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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Why Is Mounjaro Making Me Sick?
If you're experiencing nausea, vomiting, or feeling unwell after starting Mounjaro (tirzepatide), you're not alone — these are among the most commonly reported side effects in the UK. Nausea affects more than 1 in 10 people taking this prescription weight loss injection, particularly during the first few weeks or when your dose is increased. Understanding why this happens, how long it typically lasts, and what practical steps you can take to manage it is essential. Your GP or prescriber can offer personalised advice, but this guide explains the NHS and UK context around Mounjaro-related sickness and what to expect.
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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.
Why Mounjaro Causes Nausea and Sickness
Mounjaro works as a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways that help regulate appetite and blood sugar. One of the ways these hormones work is by slowing gastric emptying (how quickly food moves through your stomach). This slower digestion can trigger nausea, especially in the first few weeks of treatment or when your dose increases. Your body typically adjusts over time as it becomes accustomed to the medication.
The nausea is usually temporary and dose-dependent — it often improves within a few days or weeks as your body adapts. However, some people experience prolonged nausea, vomiting, constipation, or diarrhoea. The Mounjaro KwikPen is administered once weekly, and many people find symptoms are worst in the first 24–48 hours after injection, then gradually improve. Individual responses vary significantly — some people experience minimal nausea, whilst others find it more challenging.
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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 Nausea and When to Contact Your GP
Practical steps to manage Mounjaro sickness: Eat smaller, more frequent meals; avoid fatty or greasy foods; stay hydrated; eat slowly; avoid strong smells; rest after eating. Many people find eating bland foods like crackers, rice, or broth helpful.
If nausea is severe, persistent, or affecting your ability to eat and drink, contact your GP or prescriber immediately. They may adjust your dose, recommend anti-nausea medication, or suggest temporarily pausing treatment. Seek urgent medical attention if you experience severe, unrelenting stomach pain (possible pancreatitis), persistent vomiting, or signs of dehydration. Your GP can assess whether Mounjaro is right for you and explore alternatives if side effects become unmanageable.
- 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.
How long does nausea last when starting Mounjaro?
Nausea typically appears within hours of injection and often improves within 24–48 hours as your body adjusts. Most people find nausea decreases over the first 2–4 weeks. However, it can persist longer for some individuals, especially at higher doses. Your prescriber can advise on managing symptoms or adjusting your dose schedule.
Is there anything I can take to stop feeling sick on Mounjaro?
Your GP or prescriber may recommend anti-nausea medication (anti-emetics) alongside Mounjaro if nausea is significant. Dietary adjustments — smaller meals, avoiding fatty foods, staying hydrated — help many people. Always discuss medication options with your prescriber rather than self-treating, as some medications may interact with Mounjaro.
Can I stop taking Mounjaro if it's making me too sick?
Do not stop abruptly without speaking to your prescriber. If Mounjaro is causing unmanageable side effects, contact your GP or the service that prescribed it. They may recommend dose adjustments, a temporary break, or switching to an alternative treatment. Stopping suddenly is not recommended without professional guidance.
Is nausea on Mounjaro a sign something is seriously wrong?
Mild to moderate nausea is a common expected side effect, not a sign of serious problems. However, seek urgent medical attention if you experience severe, unrelenting stomach pain (possible pancreatitis), persistent vomiting, signs of dehydration, or difficulty keeping any food or fluids down. These require immediate GP assessment.
How does Mounjaro compare to Wegovy for nausea and sickness?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) commonly cause nausea, particularly at the start of treatment. Mounjaro uses a dual GIP and GLP-1 mechanism versus Wegovy's single GLP-1 mechanism — individual tolerance varies. Some people experience less nausea on one versus the other, but there's no universal pattern. Your prescriber can discuss which might suit you better.
Ready to take control of your weight loss journey?
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