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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Week 1 Mounjaro vs Wegovy — What's the Difference?
Starting weight loss medication can feel overwhelming — you may have heard about both Mounjaro and Wegovy and wondered which is right for you. Both are prescription-only, once-weekly injections licensed in the UK for weight management, but they work in different ways and have different starting regimens. This guide explains how week 1 Mounjaro compares to Wegovy, covering mechanism, initial doses, and what to expect during your first week on either medication. The choice between them is made by your prescriber following a clinical assessment of your individual health.
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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 Week 1 Differs: Starting Doses and Mechanisms
Mounjaro (tirzepatide) starts at 2.5 mg in week 1 — this is your initial dose, which you'll repeat each week for four weeks before any increase. It works as a dual GIP and GLP-1 receptor agonist, activating two natural hormone pathways in the gut. This dual mechanism may help regulate appetite and support feeling fuller after meals more potently than a single pathway alone.
Wegovy (semaglutide) starts at just 0.25 mg in week 1 — a much lower dose — and escalates more gradually over 16 weeks to reach its maximum of 2.4 mg. It works as a GLP-1 receptor agonist only, activating one hormone pathway. Both medications slow gastric emptying (how quickly food leaves your stomach), though this effect often reduces over time.
The SURMOUNT clinical trial showed Mounjaro participants lost an average of around 15–21% of body weight, whilst STEP trial data for Wegovy showed approximately 15% average weight loss — though individual results vary significantly and depend on diet, activity, and individual response.
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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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Which Should You Choose? It's Your Prescriber's Decision
There is no "better" option between Mounjaro and Wegovy — the right choice depends entirely on your individual health profile, medical history, and what your prescriber assesses as most suitable for you. Both are prescription-only, meaning you'll need a clinical assessment before either is prescribed. Some people may be offered one; others may be offered both and choose based on cost or preference; some may be advised that one is more appropriate than the other due to existing health conditions or medications.
Both require commitment to lifestyle changes alongside the injection — reduced-calorie diet and increased physical activity are essential for best results. Your GP or a licensed prescriber can discuss eligibility criteria, potential side effects, and how the medication fits into your overall weight management plan. Use our free GLP-1 weight loss calculator above to explore projected outcomes at different doses of either medication, and our BMI calculator to understand your starting point.
- 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's the main difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways. Wegovy is a GLP-1 only agonist, activating one pathway. Both are once-weekly injections licensed in the UK for weight management, but they have different starting doses — Mounjaro starts at 2.5 mg, Wegovy at 0.25 mg — and different escalation schedules.
Is Mounjaro better than Wegovy?
Clinical trials show both are effective for weight loss, though they achieved results through different mechanisms. Neither is objectively "better" — the right choice depends on your individual health profile and what your prescriber assesses as most appropriate for you following a clinical assessment.
Can I switch from Mounjaro to Wegovy in week 1?
Switching medications should only be managed by your prescriber. If you're considering a change, discuss it with your GP or the prescriber who assessed you. They can advise on whether switching is suitable and how to do so safely without health risk.
What side effects happen in week 1 of Mounjaro?
Week 1 side effects with Mounjaro's 2.5 mg starting dose are typically mild. Nausea is most common, along with possible constipation or mild stomach discomfort. Most people tolerate the starting dose well, though individual experiences vary. Speak to your prescriber if side effects are severe or concerning.
How do I know which injection is right for me?
Your prescriber will assess your medical history, current health conditions, medications, BMI, and weight-related complications to recommend the most suitable option. Both require a prescription following clinical assessment — neither is available without this evaluation. Discuss eligibility and suitability openly with your GP or prescriber.
Ready to take control of your weight loss journey?
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