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
Are Mounjaro Injections Safe?
Yes, Mounjaro (tirzepatide) is a prescription-only medicine approved by the MHRA for weight management in UK adults. Like all medications, it has known side effects, but clinical trials including the SURMOUNT study demonstrated its safety profile when used as directed. However, safety depends on proper medical assessment, correct dose titration, and individual suitability. Your GP or prescriber will evaluate whether Mounjaro is safe and appropriate for you based on your health history and current medications.
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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.
Safety Profile — SURMOUNT Trial Data and Mechanism
Mounjaro's safety was established through the SURMOUNT-1 clinical trial, which involved over 2,500 participants across 72 weeks. The trial showed that tirzepatide (Mounjaro's active ingredient) was well-tolerated, with side effects generally mild to moderate and manageable through dose titration — the process of gradually increasing your dose over 16–20 weeks. Most side effects, particularly nausea and gastrointestinal symptoms, tend to improve over time as your body adjusts to the medication.
As a dual GIP and GLP-1 receptor agonist, Mounjaro works differently from other weight loss injections like Wegovy. This dual mechanism means it activates two natural hormone pathways simultaneously, which may be why some participants experienced greater weight loss in trials. However, the dual action also means the potential side effect profile differs slightly from single-mechanism GLP-1 agonists. Both approaches are safe when prescribed appropriately — the choice between them depends on individual factors your prescriber will assess.
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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.
Comparing Mounjaro and Wegovy Safety
Both Mounjaro and Wegovy are prescription-only, MHRA-approved weight management injections with established safety records. Wegovy (semaglutide) is a GLP-1-only agonist tested in the STEP clinical trials, whilst Mounjaro uses the dual GIP+GLP-1 approach. Side effect profiles are similar — nausea, diarrhoea, constipation, and vomiting are most common — but frequency and severity can differ between individuals and medications.
Neither injection is suitable for everyone. Contraindications include pregnancy, personal or family history of medullary thyroid cancer, type 1 diabetes, and certain gastrointestinal conditions. Your prescriber will ask detailed questions about your medical history and current medications. Whether Mounjaro or Wegovy is safer for you is a personalised decision made during clinical assessment — there is no blanket "safer" option.
- 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 are the most common side effects of Mounjaro injections?
The most common side effects are nausea, diarrhoea, vomiting, and constipation. These affect more than 1 in 10 people but typically improve within the first few weeks as your body adjusts. Injection site reactions (redness, bruising, mild pain) are also common. Taking the injection at the same time each week and eating smaller, lower-fat meals can help minimise symptoms.
Is Mounjaro safer than Wegovy?
Both are prescription-only MHRA-approved medications with established safety profiles from rigorous clinical trials. Mounjaro uses a dual GIP+GLP-1 mechanism; Wegovy is GLP-1-only. Neither is objectively "safer" — safety is individual. Your prescriber will assess which is more appropriate based on your health history, other medications, and personal factors during your clinical assessment.
What serious side effects should I watch for with Mounjaro?
Seek urgent medical attention if you experience severe stomach pain (possible pancreatitis), persistent vomiting, changes in vision, signs of thyroid problems (lump in neck, difficulty swallowing), or serious allergic reactions (face swelling, breathing difficulty). These are rare but require immediate evaluation. Report any new or worsening symptoms to your prescriber.
Can I use Mounjaro if I take other medications?
Mounjaro can interact with certain medications, particularly oral contraceptives and diabetes medications. Oral contraceptive users should add barrier contraception for 4 weeks after starting and after each dose increase. If you take diabetes medication, your prescriber may adjust doses. Always inform your prescriber of all medications — they'll assess safety individually.
Is Mounjaro safe if I have type 2 diabetes?
Mounjaro can be used by people with type 2 diabetes for weight management, but your prescriber will need to review your diabetes medications carefully. Tirzepatide improves blood sugar control, so doses of insulin or other diabetes drugs may need reducing to prevent low blood sugar. This requires specialist oversight — never adjust diabetes medications yourself.
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.