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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What To Take: Mounjaro vs Wegovy Explained
If your GP or prescriber has suggested weight loss medication, you might be wondering whether to take Mounjaro or Wegovy—and what the real differences are. Both are prescription-only injections licensed in the UK for weight management, but they work slightly differently and have different dosing schedules. This guide explains the key differences so you can have an informed discussion with your prescriber about which option might suit you best.
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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 Work: The Key Difference
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways in the body. This dual mechanism may help regulate appetite and support feeling fuller after smaller meals. Wegovy (semaglutide), by contrast, is a single GLP-1 receptor agonist—it targets just the GLP-1 pathway. Both medications slow gastric emptying and may reduce food cravings, but the dual action of Mounjaro is the core mechanistic difference.
In clinical trials, this difference translated into slightly different weight loss results. The SURMOUNT-1 trial for Mounjaro showed an average weight loss of approximately 15–21% of body weight, depending on dose. The STEP-1 trial for Wegovy showed an average weight loss of approximately 14.9% at the maximum dose. However, individual results vary significantly—some people respond better to one medication than another, and there is no reliable way to predict personal outcomes before starting.
Your prescriber will consider both options during your clinical assessment, based on your individual health profile, medical history, and preferences.
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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.
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Dosing, Access, and How Your Prescriber Decides
Mounjaro uses a once-weekly injection schedule with doses ranging from 2.5 mg to 15 mg maximum. Wegovy is also once-weekly, but starts at 0.25 mg and escalates to a maximum of 2.4 mg. Both require a Mounjaro KwikPen or pre-filled pen respectively, and both are available through NHS specialist weight management services or private prescribing with a UK-licensed prescriber.
Your GP or prescriber will assess your eligibility based on your BMI (typically 30 or above, or 27+ with weight-related health conditions), medical history, current medications, and contraindications. Neither medication is suitable during pregnancy or breastfeeding. Your prescriber will explain which option they recommend and why—based on clinical evidence, your personal health profile, and practical factors like cost and local availability. Use our free GLP-1 weight loss calculator to explore potential projected outcomes with either medication.
Both medications are prescription-only and available in the UK following a clinical assessment by a qualified prescriber. Neither assures specific weight loss—results depend on your body, adherence, diet, and activity level. Some people switch between medications if one doesn't suit them or loses effectiveness over time. This is a prescriber decision and should not be attempted without professional guidance. Your progress tracker can help you log and monitor any changes over time, alongside your prescriber's monitoring.
- 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.
Can I switch from Mounjaro to Wegovy?
Potentially yes, but only under prescriber guidance. There is no official conversion chart between the medications—your prescriber will assess whether a switch is appropriate based on your response, side effects, and medical circumstances. Never switch without professional advice.
Which one is available on the NHS?
Both Mounjaro and Wegovy are NHS-licensed for weight management. However, NHS access depends on your local integrated care board (ICB) and specialist tier 3 weight management service availability. Waiting lists vary significantly by region. Private prescribing is available through GPhC-registered providers.
Is one medication more effective than the other?
Clinical trials show similar average weight loss results, though Mounjaro's dual mechanism may produce slightly higher averages in some populations. However, individual response varies unpredictably—some people lose more weight on Wegovy, others on Mounjaro. Your prescriber's recommendation is based on your personal health profile, not generic trial data.
What if I'm already taking other medications?
Both Mounjaro and Wegovy can interact with other medicines—particularly oral contraceptives and diabetes medications. You must disclose all current medications during your clinical assessment. Your prescriber will advise whether either medication is safe for you and whether dose adjustments to other medicines are needed.
How long do I stay on these medications?
Duration depends on your response, side effects, and clinical judgment. Some people use them short-term to establish healthy habits; others continue longer-term. This is a prescriber decision reviewed regularly during your treatment. Stopping suddenly is not recommended without guidance.
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.