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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How Does Mounjaro Work — NHS Guide to Tirzepatide
Mounjaro (tirzepatide) is a prescription-only weight loss injection that works differently from other weight management medications available in the UK. It activates two natural hormone pathways — GLP-1 and GIP — which is why it's classified as a dual receptor agonist. This dual action helps regulate appetite, reduces cravings, and supports feeling fuller after smaller meals. Understanding how Mounjaro works can help you discuss treatment options with your GP or specialist if you're eligible for prescription weight loss support.
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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.
How Mounjaro's Dual Mechanism Works in Your Body
Mounjaro targets two hormone pathways, making it unique among weight loss injections available on the NHS and privately in the UK. First, it activates GLP-1 receptors — the same pathway targeted by Wegovy (semaglutide) — which signals to your brain that you're full and reduces hunger signals. Second, it activates GIP receptors (glucose-dependent insulinotropic polypeptide), a hormone that regulates appetite and fat storage independently.
This dual action means Mounjaro may help in multiple ways: reducing appetite, lowering food cravings, slowing how quickly food leaves your stomach, and supporting better blood sugar regulation. Clinical trials showed an average weight loss of 15–21% of body weight at the highest dose, though individual results vary significantly. The medication is given as a once-weekly injection using a KwikPen device — easier than daily injections. Mounjaro is not insulin and works by amplifying natural hormones your body already produces.
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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.
NHS Eligibility and UK Prescribing Pathways
In the UK, Mounjaro is available through NHS specialist weight management services (tier 3) and through private online prescribing following a clinical assessment. NHS access requires a referral from your GP to a specialist multidisciplinary weight management service — availability varies significantly by region. You may be eligible if you have a BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea.
Private prescription routes are available without NHS referral through GPhC-registered online prescribers. NICE has approved tirzepatide for use within NHS services, though NHS waiting lists can be long in many areas. Your GP can discuss which route is most appropriate for your situation and assess whether Mounjaro is suitable based on your individual health profile and medical history.
- 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 the same as a GLP-1 injection like Wegovy?
No. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 only agonist. Mounjaro targets two hormone pathways, which may explain why clinical trial data showed slightly higher average weight loss. Both are prescription-only weekly injections, but they work through different mechanisms.
How quickly does Mounjaro start working?
Most people notice reduced appetite within the first few days to weeks, but significant weight loss typically takes 4–12 weeks to become apparent. Doses are increased gradually every 4 weeks, so it can take 3–4 months to reach an effective maintenance dose. Weight loss is gradual and sustainable — the NHS recommends losing 0.5–1 kg per week.
Will I need to take Mounjaro forever?
This is a discussion to have with your prescriber. Some people maintain weight loss after stopping; others regain weight. Mounjaro works while you're taking it alongside lifestyle changes. Long-term use depends on your individual circumstances, health goals, and how your body responds.
Can I get Mounjaro on the NHS in the UK?
Yes, through specialist tier 3 weight management services following a GP referral. However, NHS availability varies by region and waiting lists can be very long. Private prescription is also available through online assessment with a UK-licensed prescriber, typically at a monthly cost between £150–£350 depending on dose.
What happens if I miss a dose of Mounjaro?
If you remember within 4 days, take it as soon as possible and continue your normal weekly schedule. If it's more than 4 days late, skip that dose and take the next one on your scheduled day. Never take a double dose. Always maintain at least 72 hours between injections.
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.