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 After 4 Weeks on Mounjaro?
After 4 weeks on Mounjaro, you're completing the initial dosing phase on the 2.5 mg starting dose. This is the first stage of tirzepatide treatment in the UK, designed to allow your body to adjust to the medication before dose escalation. Most people experience noticeable changes during this period, though individual responses vary significantly. Your prescriber will assess your tolerance and side effects before moving to the next dose. Understanding what to expect can help you manage this early phase of treatment effectively and know when to seek medical guidance.
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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.
The First 4 Weeks: Dosing and Adjustment
Mounjaro is prescribed as a once-weekly injection, which means you'll have received four doses by the end of week 4. Each dose uses a pre-filled KwikPen device, making self-injection straightforward. The starting dose of 2.5 mg is deliberately low to minimise early side effects and allow your body to adapt to the medication's effects on appetite and blood sugar regulation.
- Week 1–4: You remain on the 2.5 mg dose (four weekly injections)
- Week 5 onwards: Your prescriber typically escalates you to 5 mg if tolerating well
- Each dose should be taken on the same day each week for consistency
- Injection sites can be rotated between abdomen, thigh, or upper arm
At the 4-week mark, your prescriber will review your response and side effects. This assessment determines whether you proceed to the next dose or remain on 2.5 mg for longer.
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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.
Common Experiences After 4 Weeks
Many people notice changes in appetite and eating patterns by week 4. Nausea is the most frequently reported early side effect, though it typically improves as your body adjusts. Other common experiences include changes in taste preferences, mild stomach discomfort, and sometimes diarrhoea or constipation. Some people report modest weight loss during this first month, though significant results usually emerge over weeks 5–12 as doses increase.
Side effects often peak in weeks 1–2 and gradually settle by week 4. If you're experiencing severe or persistent symptoms—particularly ongoing vomiting, severe stomach pain, or signs of allergic reaction—contact your GP or prescriber immediately. Using our free GLP-1 weight loss calculator can help you project potential outcomes as you progress through the dose escalation schedule.
It's important to continue lifestyle changes alongside Mounjaro. The medication works best when combined with a reduced-calorie diet and increased physical activity. At week 4, reassess your eating habits, fluid intake (dehydration can worsen nausea), and activity levels. Many people find that Mounjaro reduces cravings and portion sizes, making dietary adjustments feel more manageable than before treatment. Track your progress using our free progress tracker to monitor changes in weight, energy levels, and how you feel—not just the number on the scales. This helps identify patterns and keeps motivation sustained through the dose titration phase.
- 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.
Is nausea normal after 4 weeks on Mounjaro?
Yes, nausea is very common in weeks 1–2 and often improves by week 4 as your body adjusts. If nausea persists severely at week 4, your prescriber may delay dose escalation. Eating smaller, lighter meals and staying hydrated can help manage it.
Will I have lost weight after 4 weeks on Mounjaro?
Weight loss varies between individuals. Some people notice modest loss in the first 4 weeks, while others see minimal change. Significant weight loss typically occurs from weeks 5–12 as doses escalate. Clinical trials showed average losses of 15–21% over 72 weeks.
What happens at the 4-week mark—do I automatically move to the next dose?
No. Your prescriber assesses your tolerance and side effects at week 4. If you're tolerating 2.5 mg well, you'll typically escalate to 5 mg at week 5. If experiencing significant side effects, your prescriber may delay escalation.
Can I use the Mounjaro KwikPen for all doses?
Each KwikPen contains 4 doses and covers 4 weeks. After 4 weeks, you'll need a new pen for doses 5–8 (weeks 5–8). Your prescriber will arrange this before your current pen runs out.
Should I change my diet during the first 4 weeks on Mounjaro?
Yes. Mounjaro works best alongside a reduced-calorie diet and increased physical activity. Focus on smaller portions, protein-rich foods, and staying hydrated. Many people find appetite naturally decreases, making dietary changes easier than before treatment.
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.