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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Understanding Mounjaro Risks and Side Effects
Mounjaro (tirzepatide) is a prescription-only weight loss injection licensed in the UK, but like all medications, it carries potential risks and side effects you should understand before starting. Most side effects are mild and tend to improve over time, but some people experience serious complications. This guide covers what you need to know about Mounjaro risks, how common they are, and when to contact your GP. Always discuss any concerns with your prescriber before beginning treatment.
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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 and Serious Mounjaro Side Effects
Most people taking Mounjaro experience gastrointestinal side effects, especially when starting or increasing doses. Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. These typically decrease as your body adjusts over weeks.
Serious side effects requiring urgent medical attention include severe, persistent abdominal pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, tongue or throat), gallbladder problems, and changes in vision. Inject site reactions like redness, bruising, or pain are common but usually mild. Hair loss has been reported in some users. Less common but important effects include indigestion, headache, dizziness, and fatigue. Use our free BMI calculator to understand your current health baseline before discussing Mounjaro eligibility with your GP.
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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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Who Should Avoid Mounjaro and Risk Factors
Mounjaro is not suitable for everyone. Do not take Mounjaro if you: are under 18, are pregnant or planning pregnancy, are breastfeeding, have a personal or family history of medullary thyroid carcinoma (MTC) or multiple endocrine neoplasia type 2 (MEN2), have severe gastroparesis or digestion problems, or are allergic to tirzepatide.
Mounjaro may worsen certain conditions. If you have a history of pancreatitis, gallbladder disease, kidney problems, diabetic retinopathy, or take diabetes medications like sulphonylureas or insulin, your prescriber needs to monitor you closely. Oral contraceptives may be less effective during treatment — discuss alternative contraception with your prescriber. Weight loss itself can cause dehydration and electrolyte changes, so adequate hydration and nutrition are essential.
Mounjaro vs Wegovy: Mounjaro is a dual GIP/GLP-1 agonist (different mechanism than Wegovy's single GLP-1 action), which may influence side effect profiles. While both carry similar gastrointestinal risks, Mounjaro's dual mechanism may cause different effects in some individuals. Neither medication assures weight loss — results vary significantly. Your prescriber will assess whether this UK prescription medication is suitable for your health profile during clinical assessment. Track your progress and monitor side effects carefully using our free progress tracker to share updates with 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.
What should I do if I experience severe stomach pain on Mounjaro?
Seek urgent medical attention or call 999. Severe abdominal pain spreading to your back may indicate pancreatitis, a serious complication. Do not wait for an appointment — go to your nearest A&E or call 111 immediately.
Are Mounjaro side effects permanent?
No. Most side effects are temporary and improve as your body adjusts over 2–4 weeks. Nausea, diarrhoea, and constipation typically reduce significantly. However, if side effects persist or worsen, contact your prescriber about dose adjustment or alternative options.
Can I take Mounjaro if I have type 2 diabetes?
Yes, but your prescriber must monitor you carefully. If you take insulin or sulphonylureas, your diabetes medication doses may need adjustment because Mounjaro affects blood sugar. Never stop diabetes medication without guidance — always inform your prescriber of all medications.
Does Mounjaro affect contraception?
Yes. Mounjaro may reduce how well oral contraceptives (the pill) work by slowing stomach emptying and affecting medication absorption. Add a barrier method (condoms) or switch to a non-oral contraceptive for 4 weeks after starting and after each dose increase.
What's the difference between Mounjaro and Wegovy in terms of risks?
Both are GLP-1-based medications with similar gastrointestinal side effects. Mounjaro is a dual GIP/GLP-1 agonist (different to Wegovy's single GLP-1 mechanism), so individual risk profiles may vary. Your prescriber can advise which is safer for your health history.
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.