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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Vomiting on Mounjaro — What You Need to Know
Vomiting is one of the most common side effects of Mounjaro (tirzepatide), a prescription-only weight management injection licensed in the UK. It affects a significant proportion of people starting treatment, particularly during dose escalation. The good news is that nausea and vomiting typically improve over time as your body adjusts to the medication. Understanding what to expect and how to manage this side effect can help you stay on track with your treatment plan. Always speak to your GP or prescriber if vomiting is severe or persistent.
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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 Vomiting?
Vomiting on Mounjaro happens because tirzepatide is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways that regulate appetite and digestion. These hormones naturally slow how quickly food moves through your stomach (gastric emptying) and signal fullness to your brain. When you first start Mounjaro or increase your dose, your digestive system is adjusting to these stronger signals, which can trigger nausea and vomiting.
The Mounjaro KwikPen delivers doses starting at 2.5 mg, with increases every 4 weeks up to the maximum 15 mg. Higher doses carry a greater risk of nausea and vomiting during the titration phase. This is a normal adjustment response and typically lessens within the first few weeks on each new dose level.
- Vomiting is very common — affecting more than 1 in 10 people in clinical trials
- It is usually most noticeable when starting or increasing doses
- Symptoms typically improve as your body adapts
- Eating small, frequent meals can help reduce severity
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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 Vomiting and When to Contact Your GP
If you experience vomiting on Mounjaro, try eating smaller portions more frequently, avoid fatty or greasy foods, and stay hydrated with water or electrolyte drinks. Some people find ginger tea or anti-nausea remedies helpful. Your prescriber may recommend taking Mounjaro at a different time of day or suggest temporary dietary changes during adjustment periods.
Contact your GP or prescriber urgently if: vomiting is severe or prevents you from keeping food and fluids down, you experience signs of dehydration (dark urine, extreme thirst, dizziness), you have severe abdominal pain (possible pancreatitis), or vomiting persists beyond 2–3 weeks on a dose. Your prescriber may adjust your treatment plan, delay your next dose increase, or suggest alternative management strategies. Use our free GLP-1 weight loss calculator above to understand what to expect at different Mounjaro doses.
- 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 vomiting last?
Nausea and vomiting typically peak in the first few days after starting or increasing your dose, then improve over 1–2 weeks as your body adjusts. Some people experience mild nausea for longer, but severe vomiting usually settles within 2–3 weeks. If symptoms persist beyond this, contact your prescriber.
Is vomiting on Mounjaro a sign something is wrong?
Vomiting is a very common side effect of Mounjaro and usually indicates your body is adjusting to the medication — not that something is wrong. However, if vomiting is severe, accompanied by severe abdominal pain, or you cannot keep fluids down, seek medical attention as these could indicate pancreatitis or dehydration.
Can I take anti-nausea medication with Mounjaro?
Some anti-nausea medications may be safe to take alongside Mounjaro, but always ask your GP or prescriber first. Certain medications can affect how Mounjaro works or be affected by tirzepatide. Your prescriber can recommend safe options if over-the-counter remedies are not enough.
How does Mounjaro compare to Wegovy for side effects?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) commonly cause nausea and vomiting. Mounjaro uses a dual GIP and GLP-1 mechanism while Wegovy uses GLP-1 only. Clinical trial data shows Mounjaro achieved slightly higher average weight loss (~21% vs ~15%), though individual responses vary greatly and side effect profiles are similar.
Should I stop taking Mounjaro if I'm vomiting?
Do not stop taking Mounjaro without speaking to your prescriber first. Stopping suddenly could affect your treatment. Instead, contact your prescriber to discuss management options. They may recommend dietary changes, dose adjustment timing, or strategies to ease symptoms while your body adapts.
Ready to take control of your weight loss journey?
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