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
Free tracker included with every download this week
What to Expect From Mounjaro After 4 Weeks vs Wegovy
After 4 weeks on Mounjaro, you'll typically be on the starting dose of 2.5 mg as part of your dose titration schedule. If you're comparing Mounjaro and Wegovy at this early stage, it's important to understand that both are prescription-only weight management injections available in the UK, but they work through different mechanisms. This guide explains what happens during week 4 on Mounjaro and how it compares with Wegovy's initial phase. Use our free GLP-1 weight loss calculator to explore potential outcomes with either medication alongside your clinical assessment.
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Calculate Your BMI Instantly
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.
How Mounjaro and Wegovy Differ at 4 Weeks
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways simultaneously, which distinguishes it from Wegovy. At week 4, you'll still be on the 2.5 mg starting dose; the minimum 4-week interval before escalation means most patients move to 5 mg in week 5.
Wegovy (semaglutide) is a GLP-1-only agonist with a different dose schedule. At week 4, you'd typically be on 0.5 mg (having started at 0.25 mg in week 1). Both medications are injected once weekly via KwikPen or pre-filled auto-injection pen. The clinical trial evidence comes from different sources: SURMOUNT trials for Mounjaro and STEP trials for Wegovy. Early side effects are common in both — nausea, diarrhoea, and appetite changes are typical at week 4 as your body adjusts to the medication.
Individual response varies significantly. Your prescriber will assess your personal eligibility and suitability based on your health profile, not just the brand name.
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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.
Eligibility and Prescriber Decision
Both Mounjaro and Wegovy require a UK prescription following a clinical assessment. Eligibility criteria are broadly similar: adults with a BMI of 30 kg/m² or above, or BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, hypertension, or cardiovascular disease. For those of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds may apply. Your GP or a licensed prescriber will decide which medication (if any) is appropriate for you. Neither medication assures specific weight loss outcomes — results depend on individual factors, diet, and physical activity. After your initial prescription, track your progress using our free progress tracker to monitor changes beyond the 4-week mark.
- 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 notice weight loss after 4 weeks on Mounjaro?
At week 4, you're still on the starting 2.5 mg dose. Some people report early changes in appetite or reduced food cravings, but significant weight loss typically occurs after dose escalation and beyond. Clinical trials showed meaningful results after 12+ weeks. Individual responses vary — your prescriber can discuss realistic timelines.
How does Mounjaro's mechanism differ from Wegovy after 4 weeks?
Mounjaro activates both GIP and GLP-1 pathways (dual action), while Wegovy activates GLP-1 only (single action). This is why Mounjaro may work slightly differently. However, at week 4, both are still in early adjustment phases. Long-term outcomes depend on dose escalation and adherence.
Can I switch between Mounjaro and Wegovy at the 4-week mark?
Switching medications is not recommended at week 4. Both medications require a minimum escalation schedule — Mounjaro needs 4 weeks per dose, Wegovy needs varied intervals. Any change should be discussed with your prescriber, who may recommend continuing your current medication.
What side effects are common at week 4 on either medication?
Nausea, diarrhoea, constipation, and stomach discomfort are very common in the first month on both Mounjaro and Wegovy. These often improve as your body adjusts. Inform your prescriber of persistent side effects — they can discuss management strategies or dosing adjustments.
Which medication should I choose — Mounjaro or Wegovy?
This is a prescriber decision based on your individual health, medical history, and suitability. Both are UK-licensed for weight management. Your GP or licensed prescriber will assess you and recommend the most appropriate option. Neither is universally "better" — individual response differs.
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.