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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Feeling Sick on Mounjaro — What You Need to Know
Nausea and feeling sick are among the most common side effects when starting Mounjaro (tirzepatide) in the UK. These symptoms often occur during the initial dose titration phase and typically improve as your body adjusts. If you're experiencing persistent nausea, it's important to understand what's normal, when to contact your GP, and what strategies may help. This guide covers NHS and UK-specific advice for managing sickness while taking Mounjaro.
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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.
Why Nausea Happens on Mounjaro and When It Typically Improves
Nausea affects more than 1 in 10 people taking Mounjaro during the dose titration phase—the first few weeks when your dose is gradually increased. This occurs because tirzepatide slows gastric emptying (how quickly food leaves your stomach) and activates GIP and GLP-1 receptors that signal fullness to your brain. Most people experience nausea most intensely in the first 4 weeks, particularly during the 2.5 mg and 5 mg starting doses. However, individual responses vary significantly.
The good news: nausea typically improves within 2–4 weeks as your body adapts. Staying on the same dose for the full 4-week period before escalating helps your system adjust. If nausea persists or worsens, your GP or prescriber may recommend dose adjustment or a slower titration schedule. Unlike some temporary side effects, persistent nausea is worth reporting—it shouldn't prevent you from continuing treatment if the medication is otherwise suitable for you.
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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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Managing Nausea and When to Contact Your GP
Simple practical strategies can help reduce nausea while taking Mounjaro. Eat smaller, frequent meals rather than large meals; fatty or greasy foods often trigger nausea more than protein-rich options. Stay well hydrated and avoid strong smells. Over-the-counter ginger supplements or peppermint tea may provide relief for some people, though discuss this with your GP first. Antiemetic medication (anti-sickness tablets) prescribed by your GP can be helpful during the adjustment phase.
Contact your GP if: nausea is severe and prevents you eating or drinking, lasts more than a week at the same dose, or you're unable to manage it with simple strategies. Your prescriber can adjust your dose or slow your titration schedule. Use our GLP-1 weight loss calculator to understand expected outcomes alongside dose adjustment—managing side effects doesn't mean you won't benefit from the medication.
- 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 on Mounjaro dangerous?
Mild to moderate nausea is a normal, temporary side effect. However, severe nausea that prevents you eating or drinking, or lasts longer than 2 weeks, requires GP assessment. Persistent inability to consume food or fluids could affect your health and nutritional intake, so report it to your prescriber promptly.
Should I stop taking Mounjaro if I feel sick?
No—stopping without advice may be counterproductive. Nausea typically improves within 2–4 weeks as your body adjusts. Contact your GP or prescriber instead. They can adjust your dose, slow your titration schedule, or prescribe anti-sickness medication to help you through the initial phase.
Does nausea mean Mounjaro isn't working?
Not at all. Nausea and weight loss benefits are independent. You may experience nausea during adjustment while still benefiting from the medication's effects on appetite and blood sugar. Many people report nausea subsides while weight management benefits continue.
Can I take anti-sickness tablets with Mounjaro?
Yes. Commonly used antiemetics like metoclopramide or domperidone can be prescribed alongside Mounjaro by your GP. Always inform your prescriber of all medications you're taking, as some interact with how Mounjaro is absorbed, so timing may need adjustment.
How does nausea on Mounjaro compare to Wegovy?
Both tirzepatide (Mounjaro) and semaglutide (Wegovy) cause nausea in similar proportions of people. Clinical trials show comparable side effect profiles. Individual tolerance varies, so what triggers nausea in one person may not affect another—your GP can discuss which option might suit you better.
Ready to take control of your weight loss journey?
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