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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How to Use Mounjaro Injections Correctly
Mounjaro (tirzepatide) is a prescription-only weight management injection that you administer yourself once a week using a pre-filled KwikPen device. If your GP or prescriber has assessed you as eligible, understanding the correct injection technique, storage requirements, and dose schedule is essential for safe and effective use. This guide covers everything you need to know about using Mounjaro in the UK, from initial setup to storage and troubleshooting common questions. Always follow your prescriber's instructions and the information leaflet included with your medication.
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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.
How to Give Yourself a Mounjaro Injection
Mounjaro comes in a pre-filled KwikPen — you do not need to draw up doses yourself. Each pen contains four weekly doses. Here's how to inject safely:
- Prepare: Check the expiry date, inspect for cloudiness or particles (should be clear), and let the pen sit at room temperature for 15 minutes before injecting
- Choose a site: Inject into your stomach (abdomen), upper thigh, or upper arm. Rotate sites each week to avoid skin irritation
- Clean: Wipe the injection site with an alcohol swab and let it dry
- Inject: Remove the needle cover, place the pen against your skin at a 90-degree angle, press the injection button and hold for 3 seconds until you hear a click
- Remove: Lift the pen away and replace the needle cover immediately
- Dispose: Place the used pen in a sharps bin — never in household waste
If you're unsure about technique, ask your pharmacist or GP practice nurse for a demonstration before your first injection.
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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.
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Mounjaro Dose Schedule and Titration
Mounjaro is started at a low dose and increased gradually over weeks to find the dose that works best for you — this is called dose titration. You stay on each dose for at least 4 weeks before increasing. The typical schedule is: 2.5 mg (weeks 1–4), 5 mg (weeks 5–8), 7.5 mg (weeks 9–12), 10 mg (weeks 13–16), 12.5 mg (weeks 17–20), and 15 mg (week 21 onwards, the maximum dose). Your prescriber may adjust this schedule based on your response and side effects. Never increase your dose without your prescriber's guidance, and always inject on the same day each week. Try our GLP-1 weight loss calculator to estimate potential results at different doses.
- 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 Mounjaro pens in the UK?
Unopened pens must be stored in a refrigerator at 2–8°C in their original carton, away from light. Do not freeze them. After first use, you can keep the pen at room temperature (below 30°C) for up to 30 days. Always check the expiry date before using, and ask your pharmacist if you're unsure about storage conditions.
What 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 schedule the following week. If more than 4 days have passed, skip that dose and take your next injection on your regular day. Never double-dose. Ensure at least 72 hours (3 days) between injections.
Can I use Mounjaro with other medications?
Mounjaro may affect how your body absorbs oral medications, including contraceptive pills. If you take oral contraceptives, use an additional barrier method for 4 weeks after starting Mounjaro and 4 weeks after each dose increase. Tell your prescriber about all medications you take — they may need adjusting, especially diabetes medications.
What should I do if I have an injection site reaction?
Minor redness, bruising, or mild pain at the injection site is common and usually resolves within a few days. Rotate your injection sites weekly to minimise irritation. If you experience severe swelling, spreading redness, warmth, or signs of infection, contact your GP or call NHS 111 for advice.
Is Mounjaro the same as Wegovy or Ozempic?
No. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 agonist only — both are licensed for weight management in the UK. Ozempic is also semaglutide but is approved only for type 2 diabetes, not weight loss. Mounjaro and Wegovy have different dose ranges, injection schedules, and clinical data. Ask your prescriber which is appropriate for you.
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