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
Average Weight Loss First Week On Mounjaro
Most people do not experience significant weight loss in the first week of Mounjaro — this is completely normal. The starting dose of 2.5 mg is designed to help your body adjust to the medication, and meaningful weight loss typically begins after several weeks of treatment. Understanding realistic expectations from day one helps you stay motivated through your weight management journey with this prescription-only injection.
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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.
What Happens in Week One of Mounjaro
During the first week on Mounjaro's starting dose of 2.5 mg, your body is adjusting to the medication rather than experiencing dramatic weight changes. Most people see minimal or no weight loss in week one — this is expected and does not indicate the medication is not working. Instead, you may notice changes like slightly reduced appetite, less interest in certain foods, or feeling fuller after smaller portions. Some people report mild nausea, constipation, or diarrhoea during this adjustment phase. The medication takes time to build up in your system, and each dose is gradually increased over 4 weeks before moving to the next level. Clinical trials show that meaningful weight loss typically becomes noticeable from week 4 onwards, after you reach the first maintenance dose of 5 mg. This gradual approach reduces side effects and allows your body to adapt safely to the medication.
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.
Factors Affecting First-Week Changes
Individual responses to Mounjaro vary widely in week one, depending on several factors. Age, metabolic rate, current diet, activity level, and how your body responds to GLP-1 hormones all influence whether you experience appetite changes or early weight shifts. Some people are more sensitive to the medication and may feel effects within days, while others take longer to notice any change. Your starting weight also matters — those starting from a higher body weight may see faster early shifts due to fluid loss or dietary changes, whilst those closer to an ideal body weight range may see slower initial changes. Frame size, muscle mass composition, and genetic factors also play a role. Additionally, what you eat and drink in week one affects the scale — eating less salt, increasing water intake, or reducing carbohydrate intake can all cause temporary fluid loss that masks or amplifies weight changes. This is why tracking weight loss over several weeks rather than daily is more useful for UK adults following a realistic healthy weight path.
- 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 lose weight in my first week on Mounjaro?
Most people experience minimal weight loss in week one on the 2.5 mg starting dose. The first week is an adjustment phase, and meaningful weight loss typically begins after 2–4 weeks once your body adapts and you progress to higher doses. Individual responses vary, so some people may notice appetite changes immediately while others see changes later.
Should I expect to see results on the 2.5 mg dose?
The 2.5 mg starting dose is designed for tolerance, not for weight loss. You remain on this dose for 4 weeks before increasing to 5 mg, the first maintenance dose where more noticeable effects appear. Clinical trials show weight loss becomes significant from week 4 onwards as doses increase.
Why is my weight not changing in week one?
Week one is your body's adjustment period. Mounjaro needs time to build up in your system and for appetite-regulating effects to develop. Additionally, early weight changes are unpredictable — fluid retention, food in your digestive system, and hormonal factors can mask true fat loss. Always track progress over weeks, not days.
When should I expect noticeable weight loss on Mounjaro?
Most people report noticeable weight loss by weeks 4–8 once they reach maintenance doses (5 mg or higher). Clinical trials showed average weight loss of around 15–21% of body weight at 72 weeks across all doses. Your progress depends on diet, activity, dose, and individual factors.
Can I use a weight loss calculator to estimate my progress?
Yes — our free GLP-1 weight loss calculator can help you project potential weight loss at different Mounjaro doses based on clinical trial data. However, remember that individual results vary significantly, and first-week expectations should be low. Use our BMI calculator alongside progress tracking for realistic goal-setting.
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.