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 in Your First Month on Mounjaro
Starting Mounjaro in the UK marks the beginning of a structured weight management journey. Your first month involves an initial low dose (2.5 mg) to help your body adjust to this dual GIP and GLP-1 receptor agonist. This guide explains the NHS prescription process, what happens during your initial four weeks, expected side effects, and practical tips for getting the most from your new treatment. Remember, individual responses vary, and your prescriber will monitor your progress carefully.
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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.
NHS Access and First-Month Eligibility
In the UK, Mounjaro is accessed through NHS specialist tier 3 weight management services, typically following GP referral, though availability varies significantly by region. NICE has approved tirzepatide for NHS use, but access depends on your local Integrated Care Board (ICB) and current service capacity. You'll need a BMI of 30 kg/m² or above, or 27–29.9 kg/m² with weight-related health conditions such as type 2 diabetes, hypertension, or sleep apnoea. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower thresholds may apply (BMI 27.5+). Your prescriber will assess your full medical history, current medications, and contraindications before your first dose. The initial 2.5 mg dose is deliberately low to allow your body to adjust and for your prescriber to evaluate your tolerance and response.
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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.
What Happens During Your First Four Weeks
Your first Mounjaro injection (2.5 mg) is typically administered by a healthcare professional or self-administered at home using the pre-filled KwikPen device. You'll inject once weekly for four weeks at this starting dose—this is your dose titration phase. Nausea is very common in the first week or two; it usually lessens as your body adjusts. Keep well hydrated, eat small frequent meals, and avoid heavy or greasy foods. You may notice reduced appetite almost immediately, or it may take a few days. After four weeks (at your week 5 appointment), your prescriber will review your tolerance and side effects before deciding whether to increase to 5 mg. Most people stay on this dose for at least four weeks before further increases. Use our free GLP-1 weight loss calculator to understand potential outcomes across different doses as you progress through your first month.
- 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 don't lose significant weight in the first four weeks because you're on the starting dose (2.5 mg). The main goal of month one is tolerance and adjustment. Some people experience modest weight loss (1–2 kg) due to reduced appetite, but substantial loss typically occurs from month two onwards as doses increase. Individual responses vary considerably.
What side effects should I expect in week one?
Nausea is the most common side effect in the first week, affecting about 1 in 3 people. You may also experience mild stomach discomfort, constipation, or reduced appetite. These typically improve within 7–10 days as your body adjusts. Staying hydrated, eating small meals, and avoiding fatty foods helps. Contact your GP if side effects are severe or persistent.
Can I get Mounjaro through the NHS in my area?
NHS access to Mounjaro varies by region. Ask your GP to refer you to your local tier 3 specialist weight management service. Some areas have waiting lists of several months. If NHS access is unavailable, you may explore private prescription options through a UK-licensed prescriber, though this involves costs. Your GP can advise on local pathways.
What's the difference between Mounjaro and Wegovy in the first month?
Mounjaro starts at 2.5 mg (dual GIP + GLP-1 action), while Wegovy starts at 0.25 mg (GLP-1 only). Mounjaro's dual mechanism may produce appetite reduction more quickly, but both require four-week dose intervals. Clinical trials show Mounjaro produces slightly greater average weight loss overall, though individual results vary. Your prescriber will choose based on your health profile.
How do I use the Mounjaro KwikPen in my first injection?
The KwikPen is pre-filled and easy to use. Rotate your injection site weekly (stomach, thigh, or upper arm). Clean the skin, pinch gently, inject at 90 degrees, and hold for 3 seconds. Store unused pens in the fridge (2–8°C), and after first use, keep at room temperature (below 30°C) for up to 30 days. Your prescriber or nurse will demonstrate the technique before your first dose.
Ready to take control of your weight loss journey?
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