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 to Expect in Your First Week of Mounjaro
Your first week of Mounjaro is typically when you'll receive your starting dose of 2.5 mg via the KwikPen injection device. This is a low dose designed to help your body adjust to the medication. Many people wonder how Mounjaro's first week compares to starting Wegovy, another weight loss injection available in the UK. Both are prescription-only medications that work differently, and understanding these differences can help you know what to expect as you begin treatment. This guide explains the key similarities and differences between the two.
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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.
Mounjaro vs Wegovy: First Week Mechanism & Dose Comparison
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — meaning it activates two natural hormone pathways rather than one. In your first week, you'll inject 2.5 mg once weekly, which is the standard starting dose. Wegovy (semaglutide) is a GLP-1-only receptor agonist, and its first-week dose is much smaller: just 0.25 mg. This is why the two medications are not directly comparable by dose number alone.
- Mounjaro first dose: 2.5 mg once weekly (weeks 1–4)
- Wegovy first dose: 0.25 mg once weekly (weeks 1–4)
- Key difference: Mounjaro's dual mechanism (GIP + GLP-1) vs Wegovy's single mechanism (GLP-1 only)
- Clinical trial data: SURMOUNT (Mounjaro) and STEP (Wegovy) trials showed different average weight loss outcomes, though individual results vary significantly
Both medications are available in the UK through NHS specialist services and private prescribing. Your GP or prescriber will assess which is most suitable for your individual health profile.
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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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First Week Side Effects & Suitability: Mounjaro vs Wegovy
In your first week of either medication, nausea is the most commonly reported side effect. With Mounjaro's higher starting dose (2.5 mg), some people report slightly stronger initial nausea compared to Wegovy's gentler 0.25 mg start — though this varies greatly between individuals. Both medications may cause stomach discomfort, diarrhoea, or constipation. These typically ease as your body adjusts over the first 2–4 weeks.
Neither Mounjaro nor Wegovy is suitable for everyone. Your prescriber will assess your UK eligibility based on BMI (typically 30 or above, or 27+ with weight-related health conditions like type 2 diabetes or high blood pressure), medical history, and current medications. Use our free GLP-1 weight loss calculator to explore projected outcomes, and our BMI calculator to understand your starting health metrics. Both inform your discussion with your GP.
- 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 actual starting dose difference between Mounjaro and Wegovy in week one?
Mounjaro starts at 2.5 mg once weekly, while Wegovy starts at 0.25 mg once weekly. The difference reflects their distinct mechanisms: Mounjaro's dual GIP+GLP-1 action vs Wegovy's GLP-1-only action. Do not compare doses directly—they are not equivalent units.
Will I experience worse side effects with Mounjaro's higher first-week dose?
Not necessarily. Individual responses vary significantly. Some people tolerate Mounjaro's 2.5 mg well, while others find Wegovy's gentler start easier. Nausea, stomach pain, and changes in digestion are common with both in week one but typically improve by week 3–4.
How long does it take to see weight loss results—is there a difference between Mounjaro and Wegovy in week one?
Weight loss in the first week is typically minimal or not noticeable because these are low starting doses. Both medications work gradually. Significant results usually appear after 4–8 weeks as doses increase. Clinical trials showed average weight loss over 68–72 weeks, not the first week.
Can I switch from Mounjaro to Wegovy (or vice versa) if I don't tolerate the first dose?
Switching between medications is possible but should only be managed by your prescriber. There is no official dose conversion chart. Your prescriber may instead adjust the current dose, or monitor side effects for 2–4 weeks before deciding on a switch. Always discuss concerns with your GP.
How do I get prescribed Mounjaro or Wegovy in the UK—is the process different for each?
Both are prescription-only and require clinical assessment. You can access Mounjaro and Wegovy through NHS specialist tier 3 weight management services (usually via GP referral) or through private online prescribing. Eligibility and availability vary by region and private provider.
Ready to take control of your weight loss journey?
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