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 Does Mounjaro Make You Feel Sick?
Nausea and feeling sick are among the most common side effects of Mounjaro (tirzepatide) in the UK. If you're experiencing sickness after starting this prescription weight loss injection, you're not alone — clinical trials show that nausea affects many people, especially when beginning treatment or increasing doses. Understanding what's happening, why it occurs, and what you can do about it will help you manage this side effect effectively and decide whether to continue treatment.
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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 Causes Nausea and Sickness
Mounjaro works by activating two hormone pathways — GIP and GLP-1 receptors — which signal fullness to your brain and slow how quickly food leaves your stomach. This dual mechanism is what makes tirzepatide different from single-action GLP-1 medications like Wegovy (semaglutide). The stomach-slowing effect, called delayed gastric emptying, is responsible for much of the nausea people experience. When food moves through your digestive system more slowly than usual, your stomach may feel uncomfortable, full, or unsettled — triggering the sensation of sickness. Additionally, the appetite-suppressing effect of Mounjaro can create an unusual relationship with food, where even favourite meals may seem unappealing. This is intentional and part of how the medication works, but it can feel strange and contribute to general queasiness. Nausea is classified as a "very common" side effect in UK clinical data, meaning it affects more than 1 in 10 people taking Mounjaro.
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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.
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When Nausea Typically Starts and How Long It Lasts
Nausea often begins within the first few days of starting Mounjaro or after a dose increase, and typically improves within 1–2 weeks as your body adjusts. However, some people experience mild, ongoing queasiness throughout their treatment. The intensity often depends on your individual sensitivity, your starting dose (2.5 mg is gentler than jumping to higher doses), and how well you manage your diet. If you're increasing through the dose titration schedule — starting at 2.5 mg and moving up every 4 weeks to 5 mg, 7.5 mg, 10 mg, 12.5 mg, and eventually 15 mg — you may notice sickness returns briefly each time you step up. This is normal. Track your symptoms using our free progress tracker to spot patterns and identify which doses or timing work best for you.
- 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 nausea from Mounjaro dangerous?
Nausea itself is not dangerous, but severe or persistent vomiting can lead to dehydration. If you're unable to keep fluids down or feel unwell beyond typical nausea, contact your GP. Seek immediate medical attention if you experience severe stomach pain — this could indicate pancreatitis, a rare but serious side effect.
What can I do to reduce nausea on Mounjaro?
Eat small, frequent meals rather than large portions; avoid fatty, greasy, or heavily spiced foods; stay hydrated with water and electrolyte drinks; eat slowly and chew thoroughly; avoid strong smells that trigger nausea; and take your injection on a day when you can rest if possible. Some people find ginger, peppermint tea, or travel sickness bands helpful.
Should I stop Mounjaro if I feel sick?
Do not stop without speaking to your prescriber first. Nausea often improves naturally within 1–2 weeks. If it's unbearable, your prescriber may slow your dose titration, pause at your current dose longer, or discuss alternative options. Always seek advice before stopping prescription medication.
How does Mounjaro nausea compare to Wegovy?
Mounjaro (dual GIP+GLP-1) and Wegovy (GLP-1 only) both cause nausea as a very common side effect. Some people report that Mounjaro nausea is more intense initially because of its dual mechanism, but individual experiences vary widely. Your prescriber can advise which is more suitable for you.
When should I contact my GP about Mounjaro sickness?
Contact your GP if nausea lasts more than 2–3 weeks without improvement, if you're unable to eat or drink, if you're losing weight unintentionally beyond expected loss, or if you develop severe abdominal pain, vomiting, or signs of allergic reaction. Always report persistent or worsening symptoms.
Ready to take control of your weight loss journey?
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