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 Happens in Week 4 of Mounjaro Treatment?
Week 4 marks an important milestone in your Mounjaro journey — you're completing the initial starting phase on the 2.5mg dose. At this stage, many people are beginning to notice changes in their appetite and food preferences, though individual experiences vary significantly. Understanding what to expect during this critical adjustment period, and how Mounjaro fits within NHS weight management services in the UK, can help you feel more confident as you progress toward your maintenance dose. This guide covers NHS eligibility, the dose titration schedule, and what medical support you should expect from your prescriber.
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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.
NHS Eligibility and Week 4 Treatment Context
To access Mounjaro (tirzepatide) through NHS specialist services, you must meet specific criteria. Adults with a BMI of 30 kg/m² or above, or a BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, prediabetes, high blood pressure, or sleep apnoea may be eligible. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS applies lower BMI thresholds — eligibility may begin at BMI 27.5 or above, reflecting higher cardiovascular risk at lower weights in these populations.
By week 4, you should be well into your treatment plan through a tier 3 specialist weight management service. These services provide medical oversight, dietary support, and behavioural guidance alongside your medication. You're still on the 2.5mg starting dose — the standard dose titration schedule requires you to remain on this dose for a minimum of 4 weeks before escalation to 5mg in week 5, allowing your body to adjust to the medication.
- Continue your 2.5mg injection once weekly on the same day
- Store your Mounjaro KwikPen in the refrigerator (2–8°C) if unopened; after first use, can be kept at room temperature (below 30°C) for up to 30 days
- Report any persistent side effects or concerns to your prescribing service
Clinically BackedWeight Loss
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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.
Managing Week 4 and Preparing for Dose Escalation
By week 4, nausea and digestive changes are common — these typically peak around weeks 2–4 and often improve as your body adapts. Stay hydrated, eat small frequent meals, and avoid high-fat or rich foods if they trigger symptoms. Use our free BMI calculator to track where you stand against NHS healthy weight targets, and our GLP-1 weight loss calculator to see projected outcomes at different doses as you move toward maintenance.
In week 5, your prescriber will assess your progress and side effect tolerance before increasing to 5mg. NICE guidance supports tirzepatide as an effective dual GIP and GLP-1 agonist — clinical trials showed average weight loss of around 15–21% body weight at maintenance doses, though results vary by individual. Keep a record of your weight, appetite changes, and any side effects using our free progress tracker, which helps your medical team make informed decisions about your ongoing treatment plan.
- 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.
Can I increase my Mounjaro dose in week 4?
No. The standard UK prescribing protocol requires you to stay on 2.5mg for a minimum of 4 weeks. This allows your body to adjust to the medication and helps your prescriber assess your tolerability. You'll typically escalate to 5mg in week 5, provided your side effects are manageable and you're tolerating the medication well.
What side effects should I expect in week 4?
Nausea, constipation, diarrhoea, and stomach discomfort are very common in weeks 2–4. Most people find these symptoms improve as their body adapts. If side effects are severe or persistent, contact your specialist service — they may adjust your approach or provide additional support without stopping treatment.
Is Mounjaro available on the NHS in week 4?
Yes, if you've been prescribed it through a tier 3 NHS specialist weight management service following proper assessment. NHS access depends on meeting BMI and health criteria and being referred by your GP. Access varies significantly by region and availability of specialist services in your area.
How does Mounjaro vs Wegovy compare for week 4 treatment?
Both are once-weekly injections, but Mounjaro is a dual GIP and GLP-1 agonist (starting at 2.5mg) whilst Wegovy is a GLP-1-only medication (starting at 0.25mg). Mounjaro's dual mechanism may produce stronger appetite suppression, though both require the same 4-week minimum on starting doses. Individual response varies.
What should I track in week 4 to monitor progress?
Record your weight weekly, note any changes in appetite or food preferences, track side effects, and monitor energy levels. Our free progress tracker makes this easy — sharing this data with your specialist service helps them optimise your treatment plan and decide on dose escalation safely.
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.