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
How Often Is Mounjaro Injection?
Mounjaro (tirzepatide) is a once-weekly weight loss injection, meaning you inject it the same day each week. Each pre-filled KwikPen contains four doses — enough for one month of treatment. Your prescriber will adjust your dose every four weeks, starting at 2.5 mg and increasing up to a maximum of 15 mg per week, depending on how well you tolerate the medication and your weight loss progress. The injection takes just a few seconds and can be given at home.
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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.
Mounjaro Weekly Dosing Schedule
Mounjaro is injected once every seven days, ideally on the same day each week for consistency. Each injection is a subcutaneous dose — meaning it goes into the fatty tissue under your skin, typically in your stomach, thigh, or upper arm. You can rotate injection sites each week to help prevent irritation.
The standard dose titration schedule works like this: you start at 2.5 mg for the first four weeks, then increase to 5 mg for weeks five to eight. Your prescriber will continue increasing the dose every four weeks (to 7.5 mg, then 10 mg, then 12.5 mg, and finally up to 15 mg) based on your tolerance and response. Most people reach their effective dose between 10–15 mg per week.
Each KwikPen is pre-filled with a single dose and is used once, then disposed of. A one-month supply (four pens) covers four weekly injections.
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.
Missed Doses and Storage Guidance
If you miss a weekly dose, NHS guidance suggests: if you remember within four days of your usual injection day, inject as soon as you remember and continue your normal schedule the following week. If more than four days have passed, skip that dose and resume on your scheduled day the following week — never inject two doses at once.
Storage matters: unopened pens must be kept in a refrigerator 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. After 30 days from first use, dispose of the pen even if it contains remaining medication. Use the GLP-1 weight loss calculator to help track your injection schedule and projected weight loss progress alongside your weekly injections.
- 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 many times per week do I inject Mounjaro?
Once per week. You choose the same day each week (e.g. every Monday) and stick to it. This consistency helps maintain steady medication levels in your body and makes it easier to remember your injection day.
What happens if I miss my Mounjaro injection?
If you remember within 4 days of your missed dose, inject as soon as possible and continue your normal schedule. If more than 4 days have passed, skip the dose and resume the following week. Never take a double dose. Contact your prescriber if unsure.
How long does each Mounjaro KwikPen last?
Each pre-filled pen contains one dose and lasts one week. A one-month supply includes four pens (four weekly injections). Once used, dispose of the pen safely — do not reuse it.
Can I change my Mounjaro injection day?
Yes. If you need to change your weekly injection day, ensure at least 3 days (72 hours) between injections. Plan the new day and inform your prescriber. Consistency is important for best results, so pick a day you can stick to long-term.
How quickly does Mounjaro start working after injection?
You may notice reduced appetite within hours of your first injection, though effects build over the first few weeks. Clinical benefits develop gradually — most weight loss appears after 4+ weeks on each dose level as your body adjusts. ```
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.