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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Second Week On Mounjaro: How It Compares To Wegovy
If you're in your second week on Mounjaro, you might be wondering how it compares to Wegovy—the other GLP-1 injection licensed for weight management in the UK. Both are prescription-only medicines that may help with weight loss, but they work slightly differently and have different dosing schedules. Understanding the differences can help you discuss your treatment with your prescriber and set realistic expectations for your weight loss journey.
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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.
Mounjaro vs Wegovy: Clinical Trial Data and Mechanism
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways. In the SURMOUNT-1 clinical trial, participants on Mounjaro 15 mg lost an average of approximately 20.9% of their body weight over 72 weeks alongside a reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. In the STEP-1 trial, participants on Wegovy 2.4 mg lost an average of approximately 14.9% of their body weight at 68 weeks with similar lifestyle support.
- Mounjaro's dual mechanism may offer greater appetite suppression for some individuals
- Wegovy has longer-established use in the UK and more real-world data
- Both require a clinical assessment before prescription
- Both are once-weekly injections given via pre-filled pen
In your second week on Mounjaro, you'll still be on the 2.5 mg starting dose—this low dose is intentional to allow your body to adjust and minimise side effects as you gradually increase.
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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.
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Dosing Schedules and Individual Suitability
Mounjaro follows a dose titration schedule where you increase every 4 weeks: 2.5 mg (weeks 1–4), then 5 mg, 7.5 mg, up to 15 mg maximum. Wegovy also titrates gradually: 0.25 mg, 0.5 mg, 1 mg, 1.7 mg, then 2.4 mg—but the steps are different and take longer overall.
Which is right for you depends on your individual health profile, any existing health conditions, and prescriber assessment. Both medicines require a UK prescription following a clinical evaluation. Your prescriber will consider your BMI, weight-related health conditions, and any medications you take. Use our free BMI calculator to understand your starting point, and our GLP-1 weight loss calculator to see projected outcomes at different doses. Individual results vary significantly—what matters most is consistency with your injection schedule and supporting lifestyle changes.
- 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 difference between Mounjaro's and Wegovy's mechanisms?
Mounjaro activates both GIP and GLP-1 hormone pathways (dual action), while Wegovy activates only GLP-1. This dual mechanism may help Mounjaro produce slightly larger average weight loss in clinical trials, though individual results vary considerably.
Am I on a different dose schedule than Wegovy users?
Yes. In week two, you're on Mounjaro 2.5 mg. Wegovy starts at 0.25 mg. Mounjaro's KwikPen doses are 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg. Wegovy reaches 2.4 mg maximum. Both increase every 4 weeks minimum.
Should I switch from Mounjaro to Wegovy if I'm not seeing results in week two?
No. Week two is far too early to judge effectiveness—you're still on the starting dose. Weight loss typically becomes noticeable after 4–8 weeks. Always discuss any concerns with your prescriber rather than switching without guidance.
Do Mounjaro and Wegovy have the same side effects?
Both commonly cause nausea, diarrhoea, and constipation, especially in the first few weeks. Mounjaro's side effect profile is very similar to Wegovy, though individual experiences vary. Most side effects improve as your body adjusts.
Can my GP prescribe either medication on the NHS?
Both are available through NHS specialist tier 3 weight management services following referral from your GP, though access and waiting times vary significantly by region. Private prescribing through online assessments is also available if you meet eligibility criteria.
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.