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
Should You Take Mounjaro On An Empty Stomach?
The short answer: food does not affect how Mounjaro (tirzepatide) works, so you can take your weekly injection with or without food. Unlike some oral medications, Mounjaro is injected directly under the skin, bypassing your digestive system — meaning stomach contents are irrelevant to its effectiveness. This is one of the key practical advantages of once-weekly injectable weight loss treatment. Both Mounjaro and its alternative, Wegovy (semaglutide), work the same way regarding food timing. Your UK prescription should include clear injection guidance from your prescriber.
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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 vs Wegovy: Injection Timing and Mechanism
Mounjaro (tirzepatide) is injected once weekly into the fatty tissue under your skin — typically your stomach, thigh, or upper arm. Because it's a subcutaneous injection, it doesn't pass through your digestive system. Food, stomach acid, and gastric emptying have no bearing on how the medication is absorbed. You can inject on a full stomach, an empty stomach, or any time in between.
Wegovy (semaglutide) works identically in this respect — it's also a once-weekly subcutaneous injection that bypasses the digestive tract entirely. The key difference between these two is their mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy activates GLP-1 only. Clinical trial data from SURMOUNT (Mounjaro) and STEP (Wegovy) showed both achieved meaningful weight loss, though individual results vary. Dose titration schedules differ between the two — Mounjaro starts at 2.5 mg, Wegovy at 0.25 mg.
Neither requires food timing adjustments for effectiveness. The practical advantage is consistency — inject on the same day each week at a time that suits you.
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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.
Choosing Between Mounjaro and Wegovy: What Your Prescriber Considers
Eligibility for Mounjaro UK prescription and Wegovy is similar: adults with a BMI of 30 or above (or 27+ with weight-related health conditions such as type 2 diabetes, hypertension, or sleep apnoea) may qualify. Your GP or a licensed prescriber will assess you individually — neither medication is suitable for everyone. Pregnancy, breastfeeding, and certain medical conditions rule out both. The choice between them depends on your personal health profile, previous medication responses, and prescriber recommendation.
Use our GLP-1 weight loss calculator above to explore potential outcomes at different dose levels. Try our BMI calculator to check whether you fall within the eligibility range. Your prescriber will manage dose titration carefully — Mounjaro escalates from 2.5 mg weekly, while Wegovy escalates from 0.25 mg. Both are prescription-only medicines requiring clinical assessment.
- 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.
Does eating before my Mounjaro injection affect how well it works?
No. Because Mounjaro is injected directly under your skin (subcutaneous injection), stomach contents and food do not affect absorption or effectiveness. You can inject at any time, with or without food. Unlike oral medications, injection timing is not influenced by meals.
Is the injection timing different for Wegovy compared to Mounjaro?
No. Both Mounjaro and Wegovy are once-weekly subcutaneous injections that are not affected by food or stomach contents. You can inject either medication at any time of day that suits your schedule, on any day of the week you choose for consistency.
What time of day should I take my Mounjaro injection?
Choose a time that works for your routine and stick with it each week for consistency. Morning, evening, or any other time is equally effective. Many people choose the same day and time weekly for easier remembering — for example, every Monday morning with your Mounjaro KwikPen.
Can I take other oral medications around the time of my Mounjaro injection?
Yes, but oral contraceptives and some diabetes medications may have reduced effectiveness due to slower gastric emptying. Inform your prescriber of all medications you take. Allow at least 4 weeks after starting Mounjaro (or after each dose increase) before relying on oral contraceptives without backup protection.
What if I forget my weekly Mounjaro injection?
If you miss your injection, take it as soon as you remember — provided it's within 4 days of your scheduled time. If more than 4 days have passed, skip that dose and take your next injection on your normal scheduled day. Never double-dose. Your prescriber will provide detailed guidance on your UK prescription.
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.