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
Is The Mounjaro Jab Safe — What Clinical Evidence Shows
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections approved and licensed in the UK for safe use. Both have been extensively studied in clinical trials and show similar safety profiles overall, though they work slightly differently. The choice between them depends on your individual health profile and how your body responds — a prescriber will assess which is more suitable for you. Both carry risks and benefits that need careful consideration.
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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.
How Mounjaro and Wegovy Compare — Clinical Trial Safety Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist tested in the SURMOUNT-1 clinical trial. Wegovy (semaglutide) is a single GLP-1 receptor agonist tested in the STEP-1 trial. Both showed similar rates of gastrointestinal side effects — nausea, diarrhoea, constipation, and vomiting — with severity typically decreasing over time as your body adjusts.
- Mounjaro: Most common side effects affected more than 1 in 10 participants; serious side effects were rare
- Wegovy: Side effects followed a similar pattern; both drugs carry the same absolute contraindications (pregnancy, breastfeeding, personal or family history of thyroid cancer)
Neither medication is superior in safety terms — both require careful dose titration and medical monitoring. The SURMOUNT and STEP trials showed comparable tolerability profiles, meaning most people adjust to either medication without serious harm.
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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.
Which Is Right For You — Individual Suitability Matters
Safety is not just about side effects — it's also about whether the medication suits your individual health profile. Your prescriber will assess your medical history, current medications (especially oral contraceptives, which Mounjaro may affect), and any conditions like gastroparesis or pancreatitis risk. Both Mounjaro and Wegovy require a clinical assessment before prescription — this is your protection.
Use our free GLP-1 weight loss calculator to explore projected outcomes with either medication, and discuss your specific safety concerns with your prescriber. They will recommend the option best suited to your circumstances, not just general safety data.
- 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?
No — both are similarly safe based on clinical trial data. The SURMOUNT and STEP trials show comparable side effect rates and safety profiles. Safety depends more on whether the medication suits your individual health profile, which your prescriber will assess.
What are the most serious side effects of Mounjaro?
Serious but rare side effects include severe stomach pain (possible pancreatitis), gallbladder problems, and allergic reactions. Most side effects are gastrointestinal (nausea, diarrhoea) and typically mild to moderate, improving over weeks as your body adjusts.
Can I switch from Mounjaro to Wegovy if I don't feel safe?
Yes — if you experience intolerable side effects or have concerns, discuss switching with your prescriber. Any medication change must be managed medically. There is no standard conversion formula, so your prescriber will advise on safe transition.
Does the Mounjaro KwikPen injection itself pose safety risks?
The KwikPen injection device is designed for safe self-injection with a very short needle. Injection site reactions (mild redness or bruising) occur in some users but are usually minor. Rotate injection sites weekly to minimise this risk.
Who should NOT use Mounjaro or Wegovy?
Both are contraindicated if you're pregnant, breastfeeding, have a personal or family history of thyroid cancer, severe gastroparesis, or previous pancreatitis. Your GP or prescriber will confirm eligibility during clinical assessment.
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.