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
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Mounjaro First Month Weight Loss — What to Expect
Weight loss during your first month on Mounjaro is typically modest because the starting dose (2.5 mg) is lower than the maintenance dose. Most people don't experience significant weight loss in weeks one to four — the first month is primarily about allowing your body to adjust to the medication and assess tolerance. Clinical trial data and real-world feedback suggest that meaningful weight loss usually begins in month two or three, once your dose has been increased. This guide explains what to expect from Mounjaro in your first month, how dose escalation works, and why patience is important for sustainable results.
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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.
How Mounjaro Dosing Works in the First Month
Mounjaro is a prescription-only weekly injection containing tirzepatide, a dual GIP and GLP-1 receptor agonist. Unlike other weight loss injections, Mounjaro follows a specific dose titration schedule that means your dose increases every four weeks. During your first month, you'll inject 2.5 mg once weekly — this is the starting dose for all patients, regardless of final target dose.
- Week 1–4: 2.5 mg weekly injection
- Weeks 5–8: Increases to 5 mg weekly
- Weeks 9–12: Increases to 7.5 mg (if tolerated)
- Beyond: Continues increasing every 4 weeks until maintenance dose is reached
The lower starting dose allows your body to adjust to the medication and helps minimise side effects like nausea. It also means that the first month is not when maximum weight loss happens — that typically occurs at higher doses in months two, three, and beyond. Your GP or prescriber will monitor your tolerance at each stage.
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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.
Weight Loss Expectations: Month One vs Later Months
Clinical trials (SURMOUNT-1) showed that participants on Mounjaro lost an average of 15–21% of body weight over 72 weeks at maximum doses — but this weight loss was not evenly distributed across the treatment timeline. In the first month on the 2.5 mg starting dose, weight loss is often minimal or even unmeasurable for some people. This is completely normal and not a sign the medication isn't working.
Most meaningful weight loss occurs once you've reached higher doses (10–15 mg). Between months two and six, as your dose increases, you'll typically notice appetite reduction, reduced cravings, and gradual weight loss. Try our free GLP-1 weight loss calculator to estimate potential weight loss at different doses based on your starting weight and expected timeline. Individual results vary significantly — some people see faster loss, others more gradual — and this depends on diet, activity level, genetics, and how well your body responds to the medication.
- 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 month on Mounjaro?
Weight loss in the first month is typically minimal because you're on the lowest starting dose (2.5 mg). Most people experience greater weight loss in months two through four as the dose increases. Focus on tolerance and adjusting to the medication rather than expecting rapid results week one.
Why does Mounjaro start at 2.5 mg if the maximum is 15 mg?
The lower starting dose allows your body to adapt and minimises side effects like nausea and stomach upset. It also lets your prescriber assess how you respond before increasing the dose. This gradual approach supports safer, more sustainable weight loss.
What side effects might I experience in month one?
Common first-month side effects include nausea, mild stomach discomfort, constipation, and loss of appetite. Most side effects are temporary and improve as your body adjusts. If side effects are severe, contact your GP or prescriber — they may adjust your dose or provide management strategies.
How can I track my progress if I'm not seeing weight loss yet?
Use our free progress tracker to log your weight weekly and watch for non-scale improvements — reduced appetite, fewer cravings, or better energy levels. These are signs the medication is working, even if the scales haven't moved. Many people see measurable weight loss begin in month two.
Should I diet and exercise in my first month on Mounjaro?
Yes — prescription weight loss medications work best alongside a reduced-calorie diet and increased physical activity. Your GP can advise on suitable calorie targets. Exercise need not be intense; even 150 minutes of moderate activity per week (brisk walking) supports better results.
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.