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
What is Mounjaro? The Active Ingredient Explained
Mounjaro is the brand name for tirzepatide, a prescription-only weight management injection licensed in the UK. While Mounjaro is the commercial name you'll see advertised, the actual medication inside the Mounjaro KwikPen is tirzepatide — a dual GIP and GLP-1 receptor agonist. Understanding both names helps you navigate prescriptions, clinical information, and discussions with your healthcare provider. This guide explains what tirzepatide is, how it works, and how to access it in the UK through NHS or private routes.
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 Tirzepatide Works and Why It's Different
Tirzepatide is a dual-action medication, which sets it apart from single-mechanism weight loss injections like semaglutide (Wegovy). It activates two natural hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This combination may help regulate appetite more effectively by signalling to your brain that you're full, reducing food cravings, and supporting feeling satisfied after smaller meals. Tirzepatide also slows gastric emptying — how quickly food leaves your stomach — though this effect typically reduces over time as your body adjusts.
Clinical trials (the SURMOUNT studies) showed that tirzepatide helped participants lose an average of 15–21% of their body weight over 72 weeks, though individual results vary significantly. The medication is administered as a once-weekly injection via the Mounjaro KwikPen. Doses range from 2.5 mg (starting dose) to 15 mg (maximum dose), with increases every 4 weeks based on how well you tolerate it and your response.
Use our free GLP-1 weight loss calculator to explore projected weight loss outcomes at different doses.
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.
Accessing Tirzepatide in the UK: NHS vs Private Routes
NHS Access: Tirzepatide (Mounjaro) is approved by NICE for use within NHS specialist weight management services. However, access is through tier 3/4 specialist services, not standard GP prescription. You'll need a referral from your GP, and availability varies significantly across different regions and integrated care boards. Waiting lists in many areas are several months to over a year.
Private Access: You can access tirzepatide privately through a UK-licensed prescriber via online consultation. After clinical assessment, if appropriate, a prescription is issued and dispensed by a GPhC-registered pharmacy. Costs vary by dose but typically range from £129–£320 per month. This route is faster — often accessible within days — and doesn't require NHS referral. Private prescribers assess your BMI, health history, and eligibility independently.
Check your personal BMI using our free BMI calculator to understand your starting point for eligibility discussions.
- 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 tirzepatide the same as Mounjaro?
Yes. Mounjaro is the brand name; tirzepatide is the active ingredient inside the Mounjaro KwikPen. When you're prescribed Mounjaro, you're receiving tirzepatide 2.5 mg to 15 mg depending on your dose. Other brand names may exist in different countries, but in the UK, tirzepatide for weight management is branded as Mounjaro.
How does tirzepatide differ from semaglutide (Wegovy)?
Tirzepatide is a dual GIP+GLP-1 agonist; semaglutide is a GLP-1 agonist only. Clinical trials suggest tirzepatide may produce slightly greater average weight loss (15–21% vs ~15% with semaglutide), but individual results vary. Both are once-weekly injections. Your prescriber will assess which is more suitable for you based on your health profile.
How long does tirzepatide stay in your system?
Tirzepatide is typically detected in your bloodstream for about 5 days after injection, though some effects on appetite and blood sugar may persist slightly longer. Since it's a once-weekly injection, the next dose is given before it fully clears. If you stop taking it, appetite suppression effects usually fade within 1–2 weeks.
Can I get tirzepatide on the NHS quickly?
NHS access depends on your region and tier 3 service capacity. Many areas have waiting lists of 6–12 months or longer. Private prescription routes are typically faster — often within days or weeks. If speed is important, discuss private options with a UK prescriber.
What happens if tirzepatide doesn't work for me?
If tirzepatide isn't producing expected results after 12 weeks on the target dose, your prescriber may investigate underlying factors (diet, activity, medication interactions) or explore alternative medications like semaglutide. Weight loss response varies; some people respond very well, others modestly. Your prescriber will personalise your treatment plan.
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.