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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What Happens When You Take Mounjaro
When you take Mounjaro (tirzepatide), a dual GIP and GLP-1 receptor agonist, your body experiences changes in appetite regulation, how quickly food leaves your stomach, and how fat is utilised. This prescription injection works differently from Wegovy and other single GLP-1 medications. Understanding what happens in your body helps you recognise expected effects and side effects, and whether this medication suits your individual health needs. This guide compares Mounjaro's mechanism to Wegovy so you can understand the practical differences.
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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 activates two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism is what distinguishes tirzepatide from Wegovy, which activates GLP-1 only. When you take Mounjaro, your brain receives stronger signals that you are full, reducing appetite and food cravings. The medication also slows gastric emptying — how quickly food leaves your stomach — which helps you feel fuller for longer after smaller meals.
Expected timeline: Most people notice reduced appetite within days of the first dose, though this effect may reduce over weeks or months as your body adapts. Clinical data from the SURMOUNT trials showed average weight loss of approximately 15–21% of body weight over 72 weeks, depending on dose. However, individual results vary significantly — some people lose more, some less.
Mounjaro requires a dose titration schedule, starting at 2.5 mg and increasing weekly until reaching your maintenance dose (typically 5–15 mg). This gradual approach helps minimise initial side effects. The medication is injected once weekly using a KwikPen device into the stomach, upper thigh, or upper arm.
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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 vs Wegovy: Key Practical Differences
Both Mounjaro and Wegovy are prescription-only weight loss injections available in the UK through NHS specialist services or private online prescribing. The main difference is mechanism: Mounjaro has two active pathways (GIP + GLP-1), while Wegovy has one (GLP-1 only). SURMOUNT trials showed Mounjaro achieved average weight loss of 20.9% at maximum dose, compared to approximately 15% for Wegovy in STEP trials. However, clinical trial comparisons should be interpreted carefully — different trials, populations, and timeframes mean direct equivalence cannot be assumed.
Both medications have similar common side effects: nausea, diarrhoea, constipation, and reduced appetite. The choice between them depends on individual suitability, assessed by your GP or a licensed prescriber following a clinical evaluation. Your BMI, medical history, current medications, and personal health goals all influence which medication is recommended.
- 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 does Mounjaro do in your body?
Mounjaro (tirzepatide) activates two hormone pathways — GIP and GLP-1 — to reduce appetite, slow digestion, and support weight loss. It signals your brain that you are full, helping you eat smaller portions. Clinical trials showed average weight loss of 15–21% of body weight, though individual results vary significantly.
How is taking Mounjaro different from Wegovy?
Mounjaro has dual GIP+GLP-1 action versus Wegovy's single GLP-1 action. SURMOUNT trials showed Mounjaro achieved approximately 21% average weight loss compared to 15% for Wegovy, but both require prescription and clinical assessment. Individual suitability varies.
How quickly does Mounjaro start working?
Most people notice reduced appetite within days of starting Mounjaro. However, you begin on a low dose (2.5 mg) and increase weekly — significant weight loss typically appears after 4–8 weeks of treatment at higher doses.
What are the most common side effects of Mounjaro?
Nausea, diarrhoea, vomiting, and constipation are very common, affecting more than 1 in 10 people. Most side effects improve over time as your body adapts. Serious side effects like severe stomach pain (possible pancreatitis) are rare but require immediate medical attention.
Can I use our GLP-1 weight loss calculator to estimate my Mounjaro results?
Yes. Our GLP-1 weight loss calculator can help you explore projected outcomes at different doses, based on clinical trial data. Remember that individual results vary — your prescriber can give personalised guidance based on your health profile and the dose you're prescribed.
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.