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 Do When Taking Mounjaro
Starting Mounjaro (tirzepatide) is a significant step in your weight management journey. This guide covers everything you need to know about using your KwikPen injection, managing side effects, staying on track with your treatment, and what to expect during the first few weeks. Mounjaro is a prescription-only medicine that works alongside lifestyle changes — diet and physical activity are equally important for success. If you're considering Mounjaro or have just been prescribed it, this information will help you prepare and feel confident about your treatment.
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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.
Getting Started: Injection Technique and Storage
Your Mounjaro comes in a pre-filled KwikPen, which is designed to be simple and relatively painless to use. Each pen contains four weekly doses (one month's supply). Before your first injection, your healthcare provider or pharmacist will show you the correct injection technique — watch this demonstration carefully and ask questions if anything is unclear.
Storage is important: unopened pens must be kept in the refrigerator at 2–8°C and not frozen. Once you've started using a pen, you can keep it at room temperature (below 30°C) for up to 30 days. Always keep it away from direct sunlight and heat. Rotate your injection sites weekly between your stomach (abdomen), upper thigh, or upper arm (though arm injections require assistance). This rotation helps prevent skin irritation and ensures consistent absorption. Always check the liquid in your pen before injecting — it should be clear and colourless. If it looks cloudy or discoloured, do not use it and contact your pharmacy.
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.
Managing Side Effects and Staying Healthy
Nausea, diarrhoea, and constipation are common, especially in the first 4–8 weeks as your body adjusts. These often improve over time, but several strategies can help: eat smaller, more frequent meals; avoid fatty or spicy foods; stay well hydrated; and eat foods rich in fibre gradually. If side effects are severe or persistent, speak to your GP — they may suggest eating pattern adjustments, medication, or a slower dose titration (staying at a lower dose longer before increasing). Do not stop Mounjaro suddenly without medical guidance. Report serious side effects immediately, such as severe stomach pain spreading to your back (possible pancreatitis), gallbladder pain, serious allergic reactions, or significant changes in vision. Keep your GP informed about your progress and any concerns.
- 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.
How do I inject Mounjaro correctly?
Your pharmacist or GP will demonstrate the injection technique before you start. Clean the injection site with an alcohol wipe, pinch the skin gently, insert the needle at a 90-degree angle, and press the injection button until you hear a click. Hold for 3 seconds, then remove. Always follow the instructions provided with your KwikPen carefully.
What should I eat and drink while taking Mounjaro?
Focus on balanced meals with lean protein, vegetables, and whole grains in smaller portions. Avoid high-fat, greasy, or very spicy foods, which can worsen nausea and digestive side effects. Stay hydrated with water throughout the day. Your GP or a dietitian can provide personalised nutrition guidance to support your weight loss.
When should I increase my Mounjaro dose?
Mounjaro starts at 2.5 mg and increases every 4 weeks to a maximum of 15 mg weekly. Your prescriber will decide when to increase based on your tolerance and how you're managing side effects. Never increase your dose yourself — always follow your prescriber's schedule and discuss any concerns with them.
Can I take other medications while on Mounjaro?
Mounjaro may affect how your body absorbs some oral medications, particularly contraceptive pills. If you take contraceptives, add a barrier method for 4 weeks after starting Mounjaro and 4 weeks after each dose increase. Always tell your GP or prescriber about all medications you take before starting Mounjaro.
How can I track my progress with Mounjaro?
Use our free progress tracker to log your weight, energy levels, and side effects week by week. Track your BMI using our BMI calculator and weight converter to see changes across different units. Regular monitoring helps you and your GP assess how well Mounjaro is working for you and whether any adjustments are needed.
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.