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
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Mounjaro and Fasting — What You Need to Know
If you're taking Mounjaro (tirzepatide) and considering intermittent fasting, it's important to understand how these work together. Mounjaro is a prescription-only GIP and GLP-1 receptor agonist injection that may support weight loss alongside reduced-calorie eating and increased physical activity. Fasting is a common dietary approach in the UK, but combining it with prescription weight loss medication requires careful consideration. Always discuss any fasting plans with your GP or prescriber before starting, as they can assess what's safe and appropriate for your individual health situation. Our GLP-1 weight loss calculator can help you understand potential outcomes with Mounjaro at different doses.
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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 Works With Intermittent Fasting
Mounjaro works by activating two hormone pathways — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 — which help regulate appetite and support feeling fuller after meals. Intermittent fasting (such as 16:8 or 5:2 patterns) involves eating within specific time windows or on certain days. When combined, Mounjaro's appetite-suppressing effects may enhance the results of fasting, but this also means you need to be careful about nutrient intake and energy levels.
- Mounjaro slows gastric emptying and may reduce food cravings throughout the day, making fasting windows potentially easier to follow
- Intermittent fasting creates a calorie deficit during fasting windows, which is essential for weight loss
- Together, they may support faster results, but prescribers need to monitor your health — muscle loss, nutritional deficiencies, and blood sugar levels matter
- Your prescriber may adjust your approach based on your individual health, other medications, and weight loss progress
The SURMOUNT clinical trials tested Mounjaro alongside reduced-calorie diets, not specifically with intermittent fasting. Individual responses vary, and fasting is not recommended for everyone taking weight loss medication.
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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
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Mounjaro vs Wegovy — How Fasting Differs
Wegovy (semaglutide) is a GLP-1 only agonist, whereas Mounjaro is a dual GIP+GLP-1 agonist. This means Mounjaro has two mechanisms working together, which may result in stronger appetite suppression compared to Wegovy's single mechanism. In clinical trials, Mounjaro showed average weight loss of around 20.9% at maximum dose (SURMOUNT-1), while Wegovy showed approximately 14.9% (STEP-1). When fasting with either medication, the stronger appetite suppression from Mounjaro may make fasting easier, but it also increases the risk of undereating or nutrient deficiency. Whether you use Mounjaro or Wegovy, fasting should only be attempted with your prescriber's explicit approval and regular monitoring. Both are prescription-only and require clinical assessment before use.
- 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 do intermittent fasting while taking Mounjaro?
You can, but only with your prescriber's approval. Mounjaro's appetite-suppressing effects combined with fasting may lead to very low calorie intake, muscle loss, or nutritional deficiencies. Your GP needs to monitor your health, blood sugar, and nutrient status. Never start fasting without discussing it with your prescriber first.
Will Mounjaro make fasting easier?
Yes, likely. Mounjaro reduces appetite and slows stomach emptying, which means you may feel less hungry during fasting windows. However, this doesn't mean fasting is safe for you — it still requires prescriber guidance. Just because you don't feel hungry doesn't mean your body is getting adequate nutrition.
Is Mounjaro or Wegovy better for fasting?
Mounjaro (dual GIP+GLP-1) may suppress appetite more strongly than Wegovy (GLP-1 only), potentially making fasting feel easier. However, "easier" doesn't mean "better" — stronger appetite suppression increases undereating risks. Your prescriber will recommend what's safest for your health, based on your individual situation, not on how well fasting feels.
What's the difference between Mounjaro and tirzepatide?
Mounjaro is the brand name; tirzepatide is the active ingredient. Mounjaro (tirzepatide) is available as a KwikPen pre-filled injection with doses from 2.5mg to 15mg once weekly. Your prescriber may refer to it by either name — they mean the same medication.
Should I use my BMI to decide on fasting with Mounjaro?
No. BMI is useful for screening but doesn't account for muscle mass, overall fitness, or nutritional status. Use our free BMI calculator to understand your baseline health category, but your prescriber's decision about whether to combine Mounjaro with fasting depends on your full health picture, not BMI alone.
Ready to take control of your weight loss journey?
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