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
Do You Take Mounjaro Once a Week?
Yes, Mounjaro (tirzepatide) is a once-weekly injection prescribed for weight management in UK adults. Each dose is given as a single injection into the stomach, thigh, or upper arm, using a pre-filled KwikPen device. The injection schedule remains the same throughout treatment — you take one dose every 7 days. Your prescriber will guide you through the correct injection technique and storage requirements. Individual results vary, and Mounjaro is always used alongside dietary changes and increased physical activity for best results.
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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.
How Mounjaro Dosing Works
Mounjaro follows a structured dose titration schedule, meaning your dose increases gradually over time rather than starting at maximum strength. You begin with a 2.5 mg dose for 4 weeks, then your prescriber may increase it based on your response and tolerance. The doses progress through 5 mg, 7.5 mg, 10 mg, 12.5 mg, and up to the maximum of 15 mg once weekly. Each step takes a minimum of 4 weeks before increasing further. This gradual approach helps your body adjust and reduces side effects like nausea. The KwikPen — a pre-filled injection pen — contains 4 doses (4 weeks' supply). You'll need to order a new pen every month. If you miss a dose, you can take it within 4 days and resume your normal schedule. Never skip to catch up or double-dose.
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.
Storage and Practical Tips
Store unopened Mounjaro pens in a refrigerator between 2°C and 8°C in the original carton. After first use, a pen can be kept at room temperature (below 30°C) for up to 30 days. Never freeze your medication — if frozen, it cannot be used. Rotate your injection sites each week to avoid irritation: stomach (abdomen), upper thigh, or upper arm are suitable locations. Some people experience mild redness or bruising at the injection site, which usually settles within hours. Keep your progress tracker updated to monitor your weight loss journey. Your GP or prescriber should review your progress regularly, especially during dose increases, to ensure the medication is working well for you.
- 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.
Is Mounjaro definitely taken once a week?
Yes, Mounjaro is a once-weekly injection. The standard schedule is one dose every 7 days. You use a pre-filled KwikPen device to inject it into your stomach, thigh, or upper arm. Your prescriber will show you how to use it correctly. The weekly routine stays the same even as your dose increases during the titration phase.
What happens if I miss a Mounjaro injection?
If you miss your dose within 4 days of when it was due, take it as soon as you remember and resume 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 double-dose. Always contact your prescriber if you're unsure about timing.
How long does a Mounjaro pen last?
Each pre-filled KwikPen contains 4 doses, meaning each pen lasts 4 weeks. You'll need a new pen every month. Your prescriber or pharmacy will arrange repeat supplies. Make sure you always have your next pen before your current one runs out to avoid missing doses.
Can I switch from Wegovy to Mounjaro and stay on the same schedule?
Both Mounjaro and Wegovy are once-weekly injections, so the injection frequency stays the same. However, the doses are not directly equivalent — Wegovy starts at 0.25 mg while Mounjaro starts at 2.5 mg. Any switch between medications must be managed by your prescriber, who will guide the transition safely.
What are the main side effects of weekly Mounjaro injections?
Very common side effects include nausea, diarrhoea, vomiting, and stomach pain — these often improve as your body adjusts. Less common effects include headache, fatigue, and injection site reactions. Serious side effects are rare but include severe stomach pain (possible pancreatitis). Always report persistent or concerning symptoms to your GP or prescriber immediately.
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.