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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Mounjaro vs Wegovy: Ingredients and How They Work
Both Mounjaro and Wegovy are prescription-only weight management injections licensed in the UK, but they work differently due to their active ingredients. Mounjaro contains tirzepatide, a dual GIP and GLP-1 receptor agonist, while Wegovy contains semaglutide, a GLP-1 receptor agonist only. Understanding how these ingredients work and comparing clinical trial data can help you and your GP decide which medication may be more suitable for your individual circumstances.
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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.
Active Ingredients and How Each Works
Mounjaro (Tirzepatide) is a dual-action medication that activates two natural hormone pathways simultaneously. It targets both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) receptors. This dual mechanism means it works on appetite regulation through two routes: reducing hunger signals, slowing stomach emptying, and helping regulate fat utilisation. The tirzepatide UK prescription is available in doses from 2.5 mg up to 15 mg once weekly.
Wegovy (Semaglutide) activates the GLP-1 receptor pathway only, making it a single-mechanism medication. It works by enhancing the body's natural GLP-1 hormone response to food, signalling fullness to the brain and slowing gastric emptying. Wegovy titrates from 0.25 mg up to a maximum dose of 2.4 mg once weekly. Both are delivered via pre-filled injection pens, though the dose escalation schedules differ significantly.
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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.
Clinical Trial Data and Weight Loss Outcomes
The SURMOUNT-1 trial for Mounjaro showed average weight loss of approximately 20.9% at the maximum 15 mg dose over 72 weeks, compared to placebo. The STEP-1 trial for Wegovy showed average weight loss of approximately 14.9% at 2.4 mg over 68 weeks. However, individual results vary considerably. Both trials included reduced-calorie diet and increased physical activity alongside medication. These are average outcomes — some participants lost more, others less. Your prescriber can discuss what realistic expectations might be based on your health profile and starting weight.
- 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 is the main active ingredient in Mounjaro?
Mounjaro's active ingredient is tirzepatide. It is a dual GIP and GLP-1 receptor agonist, meaning it activates two different hormone pathways in the body to reduce appetite and support weight loss. Tirzepatide is the first medication in its class approved for weight management in the UK.
What is the active ingredient in Wegovy?
Wegovy contains semaglutide as its active ingredient. Semaglutide is a GLP-1 receptor agonist, activating one hormone pathway. It was originally developed for type 2 diabetes management and is also available as Ozempic — but Wegovy is the licensed weight management version available in the UK.
Which medication is more effective for weight loss?
Clinical trials showed Mounjaro (tirzepatide) produced slightly higher average weight loss than Wegovy (semaglutide) — around 20.9% versus 14.9%. However, individual results vary significantly. Neither medication is assured to work the same way for every person. Your GP will assess which is more suitable based on your health, medical history, and individual circumstances.
Can I switch between Mounjaro and Wegovy if one doesn't work?
Switching between medications is possible but must be managed by your prescriber. There is no official conversion chart between the two — your GP will make recommendations based on your response to the first medication and your individual health. Any switch requires a new clinical assessment and prescription.
Do both medications require a prescription in the UK?
Yes, both Mounjaro and Wegovy are prescription-only medicines in the UK. Access is either through NHS specialist tier 3 weight management services (with potentially long waiting lists depending on your region) or through private prescribing following an online clinical assessment with a UK-licensed healthcare provider.
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.