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 in Your First Week on Mounjaro
Starting Mounjaro (tirzepatide) can feel daunting, but knowing what to expect in your first week helps you feel more confident. Most people don't experience significant weight loss in week one — the starting dose is 2.5 mg, designed for your body to adjust gradually. You may notice changes in appetite or mild side effects instead. This guide explains what's normal, when to contact your GP, and how the first week fits into your longer weight loss journey with this prescription weight loss injection.
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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.
First Week on Mounjaro: What Happens
Your first Mounjaro injection is a 2.5 mg starting dose, given once weekly. This is not a weight loss dose — it's an adjustment dose to help your body tolerate the medication. Most people remain on this dose for 4 weeks before increasing.
What to expect in week one:
- No significant weight loss — weight loss typically becomes noticeable weeks 4–8 when doses increase
- Possible nausea, especially if you eat large meals or fatty foods — this usually improves within 2–3 days
- Reduced appetite — some people notice this immediately, others don't for several weeks
- Mild stomach discomfort or constipation — stay hydrated and eat smaller, frequent meals
- Injection site reactions — mild redness or bruising at the injection site (stomach, thigh, or upper arm) is common and settles within hours
According to NICE guidance and NHS specialist services, weight changes in week one are typically minimal. Clinical trials show meaningful weight loss begins as doses increase during weeks 5–12. Use our GLP-1 weight loss calculator to understand realistic timelines for your dose.
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.
Managing Your First Week: Practical Tips
The first week is about tolerance, not results. Inject on the same day each week — many people choose Monday morning for consistency. Take your injection at least 2 hours before a meal if possible. Store your KwikPen in the fridge until use, then at room temperature for up to 30 days after opening.
Nausea management: Eat small, frequent meals with lower fat content. Avoid sugary, fried, or heavily seasoned foods. Ginger tea and plain biscuits often help. Most nausea resolves by day 3–4, though some people experience it intermittently for 1–2 weeks.
Contact your GP if you experience severe stomach pain, signs of allergic reaction, or persistent vomiting. For minor side effects, patience usually helps — your body is adjusting to a new medication. Track your symptoms using our free progress tracker to identify patterns.
- 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 my first week on Mounjaro?
Unlikely. The starting 2.5 mg dose is for tolerance, not weight loss. Clinical trials show weight loss becomes noticeable weeks 4–8 as doses increase to 5 mg and above. Individual results vary, but expect minimal change in week one. Use our weight loss calculator to see realistic timelines based on your height, weight, and target dose.
Is nausea normal in the first week?
Yes. Nausea affects most people starting Mounjaro, especially in days 1–3. It usually improves by day 4–5. Eating smaller, low-fat meals helps significantly. If nausea persists beyond 7 days or is severe, contact your GP — they may suggest adjusting meal timing or temporary anti-nausea support.
What if I have an allergic reaction to my first injection?
Seek emergency care immediately if you experience swelling of the face, lips, tongue, or throat; difficulty breathing; or severe rash. These are rare but serious. For mild itching or minor redness at the injection site, this is normal and settles within hours. Always report any concerns to your GP.
How do I store my Mounjaro KwikPen in the first week?
Keep unopened pens in the fridge at 2–8°C. After your first injection, you can store the used pen at room temperature (below 30°C) for up to 30 days. Never freeze it. Keep pens away from direct sunlight and heat. NHS specialist services will provide storage guidance when you receive your prescription.
Should I change my diet in week one?
Yes. Eat smaller meals (about 3 hours apart), avoid high-fat and sugary foods, and stay well hydrated. Many people naturally eat less due to reduced appetite, but don't force restriction — let the medication work. NHS guidance recommends combining Mounjaro with a modest calorie deficit and increased physical activity for best results.
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.