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
Mounjaro Slimming Injections in the UK
Mounjaro (tirzepatide) is a prescription-only slimming injection licensed in the UK for weight management in adults. It works differently from other weight loss injections because it activates two natural hormone pathways — GIP and GLP-1 — rather than just one. Clinical trials have shown that Mounjaro may help people lose an average of 15–21% of their body weight when used alongside a reduced-calorie diet and increased physical activity. Access in the UK is through NHS specialist weight management services or private online prescribing following a clinical assessment. Individual results vary significantly.
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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 UK Eligibility
Mounjaro's dual GIP and GLP-1 mechanism is what distinguishes it from other slimming injections. It helps regulate appetite, reduce food cravings, and support feeling fuller after smaller meals. The medication also slows how quickly food leaves the stomach, though this effect typically reduces over time.
UK eligibility: You may qualify for Mounjaro if you have a BMI of 30 kg/m² or above, or a BMI of 27–29.9 kg/m² with a weight-related health condition such as type 2 diabetes, high blood pressure, or cardiovascular disease. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds may apply (27.5 or above). Mounjaro is not suitable during pregnancy, breastfeeding, or for people under 18. Use our free BMI calculator above to check your current BMI and understand where you sit on the NHS scale.
The tirzepatide UK prescription pathway requires assessment by a licensed prescriber to confirm suitability before dispensing.
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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, Side Effects, and Storage
Dosing: Mounjaro starts at 2.5 mg once weekly for 4 weeks, then increases gradually — 5 mg, 7.5 mg, 10 mg, 12.5 mg, and up to 15 mg as the maximum dose. Each KwikPen contains 4 weekly doses.
Common side effects include nausea, diarrhoea, constipation, and stomach pain — typically mild and reducing over time. Store unopened pens at 2–8°C; after first use, they can be kept at room temperature for up to 30 days.
Mounjaro is injected once weekly into your stomach, thigh, or upper arm. Try our free GLP-1 weight loss calculator to estimate potential results at different doses, and track your progress with our progress tracker as you continue treatment.
- 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 "slimming jab"?
Yes. "Slimming jab", "fat jab", and "weight loss injection" are colloquial UK terms for prescription weight loss medications like Mounjaro. Mounjaro is a prescription-only medicine, not a cosmetic injection. It requires a clinical assessment before being prescribed and is used alongside diet and exercise changes.
How does Mounjaro compare to other weight loss injections like Wegovy?
Mounjaro uses a dual GIP and GLP-1 mechanism, while Wegovy (semaglutide) is a GLP-1-only injection. Clinical trials suggest Mounjaro may produce slightly higher average weight loss (15–21%) compared to Wegovy's 15%, but individual results vary. Both are prescription-only and require clinical assessment. Your prescriber can advise which is more suitable for you.
Can I get Mounjaro on the NHS in the UK?
Mounjaro is approved for NHS use through specialist tier 3 weight management services. However, NHS availability varies significantly by region and ICB. Many areas have long waiting lists. Private prescribing through GPhC-registered online pharmacies is also available if you meet the clinical criteria, though costs apply.
What happens if I miss a Mounjaro injection?
If you miss a dose within 4 days, take it as soon as you remember and continue your normal schedule. If it's been more than 4 days, skip that dose and take your next dose on the usual scheduled day. Never take a double dose. Contact your prescriber if you're unsure.
Will Mounjaro affect my oral contraceptive pill?
Yes. Mounjaro may reduce the absorption of oral medications, including the contraceptive pill. If you take oral contraception, use an additional barrier method or switch to a non-oral contraceptive for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. Always discuss this with your prescriber.
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.