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 Side Effects — What UK Patients Need to Know
Mounjaro (tirzepatide) is a prescription-only weight loss injection now available through NHS tier 3 specialist services and private prescribing in the UK. Like all medications, it can cause side effects — though many are temporary and mild. This guide explains what side effects to expect, how they're managed, and when to seek medical help. Understanding the full safety profile helps you make an informed decision with your GP or prescriber about whether Mounjaro 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 UK Treatment
The most frequently reported side effects during Mounjaro treatment are gastrointestinal — affecting the stomach and digestive system. Very common effects (occurring in more than 1 in 10 people) include nausea, diarrhoea, vomiting, and constipation. These typically occur during dose escalation phases and often improve as your body adjusts.
- Nausea: Most common initially; usually reduces over time as your body adapts to the dose
- Diarrhoea: Often temporary; drinking plenty of fluids helps
- Stomach pain: Usually mild to moderate; severe pain requires immediate medical attention
- Vomiting: Affects approximately 1 in 10 people; managing food intake can help
- Constipation: Can alternate with diarrhoea; staying hydrated and active helps
Other common effects include indigestion, injection site reactions (redness or bruising), fatigue, dizziness, and hair loss (alopecia). These usually resolve within days or weeks. Your prescriber will discuss management strategies — such as dietary adjustments, staying hydrated, and taking smaller meals — to help minimise discomfort during treatment.
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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
While most side effects are mild, seek immediate medical attention for these warning signs: severe, persistent abdominal pain radiating to the back (possible pancreatitis), serious allergic reactions (swelling of face, lips, or throat), significant changes in vision, or signs of gallbladder problems (pain in upper right abdomen). Additionally, if you have a personal or family history of medullary thyroid cancer or multiple endocrine neoplasia syndrome type 2, Mounjaro is not suitable for you — discuss this with your GP.
Mounjaro carries a Black Triangle (▼) designation from the MHRA, meaning it requires additional monitoring. This doesn't mean the medication is unsafe — it means healthcare professionals actively track safety data. Tell your prescriber about all existing medical conditions, other medications, and any symptoms you experience during treatment.
In the NHS, Mounjaro side effects are managed within tier 3 specialist weight management services. Your specialist team provides ongoing support, including dietary advice, hydration guidance, and dose adjustment if side effects are significant. Many patients find that taking time between dose increases allows their body to adapt, reducing side effect severity.
Use our free BMI calculator above to monitor your starting weight and track progress, and consider using our progress tracker to log any side effects you experience alongside your weight changes. This data helps your prescriber adjust your treatment plan. Always report new or worsening side effects at your next appointment — your medical team can offer practical solutions or modify your dose.
- 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.
What side effects does Mounjaro cause in the UK?
The most common side effects are gastrointestinal — nausea, diarrhoea, vomiting, and constipation affect more than 1 in 10 people. These typically occur during dose titration and often improve with time. Less common effects include injection site reactions, fatigue, and hair loss. Most side effects are temporary and reduce as your body adjusts.
How long do Mounjaro side effects last?
Most side effects resolve within days to weeks. Gastrointestinal effects like nausea typically improve as you progress through dose escalation. If side effects persist beyond 2-3 weeks at the same dose, or worsen significantly, contact your prescriber — they may adjust your dose or recommend management strategies.
When should I seek urgent medical help with Mounjaro?
Seek immediate medical attention for severe abdominal pain radiating to the back (possible pancreatitis), serious allergic reactions (facial swelling, throat tightness), significant vision changes, or signs of gallbladder problems. These are rare but serious. Call 999 or go to A&E if you experience these symptoms.
Can I use other medications while taking Mounjaro?
Tell your prescriber about all medications you take. Mounjaro may affect how your body absorbs oral contraceptives — women using the pill should add a barrier method for 4 weeks after starting and after each dose increase. If you take diabetes medications like insulin or sulphonylureas, your doses may need adjusting.
Is Mounjaro safe long-term in the UK?
Mounjaro is increasingly prescribed through NHS tier 3 services and has a strong safety record from clinical trials. It carries MHRA Black Triangle status, meaning additional safety monitoring occurs — this is routine for newer medications. Long-term safety is supported by ongoing trial data, but your GP or specialist will monitor you regularly.
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.