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
Free tracker included with every download this week
What Are the Dangers and Risks of Mounjaro?
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK, but like all medications, it carries potential risks and side effects. Understanding these dangers is essential before starting treatment. Most side effects are mild and manageable, but some serious complications can occur. Your GP or prescriber will assess whether Mounjaro is safe and appropriate for you based on your individual health profile and medical history.
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Calculate Your BMI Instantly
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 Side Effects of Mounjaro
Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. These typically occur during dose escalation and often improve over time as your body adjusts to the tirzepatide injection. Nausea is the most frequently reported side effect in clinical trials.
Serious side effects requiring urgent medical attention include severe, persistent stomach pain radiating to the back (possible pancreatitis), serious allergic reactions (swelling of face, lips, or throat), sudden vision changes, rapid or irregular heartbeat, and signs of gallbladder problems. If you experience any of these, contact your GP or NHS 111 immediately. Less common but significant risks include thyroid tumours (based on animal studies — human risk unknown), and complications if you have a personal or family history of medullary thyroid cancer or multiple endocrine neoplasia syndrome type 2 (MEN2).
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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.
Who Should Avoid Mounjaro?
Mounjaro is not suitable for everyone. You cannot take tirzepatide if you are under 18, pregnant, breastfeeding, or allergic to the active ingredient. People with severe gastroparesis or serious digestion problems should avoid it. Those with a personal or family history of medullary thyroid cancer or MEN2 syndrome should not use Mounjaro due to theoretical thyroid risks. If you have diabetic retinopathy (eye complications from diabetes), your prescriber must monitor you closely as gradual weight management can occasionally worsen this condition. Always disclose your complete medical history, including all medications and supplements, to your prescriber before starting 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.
Is Mounjaro safer than Wegovy?
Both are prescription-only GLP-1/GIP medications with similar safety profiles. Mounjaro uses a dual GIP+GLP-1 mechanism while Wegovy (semaglutide) is GLP-1 only. Clinical trials show comparable side effect rates. Safety depends on individual health factors — your prescriber will assess which is more appropriate for you.
Can Mounjaro cause pancreatitis?
Acute pancreatitis is a rare but serious side effect. Severe, persistent stomach pain radiating to your back is a warning sign requiring immediate medical attention. Contact NHS 111 or your GP urgently if you experience this. The risk is low but real — previous pancreatitis history may be a contraindication.
Does Mounjaro affect thyroid health?
Animal studies showed tirzepatide may increase thyroid tumour risk at very high doses. The human risk is unknown. If you have thyroid cancer history or MEN2 syndrome, you cannot take Mounjaro. If you have other thyroid conditions, your prescriber will monitor you. Report any neck lumps or swallowing difficulties.
What happens if I miss a Mounjaro dose?
If you remember within 4 days, take it as soon as possible and continue your normal schedule. If more than 4 days have passed, skip that dose and take your next injection on the regular day. Never double-dose. Consistent dosing is important for safety and effectiveness.
Can I use Mounjaro with other medications?
Mounjaro may reduce oral medication absorption, including oral contraceptives — add barrier contraception for 4 weeks after starting and after each dose increase. Diabetes medications like insulin or sulphonylureas may need adjustment. Always tell your prescriber about all medications you take before starting tirzepatide.
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.