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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Comparing Mounjaro and Wegovy After 4 Weeks
If you're considering weight loss injections in the UK, you may be comparing Mounjaro (tirzepatide) and Wegovy (semaglutide). After 4 weeks on either medication, you'll typically be on a starting or low maintenance dose. Both are prescription-only GLP-1-based medications licensed for weight management, but they work through slightly different mechanisms. This guide breaks down what happens in the first 4 weeks, how they compare, and what to expect. Always consult your GP or a licensed prescriber to determine which is suitable for you personally.
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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 and Wegovy Differ — Clinical Trial Evidence
Mounjaro (tirzepatide) works as a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways rather than one. After 4 weeks, most people are on the 2.5 mg starting dose. Wegovy (semaglutide) is a single GLP-1 agonist, and after 4 weeks, users are typically on the 0.25 mg starting dose.
Clinical trial data shows different average weight loss profiles: the SURMOUNT-1 trial for Mounjaro showed approximately 15–21% average body weight loss over 72 weeks at higher doses. The STEP-1 trial for Wegovy showed approximately 14.9% average weight loss at the 2.4 mg dose over 68 weeks. Both trials involved participants following reduced-calorie diets and increased physical activity alongside injections.
- Mounjaro: dual mechanism (GIP + GLP-1), larger maximum dose (15 mg)
- Wegovy: single mechanism (GLP-1 only), lower maximum dose (2.4 mg)
- Both: once-weekly injections, prescription-only in the UK
- Both require clinical assessment before prescription
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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.
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What to Expect After 4 Weeks — Suitability and Side Effects
At 4 weeks into either medication, you're typically at a starting dose. Many people experience nausea, changes in appetite, and digestive changes (constipation or diarrhoea) during this initial phase. These side effects often improve as your body adjusts, though they can persist. Individual experiences vary widely.
Both medications require a prescriber's clinical assessment to determine which is appropriate for you, based on your BMI, health conditions, medication interactions, and personal circumstances. Mounjaro may be chosen if your prescriber believes the dual mechanism offers better suitability for your situation. Wegovy may be preferred if you have a history of particular conditions or medication sensitivities. Neither is universally "better"—suitability is personal and clinically determined.
Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses, and track your progress with our progress tracker tool.
- 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.
Will I see weight loss results after 4 weeks on Mounjaro or Wegovy?
Significant weight loss typically takes longer than 4 weeks—most noticeable changes occur after 8–12 weeks. At 4 weeks, many people report reduced appetite and changes in food preferences, but visible weight loss varies. Individual results depend on dose, diet adherence, and personal metabolism. Clinical trials measured results over weeks and months, not days or weeks.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways. Wegovy (semaglutide) activates only the GLP-1 pathway. The dual mechanism may help some people achieve better appetite regulation, though both medications support weight loss effectively. Your prescriber will determine which mechanism suits your individual health profile.
Are side effects worse at 4 weeks on Mounjaro vs Wegovy?
Both medications commonly cause nausea, constipation, or diarrhoea in the first 4 weeks. Side effect severity varies individually and typically improves as your body adjusts. There's no clinical evidence that one causes significantly worse early side effects than the other—it's highly personal and depends on your individual tolerance.
Can I switch from Mounjaro to Wegovy after 4 weeks?
Switching between medications should only be managed by your prescriber. There's no official conversion chart between Mounjaro and Wegovy doses, though approximate equivalences exist (5 mg Mounjaro ≈ 0.5 mg Wegovy). Any switch requires clinical assessment to ensure safe transition and appropriate dosing.
Do I need a prescription for Mounjaro or Wegovy in the UK?
Yes, both are prescription-only medicines in the UK. You'll need a clinical assessment from a licensed prescriber (either through NHS specialist services or private prescribing) before either medication can be dispensed. Your GP can refer you to NHS tier 3 weight management services, or you can access private prescribing through GPhC-registered pharmacies online. ```
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