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 Mounjaro? The Slimming Drug Explained
Mounjaro is a prescription-only slimming drug licensed in the UK for weight management in adults. The active ingredient is tirzepatide, a dual GIP and GLP-1 receptor agonist that works differently from other weight loss injections like Wegovy. It's administered as a once-weekly injection using a pre-filled KwikPen device. Mounjaro may help reduce appetite and support feeling fuller after meals, and clinical trials showed it helped participants lose an average of 15–21% of their body weight — though individual results vary significantly and depend on lifestyle changes alongside the medication.
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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 Does Mounjaro Work as a Slimming Drug?
Mounjaro's mechanism is unique because it activates two natural hormone pathways simultaneously: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action sets it apart from single-action GLP-1 drugs. The medication signals to your brain that you're fuller sooner, may reduce cravings for food, and slows how quickly food leaves your stomach — though this effect typically reduces over time.
In the SURMOUNT-1 clinical trial, participants taking the maximum 15 mg dose lost an average of 20.9% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. Importantly, Mounjaro is not insulin and doesn't work the same way as diabetes medications, even though it was originally developed for type 2 diabetes management. It's a prescription-only medicine subject to additional MHRA monitoring (Black Triangle status).
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No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 Eligibility, Doses, and UK Access
You may be eligible for Mounjaro if you have a BMI of 30 or above, or 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 qualify at lower BMI thresholds due to higher health risks at the same BMI. Check your BMI using our free BMI calculator above.
Doses start at 2.5 mg weekly and increase gradually to a maximum of 15 mg. Access in the UK is through NHS specialist tier 3 weight management services (availability varies by region with long waiting lists in many areas) or through private online prescribing following a clinical assessment. Our GLP-1 weight loss calculator can help you understand potential weight loss at different doses based on clinical trial data.
- 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. Mounjaro contains tirzepatide and is a dual GIP+GLP-1 agonist. Wegovy contains semaglutide and is a GLP-1 agonist only. Ozempic also contains semaglutide but is approved only for diabetes, not weight loss in the UK. Mounjaro showed slightly higher average weight loss (20.9% at 15 mg) compared to Wegovy (around 15%), but individual results vary.
What are the main side effects of Mounjaro as a slimming drug?
Most common side effects are nausea, diarrhoea, constipation, vomiting, and stomach pain — typically mild and improving over time. Inject site reactions, fatigue, and hair loss occur in some users. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and gallbladder problems. Discuss any concerns with your prescriber.
How do I get Mounjaro prescribed in the UK?
Via NHS: referral from your GP to a tier 3 specialist weight management service, though waiting lists are often lengthy and availability varies by region. Via private: online consultation with a UK-licensed prescriber, followed by prescription and dispensing by a GPhC-registered pharmacy. Private access is faster but involves costs typically between £129–£320 monthly depending on dose.
How long does it take to see weight loss results with Mounjaro?
Most people notice early effects (reduced hunger, feeling fuller sooner) within the first 1–2 weeks, but meaningful weight loss typically appears after 4–8 weeks. Clinical trials measured results over 72 weeks. Weight loss depends on dose, adherence, calorie deficit, and physical activity. Use our progress tracker to log your journey over time.
Can I take Mounjaro if I'm on other medications?
Mounjaro may affect how your body absorbs oral medications. Women on oral contraceptives should use barrier methods or switch to non-oral contraception for 4 weeks after starting and after each dose increase. If you take diabetes medications, your prescriber may adjust doses. Always inform your prescriber of all medications you're taking before starting Mounjaro.
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.