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 — A Complete UK Guide
Mounjaro (tirzepatide) is a prescription-only weight loss injection that may cause side effects, particularly when starting or increasing doses. Most side effects are mild and temporary, but it's important to understand what to expect and when to seek medical advice. This guide explains the common, less common, and serious side effects based on UK safety data and patient experience, so you can make an informed decision about whether this treatment is right for you.
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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
Gastrointestinal side effects are the most frequently reported with Mounjaro. Nausea is the most common, affecting more than 1 in 10 users, particularly in the first few weeks or when doses increase. Diarrhoea, vomiting, and constipation are also very common. Stomach (abdominal) pain occurs frequently but is usually mild and settles as your body adjusts.
- Nausea: Most common in weeks 1–4; often reduces as your body adapts
- Diarrhoea and constipation: May alternate; staying hydrated and eating slowly helps
- Vomiting: Less common than nausea; seek advice if persistent
- Indigestion and acid reflux: Can occur; eating smaller meals may help
- Injection site reactions: Redness, bruising, or mild pain at injection site; usually settle within hours
Other common effects include fatigue, dizziness, headache, and hair loss (alopecia). These typically improve within 2–4 weeks as your body adjusts to the medication.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Serious Side Effects — When to Seek Help
Serious side effects are rare but require urgent medical attention. Severe, persistent stomach pain that spreads to your back may indicate pancreatitis — contact your GP or call 111 immediately. Changes in vision, particularly if you have diabetes or diabetic retinopathy, need urgent review. Gallbladder problems (pain in the upper stomach area, yellowing of skin or eyes) require prompt assessment.
A serious allergic reaction is uncommon but presents as swelling of the face, lips, tongue, or throat — call 999 if this occurs. Rapid or severely abnormal heart rate changes should also be reported urgently to your GP. Most people experience only mild, manageable side effects that resolve within weeks. Use our free GLP-1 weight loss calculator to explore projected outcomes, and discuss side effect tolerance with your prescriber before starting.
Most Mounjaro side effects are dose-dependent and temporary. They are most noticeable when starting (2.5 mg dose) and when increasing to each new dose level. Many users find side effects reduce significantly after 2–4 weeks at the same dose. Staying well hydrated, eating smaller meals, avoiding fatty or greasy foods, and eating slowly can help minimise gastrointestinal effects. If side effects are severe or don't improve, your prescriber may slow your dose progression or adjust your dosing schedule. Never skip doses to avoid side effects — speak to your healthcare provider instead. Using our BMI calculator alongside your weight tracking helps monitor whether Mounjaro is working effectively for you, even if side effects occur initially.
- 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 do Mounjaro side effects last?
Most side effects are temporary and improve within 2–4 weeks as your body adjusts. Nausea is typically the most noticeable in the first 1–2 weeks. If side effects persist beyond 4 weeks, contact your GP or prescriber — they may adjust your dose progression or suggest strategies to help manage them.
Are Mounjaro side effects worse than Wegovy side effects?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) cause similar gastrointestinal side effects. Clinical trial data suggests Mounjaro may cause slightly more nausea initially, but individual experiences vary widely. Your prescriber can discuss which medication might suit you better based on your medical history.
Can I prevent Mounjaro side effects?
Whilst you cannot prevent side effects entirely, you can minimise them by eating smaller, slower meals, avoiding fatty foods, staying hydrated, and resting when fatigued. Gradual dose escalation (staying 4 weeks at each dose) also helps your body adjust more comfortably before increasing further.
What should I do if I have severe stomach pain on Mounjaro?
Severe stomach pain spreading to your back is a medical emergency and may indicate pancreatitis. Call 111 or go to A&E immediately. Do not wait for a GP appointment. Stop taking Mounjaro and inform the healthcare team you are taking this medication.
Can I take anti-sickness medication with Mounjaro?
Some anti-sickness medications may be used, but discuss this with your prescriber first. Certain interactions are possible. Your prescriber may recommend specific anti-sickness medications that are safe to take alongside Mounjaro, or suggest dietary adjustments instead.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.