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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What to Do When Taking Mounjaro — A Practical UK Guide
Starting Mounjaro (tirzepatide) requires practical preparation and ongoing care to get the best results and manage side effects safely. This guide covers everything you need to know about using your Mounjaro KwikPen, managing common side effects, storing your medication properly, and monitoring your progress while taking this prescription-only GLP-1 and GIP receptor agonist. Whether you're taking Mounjaro through NHS specialist services or via private prescription in the UK, following these steps will help ensure safe and effective use.
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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: Preparation and Injection Technique
Before your first injection, read the patient information leaflet carefully and attend any training offered by your GP or specialist clinic. Your Mounjaro KwikPen arrives as a pre-filled device containing four weekly doses. Store unopened pens in the refrigerator at 2–8°C in their original carton. Once you've used a pen for the first time, you can keep it at room temperature (below 30°C) for up to 30 days.
- Inject into your stomach (abdomen), upper thigh, or upper arm — rotate sites weekly to prevent irritation
- Allow the pen to rest at room temperature for 30 minutes before injecting if it's been refrigerated
- Inject slowly over 5 seconds, keeping the needle in your skin for 10 seconds after pressing the button
- Check your skin for reactions; minor redness typically fades within hours
- Write down your injection day (e.g. every Monday) and set a phone reminder
Your first few injections may feel daunting, but most people find the technique straightforward after the first attempt. Ask your healthcare provider to demonstrate the injection technique before starting.
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 Monitoring Your Progress
Common side effects — nausea, diarrhoea, constipation, and stomach discomfort — are most noticeable during the first 2–4 weeks and often improve as your body adjusts. Stay hydrated, eat small frequent meals, and avoid fatty or sugary foods which can worsen symptoms. If side effects are severe or persistent beyond 4 weeks, contact your GP or prescriber immediately.
- Weigh yourself weekly on the same day and time; use our free weight converter and GLP-1 weight loss calculator to track progress in kg, stone, or lbs
- Monitor your waist circumference and how your clothes fit — these matter as much as the scales
- Keep a simple log of side effects and energy levels to share with your prescriber at review appointments
- Report any serious concerns (severe stomach pain, vision changes, rapid heartbeat) to your GP straight away
Regular NHS or private review appointments ensure your dose is adjusted safely and your progress is on track.
- 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 store my Mounjaro KwikPen safely?
Unopened pens must be refrigerated at 2–8°C in their original carton. Once you start using a pen, you can store it at room temperature (below 30°C) for up to 30 days. Never freeze your pen — if it has been frozen, discard it. After 30 days of room temperature storage, dispose of the pen safely even if it contains unused doses.
What should I do if I miss a Mounjaro injection?
If you remember within 4 days of your missed dose, inject as soon as possible and continue your normal weekly schedule from that date. If more than 4 days have passed, skip that dose and inject on your next scheduled day. Never take a double dose. Maintain at least 72 hours between injections.
How can I manage nausea when taking Mounjaro?
Eat smaller, more frequent meals rather than large portions. Avoid fatty, fried, or very sugary foods which worsen nausea. Stay well hydrated by sipping water throughout the day. Ginger tea or anti-nausea medications recommended by your GP may help. Nausea typically improves within 2–4 weeks as your body adjusts to the medication.
Should I change my diet and exercise when taking Mounjaro?
Yes — Mounjaro works best alongside a reduced-calorie diet and regular physical activity. The NHS recommends aiming for a 500 kcal daily deficit for sustainable weight loss of approximately 0.5 kg per week. Your GP or a dietitian can provide personalised meal and exercise guidance tailored to your health and fitness level.
When should I contact my GP about concerns with Mounjaro?
Contact your GP immediately if you experience severe stomach pain (possible pancreatitis), changes in vision, rapid heartbeat, signs of allergic reaction (swelling of face or throat), or gallbladder problems. For persistent or concerning side effects, contact your prescriber to discuss whether a dose adjustment or review is needed. Never stop Mounjaro without medical guidance.
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.