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: Mounjaro Weight Loss in Your First Month
During your first month on Mounjaro, you'll typically receive the starting dose of 2.5 mg as a weekly weight loss injection. Most people don't experience significant weight loss in month one — the focus is on tolerating the medication and allowing your body to adjust. The real weight loss journey begins as your dose increases over subsequent weeks. Understanding what's realistic in this initial period helps set proper expectations and supports your long-term success with this prescription medication.
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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.
First Month on Mounjaro: What Happens Week by Week
Your first month involves dose titration, a gradual escalation plan that helps your body adapt to Mounjaro (tirzepatide). In weeks 1–4, you inject 2.5 mg once weekly using the KwikPen device — this is the starting dose only, not the therapeutic dose.
- Week 1: Initial injection; some people feel nausea or mild stomach changes immediately
- Weeks 2–3: Side effects often peak (nausea, reduced appetite, changes in digestion)
- Week 4: Side effects typically begin to settle; some people notice modest appetite reduction
Weight loss in month one is usually minimal — often just 1–3 kg across NHS and private clinical experience. The 2.5 mg dose is not yet the active therapeutic dose; it's a tolerance-building phase. At week 5, your dose increases to 5 mg, where the more noticeable weight loss effects begin. Use our free BMI calculator and GLP-1 weight loss calculator to track your starting point and project realistic outcomes across higher doses.
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.
NHS Eligibility and Realistic First Month Expectations
Through NHS specialist tier 3 weight management services, Mounjaro is prescribed for adults with a BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, sleep apnoea). In the first month, focus on tolerating the injection rather than expecting dramatic results — clinical trials show meaningful weight loss emerges from week 5 onwards as doses increase. Many people report reduced food cravings and feeling fuller after smaller meals, but the scale may not move much yet. Keep your GP or prescriber informed of any severe side effects. Track your progress using our free progress tracker to monitor changes beyond just weight, including energy levels and appetite shifts.
- 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?
Most people experience minimal weight loss in month one (often 1–3 kg) because the 2.5 mg starting dose is a tolerance phase, not the active therapeutic dose. Real weight loss typically begins from week 5 onwards as your dose increases to 5 mg and higher. Individual results vary.
What side effects should I expect in the first month?
Nausea is very common in weeks 1–3, often alongside reduced appetite, constipation, or diarrhoea. Most people report these side effects improve by week 4. Severe or persistent side effects should be reported to your GP or prescriber immediately.
How do I inject Mounjaro during the first month?
Mounjaro comes as a pre-filled KwikPen. You inject 2.5 mg once weekly (same day each week) into your stomach, thigh, or upper arm. Rotate injection sites weekly. Your prescriber or pharmacist will show you how before your first dose.
Should I change my diet in the first month?
Yes. A reduced-calorie, balanced diet is essential from day one — Mounjaro works best alongside healthy eating and activity changes. NHS guidance recommends a calorie deficit of around 500 kcal daily for sustainable weight loss. Your GP can refer you to a dietitian for personalised advice.
What if I have side effects I can't tolerate in month one?
Contact your GP or prescriber immediately. Do not stop the injection without guidance. Your prescriber may suggest managing side effects (eating smaller meals, hydration) or, in rare cases, pausing escalation. Severe stomach pain, allergic reactions, or gallbladder pain require urgent medical attention.
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.