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 to Expect During Your First Week on Mounjaro
Starting Mounjaro (tirzepatide) is an important health decision. Your first week involves learning how to use the KwikPen, managing the starting dose of 2.5 mg, and monitoring how your body responds. Most people experience mild nausea or appetite changes in the first few days, which typically improve as your body adjusts. This guide walks you through what to realistically expect during week one, how to prepare, and when to contact your GP if you have concerns.
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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.
What Happens During Your First Mounjaro Injection
Your first Mounjaro dose is 2.5 mg, delivered via a KwikPen auto-injection device. The pen contains four weekly doses, so you'll have four weeks of treatment from one device. Before your first injection, you'll have received training from the prescriber or pharmacist on how to use the pen correctly—usually covering injection technique, storage, and disposal. The injection itself takes just a few seconds and goes into the stomach (abdomen), upper thigh, or upper arm. You'll rotate injection sites each week to minimise bruising or irritation. Most people report that the injection causes little to no pain, though you may experience mild stinging or redness at the injection site that resolves within a few hours.
During the first 24–48 hours, nausea is the most common experience—affecting roughly 40% of people starting Mounjaro. You may also notice reduced appetite, mild stomach discomfort, or slight dizziness. These effects are temporary and reflect how Mounjaro works: it's signalling to your brain that you're full and slowing how quickly food moves through your stomach. Most side effects peak around day 2–3 and improve significantly by days 5–7 as your body adjusts to the medication.
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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.
Managing Your First Week—Practical Steps
During week one, eat small, frequent meals of bland, easy-to-digest foods—think toast, rice, chicken, and vegetables rather than heavy or fatty foods, which can worsen nausea. Stay well hydrated; dehydration makes side effects worse. Keep ginger tea or peppermint on hand—both are evidence-backed for nausea relief. Avoid driving or operating machinery on days 2–3 if you experience dizziness. Most importantly, keep a simple food and side-effect diary. Note what you eat, how you feel, and any nausea severity. This information is valuable for your GP or prescriber at your follow-up appointment. If you experience severe stomach pain, persistent vomiting, or signs of allergic reaction (swelling of face, lips, or throat), seek medical advice immediately. Minor nausea and reduced appetite are expected; severe symptoms are not.
- 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.
Will I experience side effects in my first week on Mounjaro?
Most people experience some side effects, most commonly nausea, reduced appetite, or mild stomach discomfort. These typically peak on days 2–3 and improve by day 7 as your body adjusts. The starting dose (2.5 mg) is designed to minimise shock to your system. Not everyone experiences side effects—some people notice only appetite reduction, which is actually the medication working.
How do I store my Mounjaro KwikPen before my first injection?
Keep unopened pens in the refrigerator at 2–8°C in their original carton. Do not freeze them. Once you use your first pen, it can be stored at room temperature (below 30°C) for up to 30 days. Keep away from direct sunlight and heat. Always check the expiry date before injecting.
What if I feel too sick to inject on schedule during week one?
If side effects are severe enough to concern you, contact your GP or the prescriber who issued your Mounjaro. Do not skip your scheduled injection without guidance. Minor nausea is expected; severe vomiting that prevents you from keeping food down requires medical advice. Your prescriber may adjust your approach or provide anti-nausea support.
Should I change my diet during my first week on Mounjaro?
Yes. Eat smaller, frequent meals of bland foods rather than large portions of fatty or spicy meals, which can worsen nausea. Mounjaro naturally reduces appetite, so you'll feel full on smaller amounts. Focus on nutrient-dense foods: lean proteins, vegetables, whole grains, and fruits. Avoid alcohol and caffeine if they trigger nausea.
When should I contact my GP about first-week side effects?
Contact your GP if you experience severe stomach pain (possible pancreatitis), persistent vomiting lasting more than a few hours, signs of allergic reaction, or any symptom that concerns you. Minor nausea, mild dizziness, and reduced appetite are normal and expected. Your prescriber will give you specific guidance on when to seek help.
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.