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?
Feeling sick or nauseous is one of the most common side effects people experience when starting Mounjaro (tirzepatide). This happens because of how the medication works in your body — it activates GIP and GLP-1 hormone pathways that affect appetite, digestion, and how your stomach processes food. Nausea typically improves as your body adjusts, but understanding why it happens can help you manage it better. This guide explains the mechanism behind Mounjaro sickness, how long it usually lasts, and what you can do about it.
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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 as a dual GIP and GLP-1 receptor agonist, which means it activates two natural hormone pathways in your body. These pathways directly affect your gastrointestinal system. Here's what happens:
- Slowed gastric emptying: Mounjaro slows how quickly food moves from your stomach into the small intestine. This gives you a fuller feeling after smaller meals, but can cause nausea, bloating, and discomfort — especially early in treatment.
- Appetite regulation: The medication signals to your brain that you are satisfied, which can reduce hunger signals but may also trigger nausea as your body adjusts to these new signals.
- Digestive changes: Some people experience changes in bowel movements, indigestion, or stomach discomfort as their digestive system adapts to the altered pace of food processing.
Nausea is dose-dependent — meaning it's often worse when you start at the 2.5 mg dose or when you increase your Mounjaro dose titration. Many people find their symptoms improve significantly after 2–4 weeks on each dose as their body adjusts. Your GP can discuss whether staying at a lower dose longer or adjusting your eating patterns might help.
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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 Seek Help
If you're experiencing nausea with Mounjaro, several practical strategies may help: eat smaller, more frequent meals rather than large portions; avoid fatty or heavy foods that delay stomach emptying further; stay hydrated with water and electrolyte drinks; and take your weekly injection on a day when you can rest if side effects are severe.
Most people find nausea reduces within the first month, though some experience it longer. Contact your GP if: nausea is severe and persistent after 4 weeks; you cannot keep food or fluids down; you experience signs of dehydration; or you have severe stomach pain spreading to your back (which may indicate pancreatitis). Your prescriber may adjust your approach or discuss whether Mounjaro continues to be suitable for you. Remember, Mounjaro is prescription-only and requires a clinical assessment — your healthcare team can provide personalised guidance.
- 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 with Mounjaro normal?
Yes. Nausea affects more than 1 in 10 people taking Mounjaro and is considered a very common side effect. It usually happens during the first few weeks and often improves as your body adjusts to the medication. Most people find it manageable with dietary changes and time.
How long does Mounjaro nausea last?
Nausea typically peaks in the first week after injection and improves over 2–4 weeks as your body adapts. For some people it resolves completely; for others it persists mildly. When you increase your dose, nausea may return temporarily. Speak to your GP if it doesn't improve after a month.
Can I prevent nausea with Mounjaro?
You cannot prevent it entirely, but you can reduce severity. Eat smaller, frequent meals; avoid fatty or spicy foods; stay hydrated; and don't lie down immediately after eating. Taking your injection on a day when you can rest may also help. Your prescriber can advise on dose timing and adjustments.
Is Mounjaro nausea different from Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) cause nausea as a common side effect. Clinical data suggests Mounjaro may cause slightly higher rates due to its dual GIP/GLP-1 mechanism, but individual experiences vary significantly. Your GP can discuss which medication might suit you better.
Should I stop Mounjaro if I feel sick?
Do not stop without speaking to your GP. Nausea often improves with time and dietary adjustments. However, contact your GP if nausea is severe, persistent beyond 4 weeks, or you cannot eat or drink. Your prescriber may suggest dose adjustments or explore whether Mounjaro remains the right choice for you.
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