces. Clinical evidence suggests this dual-action approach may produce stronger appetite suppression than single GLP-1 receptor agonists. Results are most effective when combined with a reduced-calorie diet and increased physical activity—the jab supports your efforts rather than working alone.
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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 Jab Doses and Administration
The Mounjaro jab comes as a pre-filled KwikPen in five strength options: 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg. You start with 2.5 mg once weekly for 4 weeks, then increase gradually every 4 weeks until you reach an effective maintenance dose (typically 10–15 mg). Each pen contains 4 weekly doses (one month's supply). You inject under the skin on your stomach, thigh, or upper arm, rotating injection sites each week. Store unopened pens in the fridge at 2–8°C; after first use, you can keep the pen at room temperature for up to 30 days. Try our GLP-1 weight loss calculator to estimate potential results at different doses based on clinical trial data.
Eligibility and Getting Mounjaro on the NHSYou may be eligible for Mounjaro if you have a BMI of 30 or above, or a BMI of 27–29.9 with a weight-related health condition such as type 2 diabetes, high blood pressure, or heart disease. For South Asian, Chinese, Black African, and Black Caribbean adults, eligibility may start at a lower BMI threshold due to higher health risks at lower weights. In the UK, NHS access is through specialist tier 3 weight management services—you'll need a referral from your GP, though waiting lists vary by region and can be lengthy. Private access is available through registered online prescribers following a clinical assessment, usually within days rather than months. Use our BMI calculator above to check your personal BMI and discuss your eligibility with your healthcare provider.
- 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.
What's the difference between the Mounjaro jab and Wegovy?
Both are once-weekly weight loss injections licensed in the UK, but Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a single GLP-1 agonist. Clinical trials suggest Mounjaro may produce slightly higher average weight loss (15–21% vs around 15%), though individual results vary. Both require prescription and clinical assessment.
What are the main side effects of the Mounjaro jab?
Common side effects include nausea, diarrhoea, vomiting, and constipation—especially when starting or increasing doses. These typically improve over time as your body adjusts. Less common side effects include fatigue, dizziness, and injection site reactions. Seek urgent medical advice if you experience severe stomach pain, gallbladder problems, or serious allergic reactions.
Can I get the Mounjaro jab on the NHS in my area?
NHS access to Mounjaro is through specialist weight management services (tier 3), which requires a GP referral. Availability and waiting times vary significantly by region and NHS ICB. Private access is available through registered UK prescribers without NHS referral, though this involves a cost. Ask your GP about local NHS options first.
How much weight will I lose with the Mounjaro jab?
Clinical trials showed average weight loss of around 15–21% of body weight over 72 weeks when combined with a reduced-calorie diet and physical activity. However, individual results vary considerably—some people lose more, others less. Your age, starting weight, genetics, and how consistently you follow lifestyle changes all affect outcomes.
Do I need to keep taking the Mounjaro jab forever?
There's no set duration. Some people use it long-term to maintain weight loss; others stop after reaching their goal. If you stop, weight regain is possible without continued lifestyle changes. Speak with your prescriber about the best approach for your situation—they'll help you create a personalised plan.
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.