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 Best To Inject Mounjaro?
The best time to inject Mounjaro is a consistent day each week — the specific time of day is flexible and depends on your personal routine. Mounjaro is a once-weekly injection, which means you pick a day and stick to it, injecting at roughly the same time each week. Your prescriber will advise on your chosen injection day, and the Mounjaro KwikPen device is designed for easy, straightforward administration at home. Consistency matters more than the exact time, so choosing a day that fits your lifestyle is key.
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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.
Mounjaro Injection Schedule and Consistency
Mounjaro (tirzepatide) is a prescription-only, once-weekly injection administered subcutaneously — meaning under the skin. Unlike daily medications, you only need to inject once per week, always on the same day. Your prescriber will help you choose an injection day that fits your routine, whether that's Monday morning, Thursday evening, or any other day of the week.
Timing consistency is more important than the exact time of day. If you usually inject at 8am on Mondays, aim to inject between 7am and 9am on your chosen day each week. A variation of a few hours is acceptable. The key is maintaining at least 24 hours (and ideally 7 days) between doses.
- Choose a day that fits your weekly routine — work schedule, family commitments, or personal preference
- Mark it in your calendar or set a phone reminder
- Inject at approximately the same time each week (variation of a few hours is fine)
- Use the Mounjaro KwikPen at room temperature — do not inject if the pen has been frozen
If you accidentally miss a dose by a few hours, simply inject as soon as you remember. If more than 4 days have passed since your scheduled injection day, skip that dose and continue on your normal weekly schedule — never take two doses close together.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Injection Site Rotation and Administration Tips
You can inject Mounjaro in three locations: the stomach (abdomen), upper thigh, or upper arm. Rotating injection sites helps prevent skin irritation and lipodystrophy (changes to fat tissue). If you inject in your stomach this week, choose your thigh the following week, then your upper arm the next week.
The Mounjaro KwikPen is pre-filled and colour-coded by dose. Each pen contains enough medication for 4 weekly injections (one month's supply). Store unopened pens in the fridge at 2–8°C, and after first use, you can keep the pen at room temperature (below 30°C) for up to 30 days. Once 30 days have passed since first use, discard the pen safely.
- Rotate between stomach, thigh, and upper arm each week to reduce injection site reactions
- Keep injections at least 2.5cm apart from previous injection sites
- Allow the KwikPen to reach room temperature before injecting (5–10 minutes)
- Dispose of used pens in a sharps bin or as directed by your GP
- 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 each week?
Yes, you can vary the time of day by a few hours, but try to stay within the same time window (e.g., morning vs. evening) to maintain consistency. The important thing is injecting on the same day of the week, at least 7 days apart from your last dose. Set a weekly reminder to help you remember.
What happens if I inject Mounjaro late or early?
If you're a few hours early or late, it's fine — inject when you remember. However, never inject closer than 72 hours (3 days) from your last dose. If you're more than 4 days late, skip that dose and continue on your normal weekly schedule. Contact your GP if you're unsure about timing.
Is it better to inject Mounjaro in the morning or evening?
Either morning or evening is equally effective. Choose whichever fits your routine best. Some people prefer morning injections to start their week, others prefer evening. What matters is choosing one time and sticking to it each week for consistency and habit-building.
Can I change my Mounjaro injection day?
Yes, you can change your injection day if needed, but do so in consultation with your GP or prescriber. If switching days, ensure you leave at least 7 days between your last injection and your new day, then establish the new routine consistently.
Should I take Mounjaro with or without food?
Mounjaro injection timing is independent of meals — you can inject whether you've eaten or not. However, many people find it easier to remember by linking it to a routine activity (e.g., Sunday breakfast). Food does not affect how the injection works, as it acts directly on your body's hormone pathways.
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.