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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Can You Fast While Taking Mounjaro?
Fasting while taking Mounjaro (tirzepatide) is a question many people ask when combining prescription weight loss medication with dietary approaches. The short answer is that fasting is generally possible on Mounjaro, but it requires careful consideration of how the medication works, your individual health, and guidance from your prescriber. Mounjaro is a once-weekly GLP-1 and GIP receptor agonist injection that helps reduce appetite and support weight management alongside lifestyle changes. Before starting any fasting approach, always discuss your specific plans with your GP or the healthcare professional who prescribed your Mounjaro.
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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 Affects Eating and Fasting
Mounjaro works by activating two natural hormone pathways—GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1)—which help regulate appetite, reduce food cravings, and support feeling fuller after smaller meals. This dual mechanism slows gastric emptying, meaning food stays in your stomach longer, which typically reduces over time as your body adjusts.
When considering fasting on Mounjaro, it's important to understand that the medication is designed to work with reduced food intake, not to enable extreme calorie restriction. Clinical trials showed that tirzepatide works best when combined with a reduced-calorie diet and increased physical activity—fasting takes this further and may amplify effects, which can introduce risks:
- Nausea and other gastrointestinal side effects may worsen during fasting periods
- Low blood sugar (hypoglycaemia) is a risk if you're also taking diabetes medications
- Muscle loss may increase if you're not eating enough protein during eating windows
- Dehydration and electrolyte imbalances can occur during extended fasts
NHS guidance emphasises sustainable weight loss of 0.5–1 kg per week. Combining Mounjaro with fasting may accelerate weight loss beyond this, which can increase health risks if not medically supervised.
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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.
Mounjaro Dosing and Fasting Safety Considerations
Your Mounjaro dose matters when considering fasting. At lower doses (2.5–5 mg), appetite suppression is gentler and fasting may feel more tolerable. At higher doses (10–15 mg), the appetite-suppressing effects are stronger, which can make fasting easier but also increases the risk of under-eating and nutrient deficiency. If you're on a UK prescription through the NHS or private prescriber, your dose titration schedule (increasing by 2.5 mg every four weeks) means your tolerance for fasting will change over time.
Before attempting any fasting plan: inform your prescriber. They can assess whether fasting is safe given your dose, any other medications you take (especially diabetes medications), and your overall health. Some people find that intermittent fasting (such as 16:8) feels natural on Mounjaro due to reduced hunger, whilst others experience side effects. Never combine fasting with rapid dose escalation, and always monitor how you feel—fatigue, dizziness, or severe nausea warrant immediate GP advice.
- 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 taking Mounjaro?
Intermittent fasting may feel easier on Mounjaro due to reduced appetite, but it should only be attempted with your prescriber's agreement. Mounjaro is designed to support a reduced-calorie diet, not extreme fasting. Your GP needs to assess your individual dose, any other medications, and health conditions before you start fasting. Monitor yourself carefully and report any concerning symptoms.
Will fasting make Mounjaro work better or faster?
Combining Mounjaro with fasting may increase weight loss speed, but faster weight loss isn't necessarily healthier. NHS guidance recommends 0.5–1 kg per week for sustainable results. Clinical trials show tirzepatide works effectively with regular calorie deficit and lifestyle changes—fasting may cause muscle loss, nutrient deficiency, or side effects. Discuss any fasting plans with your prescriber first.
What should I eat when I'm not fasting on Mounjaro?
During your eating window, prioritise balanced meals with adequate protein (to protect muscle), plenty of vegetables, whole grains, and healthy fats. Mounjaro reduces appetite, so you may need to be intentional about nutrition to meet your daily needs. Stay hydrated throughout the day. If you're unsure, ask your prescriber or request a referral to a registered dietitian.
Can I fast if I'm also taking insulin or diabetes medication?
No—fasting combined with insulin or other blood sugar-lowering medication significantly increases hypoglycaemia risk. If you have type 2 diabetes and are considering Mounjaro, your prescriber may adjust your diabetes medications as you lose weight. Never fast without explicit approval from the healthcare professional managing your diabetes medications.
What's the difference between Mounjaro and Wegovy when it comes to fasting?
Both are prescription-only injections, but Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. Mounjaro typically produces stronger appetite suppression, which some people find makes fasting easier—but both medications should be combined with balanced eating, not extreme fasting. Your prescriber can advise which approach fits your medication and health profile.
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