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 After 4 Weeks on Mounjaro in the UK
After 4 weeks on your starting dose of Mounjaro (tirzepatide), you're at a key milestone in your weight management journey. At this point, your body is adjusting to the medication, and your prescriber will assess whether you're ready to move to the next dose. This guide explains what happens after 4 weeks on Mounjaro under NHS and UK private prescribing, what side effects typically resolve, and how the dose titration process works in UK clinical practice.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
The 4-Week Milestone and Dose Escalation in UK Practice
After 4 weeks on your starting dose of 2.5 mg Mounjaro, your prescriber will typically assess your tolerance and response before moving you to the next dose. This is why the NHS and private prescribers follow a strict minimum 4-week interval between dose increases — it allows your body time to adapt and helps identify which dose works best for you.
- Week 1–4 (2.5 mg dose): This is the adjustment phase. Many people experience gastrointestinal side effects like nausea, diarrhoea, or stomach discomfort during this period.
- Week 5 onwards (5 mg dose): If tolerated well, you'll typically escalate to 5 mg once weekly. This is the first maintenance dose in the tirzepatide dose titration schedule.
- Clinical assessment: Your prescriber will ask about side effect severity, appetite changes, and any early weight changes. If side effects are severe, you may remain on 2.5 mg for additional weeks.
- NHS tier 3 services: Specialist weight management services follow NICE guidance, which recommends this structured dose escalation approach.
Most people find that initial side effects like nausea begin to ease after 2–3 weeks, though individual experiences vary significantly.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Side Effects and What Typically Improves by Week 4
Nausea and stomach upset are the most common side effects in the first 4 weeks. By week 4, many people report these begin to improve, though some continue to experience mild symptoms. What usually improves: Initial nausea often reduces as your body adapts; appetite suppression becomes more predictable; injection site reactions typically settle if they occurred; dizziness and fatigue often ease. What may persist: Mild diarrhoea or constipation can continue longer; subtle appetite changes continue (this is the medication working); fatigue may linger if calorie intake is very low. If side effects are severe at week 4, inform your prescriber — they may recommend staying on 2.5 mg longer or, rarely, stopping treatment. Track your symptoms using our free progress tracker to help your prescriber make informed decisions about your next dose.
- 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 have lost weight by 4 weeks on Mounjaro?
Weight loss in the first 4 weeks varies significantly. Clinical trials show most people see modest weight loss by week 4, though some notice only appetite suppression with weight change appearing later. Much depends on calorie deficit, baseline weight, and individual metabolism. Your prescriber can discuss realistic timelines at your 4-week review.
What dose will I be on after 4 weeks?
You'll typically move from 2.5 mg (your starting dose) to 5 mg once weekly at week 5, provided you've tolerated the 2.5 mg dose reasonably well. This 4-week minimum between doses is standard in NHS specialist services and private prescribing. Your prescriber may delay escalation if side effects are severe.
Should I change my diet after 4 weeks on Mounjaro?
Your diet approach should remain consistent throughout — Mounjaro works best alongside a reduced-calorie, balanced diet and increased physical activity. At your 4-week review, discuss with your prescriber or a dietitian whether your current approach is sustainable and supporting weight loss.
What if I'm still feeling very sick at 4 weeks?
Persistent severe nausea at week 4 is less common but does happen. Contact your prescriber before your scheduled review. They may recommend staying on 2.5 mg longer, adjusting how you take the injection, or in rare cases, exploring whether Mounjaro is suitable for you. Do not stop injecting without guidance.
Can I use our GLP-1 weight loss calculator to estimate my progress?
Yes — our free GLP-1 weight loss calculator helps you project potential weight loss across different doses of tirzepatide-based medications. At 4 weeks, you're still early in treatment, so actual results may differ from projections as your dose increases.
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.