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 Happens in Week 1 of Mounjaro Treatment
Your first week on Mounjaro (tirzepatide) begins with the starting dose of 2.5 mg — a lower dose designed to help your body adjust to this dual GLP-1 and GIP receptor agonist. Most people tolerate this initial dose well, though some may experience mild side effects. Understanding what to expect during week 1 can help you feel more confident about your treatment and prepare properly. Use our GLP-1 weight loss calculator to see potential weight loss trajectories across different doses, and track your progress from day one with our free progress tracker.
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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.
The Week 1 Mounjaro Dose and How It Works
In week 1, you'll receive your first 2.5 mg injection of tirzepatide. This is the standard starting dose for all adults beginning Mounjaro, regardless of BMI or weight. Your KwikPen contains four pre-filled doses, with each dose lasting one week. The 2.5 mg starting dose is intentionally low — it allows your digestive system to adjust gradually to the medication's effects without overwhelming your body.
Mounjaro works by activating two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action means tirzepatide may help reduce appetite, increase feelings of fullness, and slow gastric emptying — how quickly food leaves your stomach. Even at this starting dose, you may notice subtle changes in how much you want to eat or how satisfied you feel after meals. However, full appetite-suppressing effects typically build over several weeks as you progress through dose increases.
- Dose size: 2.5 mg once weekly
- Injection site: stomach, thigh, or upper arm
- Storage: refrigerate unopened pens at 2–8°C
- Duration at this dose: weeks 1–4 minimum before escalation
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.
Common Week 1 Side Effects and Managing Them
Nausea is the most commonly reported side effect during week 1, affecting up to 1 in 3 people starting Mounjaro. This typically feels mild during the first dose and may ease as your body adjusts. Other common week 1 experiences include slight stomach discomfort, loss of appetite (which is intentional but can feel unusual), or mild indigestion. Most people find these effects manageable and temporary.
How to manage week 1 side effects: Eat smaller, lighter meals to reduce stomach strain. Stay hydrated — drink plenty of water throughout the day. Avoid fatty, greasy, or very sugary foods, which can worsen nausea. If you feel very unwell or develop severe stomach pain, contact your GP or prescriber immediately. Side effects typically reduce after a few days as your body adapts.
- Most side effects peak within 24–48 hours of injection
- Usually settle within 3–5 days
- Mild side effects are normal and expected
- Severe symptoms warrant medical advice
- 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 lose weight in week 1 of Mounjaro?
Significant weight loss in week 1 is unlikely. The 2.5 mg starting dose is designed for tolerance, not maximum effect. Most weight loss occurs gradually over weeks 5–20 as you increase doses. However, you may notice reduced appetite or eating less naturally, which can lead to modest weight loss. Use our BMI calculator to track your baseline, then monitor progress weekly.
Can I eat normally in week 1 of Mounjaro?
Yes, you can eat normally, though your appetite may naturally decrease. Listen to your body — eat when hungry, stop when satisfied. Avoid large meals or fatty foods, which can trigger nausea. Focus on balanced, nutritious meals with plenty of vegetables, lean protein, and whole grains to support your health goals alongside the medication.
When should I be concerned about week 1 side effects?
Contact your GP or prescriber if you experience severe, persistent stomach pain (possible pancreatitis), signs of allergic reaction (swelling of face or lips), or symptoms that worry you. Mild nausea, slight appetite loss, and minor indigestion are normal. If mild side effects persist beyond 5 days, mention this at your next check-in.
Do I need to do anything special before my first Mounjaro injection?
Read your KwikPen instructions carefully beforehand. Store your pen in the refrigerator until use. Plan your injection for the same day each week. Check the injection site — stomach is most common but thigh or upper arm work too. Have tissues ready and dispose of the needle safely in a sharps bin. No fasting is required.
How does the 2.5 mg starting dose compare to Wegovy or other injections?
Mounjaro's 2.5 mg starting dose is higher than Wegovy's 0.25 mg opening dose, but both are designed for gradual tolerance. Mounjaro uses a dual mechanism (GIP + GLP-1) versus Wegovy's single GLP-1 action, which is why starting doses differ. Your prescriber will explain which is most suitable for you based on individual factors.
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.