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 in Your First Month on Mounjaro?
Starting Mounjaro (tirzepatide) is a gradual process. Your first month involves a single 2.5 mg starter dose — the lowest in the KwikPen range — designed to help your body adjust to the medication. Many people experience side effects early on, particularly nausea and appetite changes, but these often settle within the first few weeks. Unlike Wegovy, which starts at 0.25 mg and escalates more slowly, Mounjaro's dual GIP and GLP-1 mechanism means you may notice appetite suppression even at the starter dose. Your prescriber will monitor your response before increasing your dose at week five.
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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.
Mounjaro vs Wegovy: First Month Differences
Mounjaro (tirzepatide) and Wegovy (semaglutide) follow different titration schedules in the first month, reflecting their different mechanisms of action. Mounjaro starts at 2.5 mg for weeks 1–4, then moves to 5 mg at week 5. Wegovy begins at 0.25 mg for weeks 1–4, progressing to 0.5 mg in week 5. In clinical trials (SURMOUNT for Mounjaro, STEP for Wegovy), both medications reduced appetite and supported weight loss, but the data comes from full treatment courses, not just the first month.
The key difference: Mounjaro activates two hormone pathways (GIP and GLP-1), whilst Wegovy activates only GLP-1. This dual action may explain why some patients report stronger appetite suppression with Mounjaro earlier in treatment, though individual experience varies considerably. Side effect profiles are broadly similar in month one — nausea, diarrhoea, and constipation are common with both — but severity differs between individuals. Your prescriber will assess which medication suits your health profile and preferences.
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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.
First Month Side Effects and When to Seek Help
In your first month on Mounjaro, nausea affects many users, typically appearing within the first few days and improving by week 2–3. Diarrhoea, constipation, and reduced appetite are also common. Most side effects are mild to moderate and resolve as your body adjusts; you should never stop the medication suddenly. However, seek urgent medical attention if you experience severe stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (face or throat swelling), or gallbladder symptoms. Keep your prescriber informed of all side effects at your follow-up appointment, usually scheduled 4–6 weeks after starting. Use our free GLP-1 weight loss calculator to set realistic expectations for the full treatment course — first-month weight loss is typically modest compared to later months.
- 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?
First-month weight loss is usually modest — typically 2–4 lbs or 1–2 kg on the 2.5 mg starter dose. Most significant weight loss occurs once you reach maintenance doses (10–15 mg) after several weeks of titration. Individual results vary based on diet, activity, and metabolism.
Is nausea normal in the first week of Mounjaro?
Yes, nausea is very common in the first few days and often the most noticeable side effect. It typically improves within 2–3 weeks as your body adjusts. Eating smaller, bland meals and staying hydrated can help. Contact your prescriber if nausea is severe or persistent beyond week 3.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between GLP-1 medications is possible but requires prescriber guidance. There's no official established conversion chart — your prescriber will advise based on your response to the first medication and individual suitability. A switch typically involves stopping one medication and restarting titration with the other.
How do I inject Mounjaro correctly in my first month?
Mounjaro comes as a pre-filled KwikPen. Inject once weekly into your stomach, upper thigh, or upper arm. Rotate injection sites each week to prevent skin irritation. Your prescriber or pharmacist will demonstrate the correct technique — ask for a demonstration if unsure.
What's the difference between Mounjaro eligibility on the NHS vs privately in the UK?
NHS access requires referral to a specialist tier 3 weight management service and has strict eligibility criteria (BMI ≥30 or ≥27 with comorbidities). Private access is through a UK-licensed prescriber following an online clinical assessment. NHS waiting times vary significantly by region; private access is typically faster.
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.