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 Jabs Side Effects — What You Should Know
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK for adults with obesity or overweight plus weight-related health conditions. Like all medications, Mounjaro can cause side effects. Most are mild and temporary, occurring as your body adjusts to the medication. Understanding what to expect, how side effects compare to other weight loss injections like Wegovy, and when to contact your GP will help you 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 Mounjaro Side Effects in the UK
Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. These typically occur during the dose titration phase—as your prescription increases from the starting 2.5 mg dose to the maintenance dose—and often improve over time as your body adjusts.
Common side effects (affecting up to 1 in 10 people) include indigestion, acid reflux (GORD), injection site reactions such as redness or bruising, fatigue, dizziness, and hair loss. Injection site reactions are usually mild and resolve within a few days. Most people find nausea and stomach symptoms are most noticeable in the first 4–8 weeks.
The NHS and MHRA advise that side effect severity often reduces once your body has fully adjusted to each dose level. Staying well hydrated, eating smaller meals, and following your prescriber's guidance on dose escalation can help minimise discomfort.
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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
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When to Seek Medical Help
Contact your GP immediately if you experience severe, persistent stomach pain spreading to your back (possible sign of pancreatitis), serious allergic reactions such as swelling of the face, lips, tongue or throat, gallbladder pain in the upper stomach area, or significant changes in heart rate or vision. These are rare but require urgent medical assessment.
If you develop moderate nausea or diarrhoea lasting more than a few days, or if side effects prevent you from eating or drinking, speak to your GP or the specialist service prescribing your Mounjaro. Your prescriber may adjust your dose schedule or offer additional support. Never stop taking Mounjaro suddenly without speaking to your healthcare team.
- 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.
Are Mounjaro side effects worse than Wegovy side effects?
Both medications cause similar common side effects (nausea, diarrhoea, constipation). Clinical trial data suggests side effect profiles are broadly comparable, though individual experiences vary widely. The main difference is Mounjaro is a dual GIP/GLP-1 agonist (potentially more effective for weight loss) while Wegovy is GLP-1 only. Your GP or prescriber can discuss which is more suitable for your individual health profile.
How long do Mounjaro side effects last?
Most side effects are temporary and improve within 1–4 weeks as your body adjusts to each dose. Some people experience mild nausea or digestive changes throughout treatment, but these often become manageable with dietary adjustments and time. Serious side effects are rare. Always report persistent symptoms to your prescriber.
Can I take over-the-counter medicines to manage Mounjaro side effects?
Speak to your GP or pharmacist before taking any additional medications, including over-the-counter remedies, as some may interact with Mounjaro or affect how well it works. Simple measures like sipping water slowly, eating smaller meals, and avoiding fatty foods often help with nausea and digestive symptoms. Your prescriber may recommend specific support.
Does the NHS prescribe Mounjaro, and will I experience side effects on the NHS?
Yes, NICE has approved tirzepatide (Mounjaro) for use in NHS specialist weight management services (tier 3). Side effect profiles are identical whether prescribed on the NHS or privately—the medication is the same. NHS prescribing requires referral to a specialist service and assessment for eligibility. Waiting times vary significantly by region (ICB).
What should I do if I have a severe allergic reaction to Mounjaro?
Seek immediate emergency care (call 999 or go to A&E) if you develop signs of serious allergic reaction: difficulty breathing, severe swelling of face or throat, or loss of consciousness. This is extremely rare. Always inform healthcare staff that you are taking tirzepatide. After emergency treatment, your prescriber will advise whether you can safely continue the medication.
Ready to take control of your weight loss journey?
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