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
When Is It Best To Take Mounjaro?
Mounjaro is a once-weekly injection, so timing is straightforward — you inject the same day each week at a time that suits your routine. Unlike daily medications, you don't need to worry about morning versus evening dosing. What matters most is choosing a consistent day and time, then sticking to it. Your prescriber will advise on the best schedule for you, and our GLP-1 weight loss calculator can help you understand what to expect from your treatment plan alongside lifestyle changes.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
Timing and Frequency — How Often to Inject
Mounjaro is administered once every seven days (once weekly) as a subcutaneous injection using a pre-filled KwikPen device. Unlike daily medications, you only need to remember one injection per week. You can inject at any time of day that suits you — morning, afternoon, or evening — as there is no clinical evidence that timing within the day affects effectiveness. What matters is consistency: choose the same day each week (for example, every Monday morning) and maintain that schedule. This regularity helps your body adjust to the medication and makes it easier to remember. Your prescriber will recommend a specific day based on your lifestyle, and you should aim to inject within a 24-hour window of that scheduled day if possible. If you miss an injection, guidance is clear: if it's within 4 days of the missed dose, inject as soon as you remember and resume your normal schedule. If more than 4 days have passed, skip the missed dose and take your next dose on the normal scheduled day.
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.
Practical Injection Tips and Storage Considerations
Before injection: Store unopened Mounjaro pens in the fridge at 2–8°C. Once you start using a pen, it can be kept at room temperature (below 30°C) for up to 30 days. Allow a cold pen to warm to room temperature for a few minutes before injecting — this may reduce discomfort. Injection sites: Rotate between your stomach (abdomen), upper thigh, or upper arm (with help) each week to prevent skin irritation. Timing around food: You can inject regardless of whether you've eaten. Some people prefer injecting on an empty stomach; others inject after meals. Choose what feels comfortable for your routine. Work with your prescriber or pharmacist to establish a system that you can sustain long-term, as adherence is key to achieving weight management goals with tirzepatide.
- 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.
Can I inject Mounjaro at different times of day each week?
Yes, you can inject Mounjaro at any time of day that suits you. What's important is injecting on the same day each week — for example, every Monday. You have flexibility within that day, so if one week you inject in the morning and another in the evening, that's fine. What matters most is weekly consistency.
Does it matter if I inject before or after eating?
No, you can inject Mounjaro regardless of whether you've eaten. Food does not affect how the medication works. Inject when it's most convenient for your routine. Some people prefer injecting on an empty stomach; others inject after meals. Choose what feels most comfortable.
What should I do if I miss a weekly Mounjaro injection?
If you remember within 4 days of missing your dose, inject as soon as possible and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and take your next injection on your normally scheduled day. Never take two doses close together — maintain at least 72 hours (3 days) between injections.
Can I change my injection day if needed?
Yes, you can change your injection day, but do so gradually. If you normally inject on Mondays but want to switch to Friday, contact your prescriber or pharmacist for guidance on how to transition safely. They'll help you adjust your schedule without compromising effectiveness.
Is there a best time of year to start Mounjaro?
There's no clinical evidence that season or time of year affects when to start Mounjaro. Start when your prescriber assesses you as eligible and you're ready to commit to the treatment alongside dietary and lifestyle changes. Use our BMI calculator to understand your current health status, then discuss timing with your GP or prescriber.
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.