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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Is Mounjaro Safe? What You Need to Know
Mounjaro (tirzepatide) is a prescription-only weight management injection that has been approved by the MHRA and assessed by NICE for use in the UK. Like all medications, Mounjaro carries both benefits and potential side effects. Clinical trials have shown it to be effective for weight management, but safety depends on individual health factors, proper medical assessment, and following your prescriber's guidance. This guide explains what the evidence shows about Mounjaro's safety profile so you can make an informed decision with your GP or healthcare professional.
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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.
What Clinical Trials Show About Mounjaro Safety
The SURMOUNT-1 clinical trial, which tested tirzepatide across 72 weeks, provides the most robust safety data for Mounjaro. In this trial, participants experienced side effects at different rates depending on the dose used. The most common side effects were nausea, diarrhoea, vomiting, and constipation — affecting many participants in the early weeks but often improving as the body adjusted. These gastrointestinal effects are typical of GLP-1 and GIP/GLP-1 medications. Serious adverse events were rare and similar between the Mounjaro group and the control group. The MHRA, which regulates all medicines in the UK, continues to monitor Mounjaro's safety through its Black Triangle (▼) programme, meaning additional safety monitoring is in place. This close oversight means any emerging safety signals are quickly identified and investigated.
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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 Not Take Mounjaro and When to Seek Help
Mounjaro is not suitable for pregnant women, those planning to become pregnant, or people breastfeeding. You cannot take Mounjaro if you have a personal or family history of medullary thyroid cancer or multiple endocrine neoplasia type 2 (MEN2). Tell your prescriber about all medications you take, as Mounjaro may affect how oral medicines are absorbed. Seek urgent medical attention if you experience severe stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face or throat), changes in vision, or gallbladder problems. Most side effects are mild to moderate and resolve with time, but your prescriber should monitor you throughout treatment. Use our free BMI calculator and weight converter to track your health metrics alongside any treatment plan.
- 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.
How common are side effects with Mounjaro?
Gastrointestinal side effects like nausea are very common (affecting more than 1 in 10 people), especially in the first weeks. Most people adapt as their body adjusts to the injection. Serious side effects are uncommon. Your prescriber will start you on the lowest dose (2.5 mg) and increase gradually to minimise side effects.
Is Mounjaro safe for people with type 2 diabetes?
Yes, but prescribers must adjust other diabetes medications if needed. If you take insulin or sulphonylureas, your dose may need to be reduced when starting Mounjaro because your blood sugar management improves. Always inform your prescriber of all your medications.
Can you stop Mounjaro suddenly?
Stopping Mounjaro suddenly is safe in terms of dangerous withdrawal effects, but weight may return quickly if lifestyle changes aren't maintained. It's best to discuss stopping with your prescriber, who can provide guidance on transitioning off the medication sustainably.
Does Mounjaro affect contraception?
Yes. The Mounjaro package insert states that oral contraceptives may have reduced effectiveness when starting Mounjaro or increasing doses. Women should add a barrier method or switch to a non-oral contraceptive for 4 weeks after starting and 4 weeks after each dose increase.
How is Mounjaro safety monitored in the UK?
The MHRA monitors all medicines through pharmacovigilance systems. Mounjaro has additional Black Triangle (▼) monitoring, meaning healthcare professionals and patients can report side effects directly. NICE also reviewed the evidence before recommending it for NHS use in specialist weight management services.
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.