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 In Fridge vs Wegovy — Storage and Comparison
Keeping your Mounjaro KwikPen in the fridge is essential for it to work effectively. Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections available in the UK, but they work differently and have different storage requirements. This guide explains how to store each medication correctly, compares their mechanisms of action, and helps you understand which might suit you best. Both require a clinical assessment before prescription, and individual results vary significantly between people.
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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 Mounjaro and Wegovy Work Differently — Trial Data and Mechanism
Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways that regulate appetite and blood sugar. Clinical trials (SURMOUNT-1) showed that at the maximum dose of 15 mg weekly, participants lost an average of approximately 20.9% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. At the 10 mg dose, average weight loss was around 19.5%.
Wegovy is a GLP-1 receptor agonist only, activating a single hormone pathway. The STEP-1 clinical trial showed average weight loss of approximately 14.9% of body weight at 68 weeks on the maximum 2.4 mg dose, also with lifestyle changes. Both medications slow how quickly food leaves the stomach and may reduce cravings, but Mounjaro's dual mechanism may offer slightly greater support for some patients—though individual responses vary considerably.
Your prescriber will assess which mechanism suits your individual health profile and goals.
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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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Storage, Eligibility, and How to Choose
Storage: Both medications must be stored in a fridge at 2–8°C unopened. Once you start using a KwikPen, you can keep it at room temperature (below 30°C) for up to 30 days. Mounjaro comes as a once-weekly 2.5 mg to 15 mg injection; Wegovy ranges from 0.25 mg to 2.4 mg weekly. Both require a UK prescription following a clinical assessment.
Eligibility: You may be eligible for either medication if you have a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, heart disease, or sleep apnoea). For South Asian, Chinese, Black African, or Black Caribbean adults, lower BMI thresholds apply. Your prescriber or GP will decide which is appropriate for you based on your medical history, not the other way around. Neither is assured to work for everyone—results depend on your individual metabolism, lifestyle, and adherence.
Use our free GLP-1 weight loss calculator to explore potential weight loss projections at different doses, and our BMI calculator to check your starting point. Both tools can help you understand your current health context before speaking to a prescriber.
Key takeaway: Mounjaro and Wegovy are both prescription-only medicines with strong clinical trial evidence. Mounjaro (tirzepatide) uses a dual mechanism; Wegovy (semaglutide) uses single GLP-1 action. Storage is identical—both require refrigeration unopened. Your GP or specialist prescriber will assess which is more suitable for your individual circumstances. Neither replaces diet and exercise; both work 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.
What's the main difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 receptor agonist (activates two appetite-regulating pathways), while Wegovy is a GLP-1 agonist only (single pathway). Clinical trials showed Mounjaro at 15 mg achieved average weight loss of approximately 20.9%, whilst Wegovy at 2.4 mg achieved approximately clinically studied weight loss. Individual results vary. Your prescriber decides which is appropriate.
Do both medications need to stay in the fridge?
Yes. Unopened Mounjaro KwikPens and Wegovy pens must be stored at 2–8°C. Once you start using a pen, you can keep it at room temperature (below 30°C) for up to 30 days. Never freeze either medication. Always keep in the original carton away from direct sunlight. Dispose of the pen 30 days after first use.
How are Mounjaro and Wegovy prescribed in the UK?
Both are prescription-only and available through NHS specialist tier 3 weight management services (with potential long waiting lists) or private online prescribing following a clinical assessment. Neither can be purchased without a prescription. Your GP or a licensed prescriber will assess your eligibility based on BMI, health conditions, and medical history.
Can I switch from Mounjaro to Wegovy or vice versa?
Switching between medications is possible but must be managed by your prescriber. Approximate dose equivalences exist (e.g., Mounjaro 10 mg ≈ Wegovy 1.7 mg), but there is no official conversion chart. Never switch without professional guidance. Your prescriber will monitor you during any transition.
Which medication will help me lose more weight?
Clinical trial data suggests Mounjaro may support slightly greater average weight loss than Wegovy, but this varies significantly between individuals. Some people respond very well to Wegovy; others may need Mounjaro's dual mechanism. Individual factors—metabolism, lifestyle adherence, and health conditions—matter far more than the medication name. Your prescriber will recommend the best option for you based on your personal health profile.
Ready to take control of your weight loss journey?
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