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 Does Mounjaro Do?
Mounjaro (tirzepatide) is a prescription-only weight loss injection that works differently from other slimming injections available in the UK. It's a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways in your body to help regulate appetite and support weight management. Clinical trials showed participants lost an average of 15–21% of their body weight when using Mounjaro alongside a reduced-calorie diet and increased physical activity. Individual results vary, and it requires a clinical assessment before prescription.
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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 in Your Body
Mounjaro contains tirzepatide, which mimics two natural gut hormones: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism is what makes Mounjaro different from other weight loss medications like Wegovy, which only activates GLP-1.
When you use Mounjaro:
- It signals your brain that you're fuller after smaller meals, reducing overall appetite
- It slows how quickly food leaves your stomach, helping you feel satisfied longer
- It helps your body better utilise fat for energy
- It supports healthy blood sugar regulation
The KwikPen is injected once weekly under the skin. Doses start at 2.5mg and increase gradually over weeks to a maximum of 15mg, depending on your response and tolerability. Most people reach an effective maintenance dose between 5mg and 10mg.
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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.
Expected Weight Loss and Eligibility in the UK
In the SURMOUNT-1 clinical trial, adults taking Mounjaro at the 15mg dose lost approximately 20.9% of their body weight over 72 weeks—though results at lower doses were also significant (around 15% at 5mg).
You may be eligible for Mounjaro in the UK if you have:
- A BMI of 30 or above, OR
- A BMI of 27–29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, sleep apnoea, or cardiovascular disease)
- Lower thresholds apply if you're of South Asian, Chinese, Black African, or Caribbean heritage
You can calculate your BMI using our free BMI calculator to check where you stand. Mounjaro is available through NHS specialist weight management services or via private prescription following an online clinical assessment.
- 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 Wegovy?
No. Both are prescription weight loss injections, but Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist, while Wegovy (semaglutide) activates only GLP-1. Clinical trials showed Mounjaro achieved slightly higher average weight loss (20.9% vs 14.9%), though individual results vary. Both require prescription and clinical assessment.
What are the main side effects of Mounjaro?
Very common side effects include nausea, diarrhoea, constipation, vomiting, and stomach pain. Most people experience mild nausea initially, which often improves over time. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and allergic reactions. Always report concerns to your prescriber. Our side effects guide covers this in more detail.
How quickly will I lose weight on Mounjaro?
Weight loss typically begins within the first few weeks but accelerates as your dose increases. Most people see meaningful weight loss by 12–16 weeks. The NHS recommends losing 0.5–1kg per week as sustainable. Results depend on your diet, activity level, starting weight, and individual metabolism. Our GLP-1 weight loss calculator can help estimate potential outcomes at different doses.
Can I get Mounjaro on the NHS in the UK?
Yes, Mounjaro is approved by NICE for use in NHS specialist weight management services (tier 3). However, access varies significantly by region, and waiting lists can be lengthy. You'll need a GP referral to a specialist service. Private prescription is also available through UK-licensed prescribers without NHS referral, though costs vary by dose and provider.
How is Mounjaro different from Ozempic?
Both contain semaglutide—wait, that's incorrect. Mounjaro contains tirzepatide (dual GIP/GLP-1), while Ozempic contains semaglutide (GLP-1 only) and is licensed for type 2 diabetes, not weight loss. Wegovy also contains semaglutide but is the licensed weight management version. Mounjaro's dual mechanism may offer different benefits, though results vary individually.
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.