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 Intermittent Fasting: What You Need to Know
Combining Mounjaro (tirzepatide) with intermittent fasting is a question many UK adults exploring weight management ask their GPs. While both approaches may support weight loss, combining them requires careful consideration. Mounjaro is a prescription-only, once-weekly GLP-1 and GIP receptor agonist that affects appetite and food intake. Intermittent fasting restricts eating to specific time windows. This guide explains how they work together, safety considerations, and when to seek medical advice. Always discuss any diet changes with your healthcare provider before starting.
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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 and Intermittent Fasting Work Together
Mounjaro works by activating GLP-1 and GIP pathways, which regulate appetite, slow gastric emptying, and increase feelings of fullness. Clinical trials showed that at maintenance doses, tirzepatide users lose an average of 15–21% body weight alongside diet and exercise. Intermittent fasting (such as 16:8 or 5:2 protocols) creates a calorie deficit by restricting eating windows.
When combined, Mounjaro's appetite-suppressing effects may make intermittent fasting easier to follow. However, this combination intensifies the overall calorie deficit. The reduced hunger from the injection may mask nutritional deficiencies if meal quality during eating windows is poor. Additionally, Mounjaro may slow stomach emptying, which can cause bloating or discomfort if eating is concentrated into shorter windows. Individual responses vary significantly—some people find the combination natural, whilst others experience nausea or digestive issues when combining both approaches.
Your prescriber should assess whether this combination is suitable for you, especially if you have a history of gallbladder problems or gastroparesis.
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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.
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Practical Considerations and Safety Points
Nutrition timing: Intermittent fasting combined with reduced appetite from Mounjaro means you must prioritise nutrient-dense meals during eating windows. Skipping meals or eating low-quality food increases risk of deficiency. Stay hydrated, particularly during fasting periods.
Side effects: Mounjaro commonly causes nausea, constipation, and stomach pain. Adding fasting may worsen these. Start with a standard eating pattern first, stabilise on your Mounjaro dose, then discuss intermittent fasting with your GP if you wish to try it.
Medication interactions: If you take diabetes medication alongside Mounjaro, fasting may affect blood sugar management. Your prescriber must adjust doses accordingly. Never fast without medical guidance if on insulin or sulphonylureas.
When to stop: If you experience severe stomach pain, persistent nausea, dizziness, or rapid heartbeat, stop fasting immediately and contact 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.
Is it safe to do intermittent fasting while on Mounjaro?
Potentially, but only with medical approval. Mounjaro combined with fasting creates a significant calorie deficit. Your GP needs to assess your individual health, current medications, and any history of stomach problems. Never start fasting without discussing it with your prescriber first. The combination is not suitable for everyone.
Will Mounjaro make intermittent fasting easier?
For many people, yes. Mounjaro reduces appetite and increases fullness, which may make fasting windows feel less difficult. However, this doesn't mean the combination is automatically better—it depends on your body's response, side effects, and nutritional needs. Some users find concentrated eating windows worsen nausea or bloating from the injection.
Can I use my GLP-1 weight loss calculator to predict results with both approaches?
Our GLP-1 weight loss calculator estimates outcomes based on Mounjaro dose and typical weight loss patterns from clinical trials. However, the calculator assumes standard eating patterns. If you're also fasting, actual results may differ. Use the calculator as a baseline, then discuss personalised projections with your prescriber.
What should I eat during eating windows on Mounjaro and intermittent fasting?
Prioritise high-protein, nutrient-dense foods: lean meats, fish, eggs, legumes, vegetables, and whole grains. Avoid ultra-processed foods, excess sugar, and alcohol, which can trigger nausea or worsen Mounjaro side effects. Drink plenty of water and electrolyte-rich beverages during fasting windows. Meal quality matters more when eating windows are restricted.
Should I try intermittent fasting if I'm just starting Mounjaro?
No—start with a standard eating pattern while your body adjusts to tirzepatide. Most people experience side effects (nausea, constipation, reduced appetite) in the first 4–8 weeks. Once stable on your maintenance dose and side effects have settled, discuss intermittent fasting with your GP or prescriber. Combining too many changes at once increases risk of adverse effects.
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