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
When to Start Mounjaro — NHS and Clinical Guidance
There's no "best" calendar day to start Mounjaro — the timing depends on your individual circumstances, NHS assessment, and readiness for the treatment plan. Once prescribed by a specialist or licensed prescriber following clinical assessment, you can start your first 2.5 mg dose on any day that works for you. What matters more is that you understand the four-week dose titration schedule, have realistic expectations about side effects and results, and are committed to combining the injection with lifestyle changes. This guide explains NHS eligibility, what to expect in your first weeks, and how to prepare for starting this tirzepatide prescription medication in the UK.
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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.
NHS Access and Starting Mounjaro in the UK
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK. Most Mounjaro prescriptions come through one of two routes: NHS specialist tier 3 weight management services, or private online prescribing following a clinical assessment. Access via the NHS typically requires a referral from your GP to a specialist weight management centre, though availability varies significantly by region and waiting lists can be lengthy. If prescribed, your first dose will be 2.5 mg weekly for four weeks, then gradually increased every 4 weeks to allow your body to adjust. The specific "day" you start depends on when you receive your KwikPen and your prescriber's instructions, but many patients choose a day they'll easily remember (e.g. Monday mornings) to maintain the weekly routine. Starting Mounjaro is not time-sensitive by calendar — starting on a Tuesday versus a Friday makes no clinical difference — but consistency matters once you begin.
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.
Preparing for Your First Mounjaro Injection
Before starting your first dose, ensure you understand the four-week dose titration schedule: week 1–4 at 2.5 mg, week 5–8 at 5 mg, and further increases if tolerated. Check your refrigeration setup (unopened pens require 2–8°C storage) and familiarise yourself with injection technique using the patient information leaflet or instructional video. Discuss any current medications with your prescriber, as tirzepatide may affect oral medication absorption, and inform them of any history of pancreatitis, gallbladder problems, or thyroid issues. Have realistic expectations: clinical trials showed average weight loss of 15–21% of body weight over 72 weeks alongside reduced-calorie diet and physical activity — individual results vary significantly. Use our free weight loss calculator to explore potential outcomes at different doses, and track your progress using our progress tracker once you start.
- 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 there a medically "best" day of the week to start Mounjaro?
No — once prescribed, you can start your first 2.5 mg dose any day that works for you. What matters is choosing a day you'll remember for consistency (e.g. every Monday), as you'll need to inject the same day each week. The clinical effect is the same regardless of which day you choose. Your prescriber will simply advise you to start once you've received your KwikPen and understand injection technique.
How long after being prescribed will I start my first Mounjaro dose?
This depends on your prescription route. Private prescriptions typically dispense within 2–5 business days. NHS prescriptions may take longer depending on your local tier 3 service's supply arrangements. Once you have your KwikPen in hand and have received instruction, you can start immediately — you don't need to wait for a specific date.
What if I miss my first Mounjaro injection?
Don't worry — if you miss your first dose by a few days, simply inject as soon as you remember and return to your normal weekly schedule. However, try to maintain a consistent day each week. Never double-dose to catch up. If you have questions about timing, contact your prescriber or pharmacist before injecting.
Can I start Mounjaro while on other medications?
You can start Mounjaro while taking most other medications, but inform your prescriber of everything you take. Tirzepatide may reduce the absorption of oral medications, particularly oral contraceptives — women should use a barrier method alongside or switch to non-oral contraception for 4 weeks after starting and after each dose increase. Diabetes medications may need adjustment.
Should I start Mounjaro if I have side effect concerns?
Starting Mounjaro does carry side effect risks — the most common are nausea, diarrhoea, and stomach pain, particularly in the first 1–2 weeks. These often settle as your body adjusts. Discuss any health concerns with your prescriber before starting. Most side effects are manageable, and your dose won't be increased until you've tolerated the current dose for four weeks, allowing your body time to adapt.
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.