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 Jab Side Effects — What UK Patients Should Know
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK for adults with obesity or overweight with weight-related health conditions. Like all medicines, Mounjaro can cause side effects. Most are mild and tend to improve over time as your body adjusts to the medication, but it's important to understand what to expect. This guide explains the common and serious side effects you should be aware of, based on UK clinical data and NHS guidance, so you can make an informed decision with your GP or prescriber.
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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.
Common Side Effects of Mounjaro in the UK
The most frequent side effects reported in UK clinical trials affect the digestive system. Nausea is very common — occurring in more than 1 in 10 people — especially when starting the medication or increasing your dose. Diarrhoea and constipation are also very common, along with stomach pain and indigestion. These effects typically improve within a few weeks as your body adjusts.
Other common side effects include fatigue, dizziness, headaches, and decreased appetite — though the appetite reduction is actually the desired effect of the medication. Hair loss (alopecia) has been reported in some UK patients, though this is usually temporary and related to gradual weight management rather than the medication itself. Injection site reactions — redness, bruising, or mild pain where you inject — are also common but typically mild.
The good news: most side effects are dose-related and often settle within the first 2–4 weeks of starting treatment or after your body adapts to a new dose during the dose titration phase.
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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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Serious Side Effects Requiring Urgent GP Attention
Whilst uncommon, some side effects require immediate medical attention. Severe, persistent stomach pain spreading to your back could indicate pancreatitis — contact your GP or A&E immediately. Gallbladder problems — including pain in the upper stomach area — should also be reported urgently to your GP.
Serious allergic reactions are rare but require emergency care: swelling of the face, lips, tongue, or throat, difficulty breathing, or severe rash. Significant changes in vision can occur, especially in patients with existing diabetes, and should be reported to your GP or eye care professional urgently.
If you experience persistent nausea or vomiting preventing you from eating or drinking, contact your GP rather than waiting for your next appointment. Your prescriber may need to adjust your dose or provide additional support. Always report any side effect that concerns you, even if not listed here.
- 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 normal and will it go away?
Yes, nausea is very common when starting Mounjaro — affecting more than 1 in 10 people. It typically improves within 2–4 weeks as your body adjusts. If nausea persists beyond this or is severe, contact your GP. They may suggest taking the injection at a different time of day or adjusting your diet. Never stop taking Mounjaro without speaking to your prescriber first.
Can Mounjaro side effects be reduced or managed?
Many side effects are dose-related and improve with time. To manage them: eat smaller, more frequent meals; stay hydrated; avoid fatty or spicy foods (which can worsen nausea); and move gently after injecting. Your prescriber may slow your dose titration if side effects are significant. Always discuss management strategies with your GP or pharmacist.
Is hair loss from Mounjaro permanent?
Hair loss (alopecia) reported with Mounjaro is usually temporary and often linked to gradual weight management rather than the medication itself. It typically reverses when weight stabilises. If you're concerned about hair loss, discuss it with your GP — they can assess whether it's related to the medication or other factors like nutritional deficiency.
When should I call my GP about Mounjaro side effects?
Contact your GP if you experience: severe, persistent stomach pain; vomiting preventing eating or drinking; serious allergic reactions; vision changes; chest pain; or rapid heartbeat. These require prompt medical assessment. For mild side effects like nausea or dizziness that are improving, you can wait to discuss them at your next review.
Are Mounjaro side effects different from Wegovy or other weight loss injections?
All GLP-1 medications cause similar digestive side effects (nausea, diarrhoea, constipation). Mounjaro is a dual GIP/GLP-1 agonist whilst Wegovy (semaglutide) is GLP-1 only — this means Mounjaro's side effect profile may differ slightly for some people. Your GP can discuss which medication suits your health profile best.
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