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 You Need to Know
Mounjaro (tirzepatide) is a prescription-only weekly injection licensed in the UK for weight management in adults. Like all medicines, this GLP-1 and GIP receptor agonist can cause side effects. Most are mild and temporary, occurring as your body adjusts to the medication. Understanding what to expect helps you decide if Mounjaro is right for you and know when to contact your GP. This guide covers common side effects, serious reactions, and NHS guidance on managing them safely.
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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 Mounjaro Side Effects and What Causes Them
The most common side effects of Mounjaro affect the digestive system because tirzepatide slows how quickly food leaves your stomach (gastric emptying). Very common side effects (affecting more than 1 in 10 people) include:
- Nausea — often worst in the first 2–4 weeks, typically improves as your body adjusts
- Diarrhoea or constipation — both are temporary in most cases
- Stomach (abdominal) pain — usually mild and linked to eating too quickly or too much
- Vomiting — less common than nausea but can occur
Common side effects (up to 1 in 10 people) include: indigestion, heartburn (gastro-oesophageal reflux), injection site reactions (redness or bruising), fatigue, dizziness, hair loss, and decreased appetite. Most improve within 4–8 weeks as your body adapts. These side effects do not mean Mounjaro is not working — they reflect how it is affecting your system.
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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 and When to Seek Help
Seek urgent medical attention if you experience:
- Severe, persistent stomach pain spreading to the back — this may indicate pancreatitis (pancreas inflammation), a serious but rare side effect
- Signs of a serious allergic reaction — swelling of the face, lips, tongue, or throat; severe breathing difficulties; anaphylaxis
- Changes in vision — in people with diabetes, Mounjaro may cause diabetic retinopathy complications
- Gallbladder problems — severe pain in the upper stomach area, nausea, yellowing of skin or eyes
Contact your GP or call 111 immediately if you have these symptoms. Do not wait for them to pass. Your prescriber needs to know about any serious side effects straight away. Using our free GLP-1 weight loss calculator can help you understand what to expect at different doses.
Managing Side Effects: Practical Tips from NHS GuidanceMost Mounjaro side effects are manageable with simple strategies. To reduce nausea: eat small, frequent meals rather than large ones; avoid fatty, greasy foods; stay hydrated; try ginger or anti-nausea remedies. For constipation: increase fibre intake, drink more water, and increase physical activity. For diarrhoea: avoid high-fat foods and sugary items. Injection site reactions: rotate injection sites weekly (stomach, thigh, upper arm) to prevent irritation. If side effects persist beyond 8 weeks or significantly impact your quality of life, your GP or prescriber can adjust your dose or explore alternatives. Keep a symptom diary to share with your healthcare team — this helps them personalise your treatment.
- 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 improve within 2–8 weeks as your body adjusts. Nausea and stomach upset are typically worst in the first month. If symptoms persist beyond 8 weeks, contact your prescriber — dose adjustment may help. Individual timelines vary; some people feel better within days, others take several weeks.
Can I take anti-nausea medicine with Mounjaro?
Yes, you can use over-the-counter anti-nausea remedies like ginger or travel sickness tablets alongside Mounjaro. Always inform your prescriber about any medicines you take, as some may interact. Your GP can also prescribe anti-nausea medication if needed, though side effects usually resolve naturally.
Are Mounjaro side effects the same as Wegovy?
Similar but not identical. Both are GLP-1-based weight loss injections causing nausea and digestive side effects. Mounjaro is a dual GIP and GLP-1 receptor agonist, so its side effect profile may differ slightly. Individual experiences vary. Compare side effects on this page or consult your prescriber.
What should I do if I have severe stomach pain?
Seek urgent medical attention — call 111 or attend your nearest A&E. Severe stomach pain spreading to the back may indicate pancreatitis, a serious condition requiring immediate investigation. Do not assume it will pass. Contact your GP or prescriber immediately, and explain your symptoms clearly.
Do side effects mean I should stop Mounjaro?
Not necessarily. Most side effects are temporary and manageable without stopping treatment. However, if you experience serious symptoms (severe pain, allergic reactions, vision changes), seek medical advice immediately. Your prescriber can adjust your dose, slow your escalation, or switch medications if needed.
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.