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 Making Me Sick — What You Need to Know
Nausea and digestive discomfort are very common side effects when starting Mounjaro (tirzepatide), affecting more than 1 in 10 people. If you're experiencing sickness, vomiting, or stomach pain on Mounjaro, you're not alone — and there are practical steps you can take. This guide explains what causes these side effects, when they typically improve, and how to manage them. If symptoms are severe or persistent, your GP or prescriber can help you decide whether to continue, adjust your dose, or explore alternatives like Wegovy.
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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 Does Mounjaro Cause Nausea and Sickness?
Mounjaro works by slowing how quickly food leaves your stomach and activating appetite-suppressing hormones in the brain. This mechanism, while effective for weight loss, commonly causes nausea, vomiting, and stomach discomfort — especially when starting or increasing your dose. Most people experience their worst symptoms in the first 2–4 weeks of each dose increase, and these typically improve within 5–7 days as your body adjusts.
Common symptoms include:
- Nausea (most common)
- Vomiting
- Diarrhoea or constipation
- Stomach or abdominal pain
- Indigestion
- Loss of appetite (which often continues)
The Mounjaro KwikPen delivers a once-weekly injection, and your prescriber will guide you through a gradual dose titration schedule — starting at 2.5mg and increasing every 4 weeks. This slower approach helps your body adapt and often reduces the severity of nausea compared to starting on higher doses.
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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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Managing Mounjaro Sickness — Practical Steps and When to Seek Help
If you're feeling sick on Mounjaro, try eating smaller, frequent meals rather than large ones; choose bland, low-fat foods like toast, crackers, or boiled rice; stay hydrated; and avoid fatty or spicy foods that can trigger nausea. Your GP may suggest anti-nausea medication (antiemetics) to help you through the adjustment period — this is common practice and doesn't mean you need to stop Mounjaro.
Seek urgent medical attention if you experience:
- Severe, persistent stomach pain spreading to your back (possible pancreatitis)
- Repeated vomiting preventing you from eating or drinking
- Signs of dehydration (dark urine, dizziness)
- Yellowing of skin or eyes
If side effects are unbearable after 7–10 days, contact your prescriber. They can delay your next dose increase, reduce your current dose, or discuss whether continuing is right for you. Stopping Mounjaro is safe — it leaves your system within days.
- 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 Mounjaro nausea usually last?
Nausea is typically worst in the first 2–4 weeks after starting or increasing your dose, and usually improves within 5–7 days as your body adjusts. Some people experience mild nausea throughout treatment, whilst others have no symptoms at all. If nausea persists beyond a week or becomes unbearable, contact your prescriber.
Can I take anti-sickness tablets with Mounjaro?
Yes. Your GP or prescriber can recommend antiemetic medications (anti-sickness tablets) to help manage nausea whilst your body adjusts to Mounjaro. This is a common and effective approach and doesn't mean you need to stop the medication. Always inform your prescriber of any other medicines you're taking.
Should I stop Mounjaro if I'm feeling very sick?
Do not stop abruptly without speaking to your prescriber. Contact them if symptoms are severe or persistent — they may suggest delaying your next dose increase, reducing your current dose, or pausing treatment. Your prescriber will help you decide the safest option based on your individual situation.
Is Mounjaro nausea a sign it's working?
Not necessarily. Nausea is a side effect caused by how Mounjaro affects your stomach and appetite hormones — it's not proof the medication is working for weight loss. Weight loss results vary between individuals. You can track your progress using our free progress tracker and BMI calculator.
How does Mounjaro compare to Wegovy for side effects?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) commonly cause nausea in the first weeks. Clinical data suggests similar rates of GI side effects, though Wegovy's slower starting dose (0.25mg vs Mounjaro's 2.5mg) may result in fewer initial symptoms. Your prescriber can discuss which option suits you best.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.