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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Mounjaro Dos and Don'ts: A UK Safety Guide
Using Mounjaro (tirzepatide) safely means understanding what to do and what to avoid. This prescription-only weight management injection has specific requirements for storage, injection technique, medication interactions, and lifestyle support. Whether you're starting Mounjaro through the NHS or private prescription, knowing these dos and don'ts helps you get the best results while minimising side effects. Use our GLP-1 weight loss calculator to understand your potential results, and always follow your prescriber's guidance.
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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.
Essential Dos When Using Mounjaro
Do store your Mounjaro KwikPen correctly: Keep unopened pens in the fridge at 2–8°C in their original carton until you're ready to use them. After your first injection, you can keep the pen at room temperature (below 30°C) for up to 30 days. Never freeze your medication — if a pen freezes, discard it.
- Do rotate your injection sites each week to prevent lipodystrophy (fatty bumps under the skin)
- Do inject into your stomach (abdomen), upper thigh, or upper arm — these are the approved sites
- Do take your weekly injection on the same day each week to maintain consistent levels
- Do combine Mounjaro with a reduced-calorie diet and increased physical activity for the best results
- Do inform your prescriber about all other medications you're taking, especially oral contraceptives, diabetes medications, or blood thinners
- Do attend follow-up appointments if you're receiving Mounjaro through an NHS specialist service
- Do report any serious side effects to your GP immediately, particularly severe stomach pain, signs of pancreatitis, or allergic reactions
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.
Critical Don'ts and Safety Warnings
Don't use Mounjaro if you're pregnant, planning pregnancy, or breastfeeding. It's not recommended in these situations. Women taking oral contraceptives should add a barrier method for 4 weeks after starting and 4 weeks after each dose increase, as Mounjaro may reduce oral contraceptive absorption.
- Don't freeze your pen — if frozen, it cannot be used safely
- Don't reuse needles or share your Mounjaro pen with anyone else, even family members
- Don't skip doses or double up if you miss your weekly injection — within 4 days of a missed dose, inject as soon as you remember; beyond 4 days, skip and continue your normal schedule
- Don't take Mounjaro if you have a personal or family history of medullary thyroid cancer or multiple endocrine neoplasia type 2
- Don't ignore warning signs like severe abdominal pain (possible pancreatitis) or vision changes
- Don't expect clinically studied weight loss — results vary, and Mounjaro works best alongside lifestyle changes
- 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?
Wash your hands first. Remove the KwikPen from the fridge 15 minutes before injecting. Pinch the skin at your chosen injection site, insert the needle at a 90-degree angle, and hold for 3 seconds to ensure the full dose delivers. Rotate sites weekly between stomach, thigh, and upper arm to prevent skin damage.
What should I do if I miss a weekly Mounjaro injection?
If you miss by 4 days or fewer, inject as soon as you remember and continue your normal schedule the following week. If more than 4 days have passed, skip that dose and resume your regular weekly schedule. Never take a double dose. Ensure at least 72 hours (3 days) between injections.
Can I take Mounjaro if I'm on other medications?
Tell your prescriber about all medications before starting, especially oral contraceptives, diabetes drugs (sulphonylureas or insulin), and blood thinners. Mounjaro may affect how some oral medications are absorbed, so timing matters. Your prescriber may adjust other medication doses if necessary.
Is Mounjaro safe if I have type 2 diabetes?
Mounjaro can be prescribed for weight management alongside type 2 diabetes management, but your prescriber must know your diabetes status. If you're on insulin or sulphonylurea medications, you may need dose adjustments because Mounjaro affects blood sugar. Never adjust diabetes medications yourself — always consult your GP.
What's the difference between Mounjaro and Wegovy for weight loss?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Clinical trials show Mounjaro achieved average weight loss of 15–21%, compared to Wegovy's approximately 15%. Both are prescription-only and require clinical assessment. Individual results vary significantly, and your prescriber will advise which suits your situation.
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.