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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How Long Does Mounjaro Delay Gastric Emptying?
Mounjaro (tirzepatide) slows how quickly food moves from your stomach into your small intestine — a process called gastric emptying. This delay typically lasts several hours and is one of the ways the medication helps reduce appetite and support weight loss. However, this effect often decreases over time as your body adjusts to the medication. Understanding how gastric emptying works can help you manage side effects and know what to expect during treatment.
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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 Gastric Emptying
Mounjaro delays gastric emptying by activating GIP and GLP-1 receptors in the gut and brain. When you eat, these receptors signal that food is present, causing your stomach to contract more slowly and release food into the intestines at a reduced rate. This delay typically lasts several hours after eating — meaning food stays in your stomach longer than normal.
- At lower doses (2.5–5 mg), gastric emptying delay is usually mild
- At higher doses (10–15 mg), the effect is more pronounced initially
- The delay is strongest in the first 4–8 weeks of starting Mounjaro or increasing your dose
- Over time — usually after 8–12 weeks — your body adapts and the gastric effect diminishes
- This adaptation is why some people notice nausea improves after their first month, even if staying on the same dose
The delayed gastric emptying contributes to the feeling of fullness that makes eating smaller portions easier. However, it's also responsible for common side effects like nausea, bloating, and constipation, especially when you first start or increase your tirzepatide dose.
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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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Managing Gastric Emptying Side Effects and Access in the UK
To manage gastric emptying side effects, eat smaller, more frequent meals and avoid fatty or greasy foods, which slow digestion further. Stay hydrated and consider ginger tea or anti-nausea remedies if side effects are bothersome. If nausea or bloating becomes severe, speak to your prescriber — they may adjust your dose or suggest you stay at your current dose longer before escalating.
In the UK, Mounjaro is available through NHS specialist tier 3 weight management services (with potential waiting lists) or through private online prescribing following a clinical assessment. Private access is typically faster, with prescriptions dispensed by GPhC-registered pharmacies. Costs vary by dose — ranging from roughly £150–£250 per month for private prescriptions. Your prescriber will assess whether Mounjaro's gastric effects are manageable for your lifestyle.
- 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 the gastric emptying effect of Mounjaro wear off?
Yes, typically. Most people find the gastric emptying effect is strongest in weeks 1–8, then gradually decreases as the body adapts. This is why nausea often improves after the first month, even without dose changes. The appetite-suppressing effect usually remains, but the digestive slowdown becomes less noticeable.
How does Mounjaro's gastric delay compare to Wegovy?
Mounjaro (tirzepatide) has a dual GIP and GLP-1 mechanism and often causes more pronounced initial gastric slowdown than Wegovy (semaglutide), which is GLP-1 only. However, individual responses vary. Both medications can cause nausea and bloating, but Mounjaro's dual action may result in a stronger effect early on.
Can I eat normally while taking Mounjaro?
Many people can, but not immediately. During the first few weeks or after dose increases, smaller meals are often easier to tolerate. Once the gastric emptying effect diminishes (usually after 8–12 weeks), most people return to more normal eating patterns, though portion sizes typically remain smaller due to reduced appetite.
Will eating fatty foods make gastric emptying worse on Mounjaro?
Yes. Fat slows digestion naturally, and Mounjaro already slows gastric emptying, so combining the two can worsen nausea and bloating. During your first weeks on Mounjaro, stick to lean proteins, vegetables, and whole grains. As your body adapts, you can gradually reintroduce richer foods.
How do I know if my Mounjaro dose is right for the gastric side effects?
If nausea or bloating is severe enough to affect your daily life, discuss with your prescriber before increasing your dose. You may benefit from staying at your current dose longer — typically 4 weeks minimum — to allow adaptation. Your prescriber will balance weight loss goals with side effect tolerance.
Ready to take control of your weight loss journey?
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