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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Monjaro Side Effects — What UK Patients Should Know
Monjaro (tirzepatide) is a prescription-only weekly weight loss injection now available through NHS specialist services and private prescribing in the UK. Like all medications, it can cause side effects—some common and manageable, others requiring medical attention. Understanding what to expect helps you make an informed decision about whether this GLP-1 receptor agonist and GIP receptor agonist medication is right for you. This guide covers the side effects reported by UK patients, how they're managed, and when to contact your GP.
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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 Monjaro in the UK
Clinical trial data and NHS prescribing experience show that nausea, diarrhoea, constipation, and stomach pain are the most common side effects of Monjaro. These typically occur during the first few weeks as your body adjusts to the medication, and often improve over time.
- Nausea: Reported by over 40% of patients in trials. Usually mild to moderate and decreases as treatment continues.
- Diarrhoea and constipation: Gastrointestinal changes are common. Staying hydrated and adjusting diet can help.
- Stomach pain: Often described as mild abdominal discomfort rather than severe pain.
- Vomiting: Less common than nausea but may occur, especially in first 4 weeks.
- Appetite loss: Expected and intended—the medication works partly by reducing hunger signals.
- Injection site reactions: Mild redness, bruising, or pain at injection sites are typical.
Most patients find these effects manageable without stopping treatment. Your prescriber can advise on dietary adjustments or temporary dose pauses if needed.
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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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When to Seek Medical Attention — Serious Side Effects
While most side effects are mild, contact your GP immediately if you experience severe or persistent symptoms. Serious side effects requiring urgent medical evaluation include:
- Severe abdominal pain spreading to your back: May indicate pancreatitis, a serious pancreas inflammation.
- Signs of allergic reaction: Swelling of face, lips, tongue, or throat; difficulty breathing.
- Gallbladder problems: Pain in upper right stomach area, yellowing of skin or eyes.
- Significant vision changes: Particularly important if you have diabetes.
- Rapid heart rate or chest pain: Seek emergency care.
The NHS encourages all patients on Monjaro to discuss any concerning symptoms with their GP or prescriber promptly. Your MHRA-monitored tirzepatide prescription includes regular safety check-ins.
- 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 Monjaro side effects worse at higher doses?
Side effects often increase with dose escalation—this is why doses start at 2.5mg and increase gradually every 4 weeks. Most patients tolerate 5mg well; higher doses (10–15mg) may have more side effects. Your prescriber monitors this and may slow escalation if needed.
How long do Monjaro side effects last?
Most common side effects peak in weeks 1–4 of each new dose, then improve as your body adapts. By week 8–12, many patients report significantly fewer symptoms. Persistent nausea or stomach issues should be discussed with your GP.
Can I take anti-nausea medication while on Monjaro?
Some anti-nausea medications can reduce Monjaro's effectiveness or interact with it. Always inform your prescriber about any other medications. Your GP can recommend safe options if needed.
What's the difference between Monjaro and Wegovy side effects?
Monjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Side effect profiles are similar, though individual tolerance varies. Both cause common gastrointestinal effects.
Should I stop Monjaro if side effects are bad?
Never stop prescription medication without medical guidance. Contact your GP or prescriber first—they may recommend a temporary pause, slower dose escalation, or dietary changes. Many side effects improve with time and supportive management.
Ready to take control of your weight loss journey?
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