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
How Does Mounjaro Work in Your Body?
Mounjaro (tirzepatide) is a prescription-only injection that works differently from other weight loss medications available in the UK. Instead of targeting just one hormone pathway, it activates two natural gut hormones — GIP and GLP-1 — which work together to regulate appetite, reduce cravings, and help you feel fuller after eating smaller meals. This dual mechanism is what makes Mounjaro unique among NHS-approved weight management options. Understanding how it works can help you decide if it's right for you alongside lifestyle changes.
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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 the Dual GIP and GLP-1 Mechanism Works
Mounjaro's dual action targets two separate hormone pathways in your body. GLP-1 (glucagon-like peptide-1) signals to your brain that you're full, slowing gastric emptying so food leaves your stomach more gradually. GIP (glucose-dependent insulinotropic polypeptide) works alongside it to regulate appetite and support healthy blood sugar control. Together, these hormones help reduce food cravings and support feeling satisfied with smaller portions.
The Mounjaro KwikPen delivers tirzepatide once weekly via subcutaneous injection. You start on 2.5 mg weekly, then increase gradually every 4 weeks — typically reaching 10 mg or 15 mg as your maintenance dose. Clinical trials showed participants lost an average of 15–21% of their body weight using Mounjaro, though individual results vary significantly.
- Reduces appetite and food cravings through dual hormone activation
- Slows gastric emptying so you feel fuller longer
- Helps regulate blood sugar and fat metabolism
- Once-weekly injection — easier dosing than daily alternatives
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.
Mounjaro on the NHS: Eligibility and Access in the UK
NICE approved tirzepatide (Mounjaro) for use within NHS specialist weight management services. To access it through the NHS, you'll typically need referral to a tier 3 specialist service from your GP — standard GPs cannot prescribe it routinely. Eligibility criteria include a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea. South Asian, Black African, or Black Caribbean adults may qualify at lower BMI thresholds due to higher health risks at the same weight.
NHS waiting lists vary significantly by region. Some specialist centres have waiting lists of several months. If NHS access is delayed, prescription Mounjaro is available privately through UK-licensed prescribers following an online clinical assessment. Use our free GLP-1 weight loss calculator to explore potential outcomes, and our BMI calculator to check your starting point.
- 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's the difference between Mounjaro and Wegovy?
Both are weight loss injections approved in the UK, but Mounjaro uses a dual GIP+GLP-1 mechanism while Wegovy (semaglutide) is GLP-1 only. Mounjaro starts at 2.5 mg and increases to 15 mg maximum; Wegovy starts at 0.25 mg and reaches 2.4 mg. Clinical trials show similar weight loss outcomes (15–21%), though individual results vary. Your prescriber will assess which suits you best.
Does Mounjaro work immediately or take time?
Mounjaro takes time to work. Most people notice appetite reduction within 2–4 weeks, but full effects develop over 12–16 weeks as doses increase. You stay on each dose for 4 weeks minimum before escalating. Consistent weight loss typically becomes visible after 8–12 weeks on the medication, alongside reduced-calorie diet and increased physical activity.
Does Mounjaro work without diet and exercise changes?
Clinical trials showed Mounjaro works best alongside a reduced-calorie diet and increased physical activity. The medication supports appetite control, but you still need to make healthier food choices and move more regularly. NHS specialist services always include dietary and lifestyle support alongside prescription medication.
Can Mounjaro stop working over time?
Some people experience reduced appetite suppression after months on the medication — this is called tachyphylaxis. If this happens, your prescriber may adjust your dose or explore alternative options. Most people maintain weight loss if they continue the medication and healthy lifestyle habits. Stopping Mounjaro typically leads to weight regain over time.
How do GIP and GLP-1 hormones help weight loss?
GLP-1 signals fullness to your brain and slows stomach emptying, reducing hunger. GIP helps regulate blood sugar and fat metabolism. Together, they reduce food cravings, support eating smaller portions, and help your body use energy more efficiently. This dual action is why Mounjaro may produce slightly greater weight loss than single-action GLP-1 medications in some people.
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.