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 Mounjaro Safe For Weight Loss In The UK?
Mounjaro (tirzepatide) is a prescription-only weight loss injection licensed in the UK for adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions. Both Mounjaro and Wegovy have been assessed for safety by UK medical regulators and have clinical trial evidence supporting their use. However, like all prescription medications, both carry potential side effects. The choice between them depends on individual factors assessed during a clinical consultation with your GP or prescriber. This guide compares safety profiles based on NHS guidance and clinical trial data.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Safe Is Mounjaro Compared To Wegovy?
Both Mounjaro and Wegovy are licensed weight loss injections with established safety profiles in UK clinical practice. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whereas Wegovy (semaglutide) is a GLP-1-only receptor agonist—this mechanism difference does not automatically make one safer than the other.
Clinical trial data shows similar safety profiles: the SURMOUNT-1 trial (Mounjaro) and STEP-1 trial (Wegovy) both reported gastrointestinal side effects as the most common adverse events. The most frequently reported side effects in both trials were nausea, diarrhoea, constipation, and vomiting. Serious adverse events were rare in both trials. A weekly weight loss injection like Mounjaro offers the same dosing convenience as Wegovy, with both using once-weekly administration via KwikPen or auto-injection pen. Your prescriber will assess which medication is more suitable based on your personal health history, other medications, and individual risk factors.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
What Serious Side Effects Should You Know About?
Serious but rare side effects reported in clinical trials of both Mounjaro and Wegovy include pancreatitis (inflammation of the pancreas), gallbladder problems, and severe allergic reactions. Symptoms to watch for include severe stomach pain spreading to the back, pain in the upper right abdomen, or swelling of the face, lips, or throat. Both medications carry a warning about diabetic retinopathy complications in people with type 2 diabetes.
Mounjaro has additional MHRA monitoring (Black Triangle ▼ status) due to being newer to the UK market, meaning any new safety signals are monitored closely. This is a precautionary measure, not an indication of unsafe medication. Both medications require a prescription-only assessment before use. If you experience any concerning symptoms while using either medication, contact your GP or prescriber immediately. Use our free GLP-1 weight loss calculator to understand potential outcomes, and discuss both options with your healthcare provider to determine which is right for you.
- 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 for weight loss in the UK?
Both are licensed and have similar safety profiles based on clinical trials. Neither is definitively "safer"—choice depends on individual health factors assessed by your prescriber. Both are prescription-only medications requiring clinical assessment before use.
What are the most common side effects of Mounjaro vs Wegovy?
Both commonly cause nausea, diarrhoea, constipation, and vomiting—especially at the start of treatment. These side effects typically reduce over time. Serious side effects are rare in both. Always report concerning symptoms to your GP.
Can I switch from Wegovy to Mounjaro if I'm not tolerating it well?
Yes, prescriber-managed switches are possible. Approximate dosing equivalences exist, but any medication change must be managed by your GP or licensed prescriber. Do not switch without medical guidance.
Is Mounjaro approved by NHS NICE guidelines for weight loss in the UK?
NICE has approved tirzepatide (Mounjaro) for NHS specialist weight management services. NHS access varies by region through tier 3 services. Private access is also available through UK-licensed prescribers.
What should I discuss with my GP before starting Mounjaro or Wegovy?
Discuss medical history, current medications, diabetes status, gallbladder or pancreas problems, pregnancy plans, and any allergies. Your GP will assess eligibility using your BMI and health conditions. Use our BMI calculator to check your starting point before your appointment.
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.