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 Mounjaro First Week
Starting Mounjaro for weight management is a significant step, and knowing what to expect in your first week can help you feel more prepared. Mounjaro (tirzepatide) is a prescription-only, once-weekly injection that works as a dual GIP and GLP-1 receptor agonist. Your first dose will be 2.5mg — a lower starting dose designed to help your body adjust while you begin your weight loss journey alongside a reduced-calorie diet and increased physical activity. Understanding the injection process, potential side effects, and what's normal can make this first week smoother.
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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.
Starting Your Mounjaro KwikPen — Dose and Timing
Your first Mounjaro dose is 2.5mg, administered as a once-weekly injection using the Mounjaro KwikPen — a pre-filled, easy-to-use device. You'll inject into your stomach (abdomen), upper thigh, or upper arm, rotating injection sites each week to prevent skin irritation. The injection itself takes only a few seconds and uses a very fine needle, so most people find it relatively painless.
Timing matters: You can take your injection any day of the week, but consistency helps — choose the same day each week. If you're concerned about the injection technique, your prescriber or pharmacist will provide clear instructions, and you can ask for a demonstration before your first dose.
It's normal to feel a little nervous about your first injection, but many users report that the anticipation is worse than the actual experience. Keep your Mounjaro KwikPen refrigerated (2–8°C) until you use it, and allow it to sit at room temperature for about 15 minutes before injecting — this makes the injection more comfortable.
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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.
Common First Week Side Effects and What's Normal
Nausea is the most common side effect in the first week — occurring in more than 1 in 10 people starting Mounjaro. You may also experience mild stomach discomfort, changes in appetite, or fatigue. These effects typically decrease within the first few days as your body adjusts to the medication.
Managing side effects: Eat smaller, more frequent meals, avoid greasy or spicy foods, stay hydrated, and take your time with meals. Most people find nausea improves significantly by day 4–5. If side effects are severe or you experience severe stomach pain spreading to your back (a possible sign of pancreatitis), contact your GP immediately. Keep in touch with your prescriber about how you're getting on — they can offer practical advice tailored to your experience.
- 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?
Weight loss typically isn't noticeable in the first week — your body is adjusting to the medication. Most significant weight loss occurs over weeks 4–12 as your dose increases and the medication's effects become more pronounced. Track your progress over weeks, not days, using our free progress tracker to see meaningful changes unfold.
What if I forget my first Mounjaro injection?
If you forget your first dose, take it as soon as you remember, then continue your normal weekly schedule from that point. It's best to choose a specific day each week to help you remember. Set a phone reminder so you don't miss future doses — consistency is important for the medication to work effectively.
Can I eat normally during my first week on Mounjaro?
Yes, but you may find you naturally eat less because Mounjaro reduces appetite. Eat smaller portions of nutritious foods — you don't need to follow a strict diet, just be mindful of hunger cues. Avoid heavy, greasy, or spicy meals if you're experiencing nausea. Drink plenty of water throughout the day.
Should I exercise during my first week on Mounjaro?
Light activity is fine — a gentle walk or stretching is safe. Avoid intense exercise if you're feeling nauseous or fatigued. As your body adjusts over the next few weeks, gradually increase activity as you feel able. Exercise alongside Mounjaro improves weight loss outcomes and supports overall health.
How do I know if Mounjaro is working in the first week?
In the first week, you may notice reduced hunger or changes in appetite — these are signs the medication is working. You might not see weight loss yet. Use our BMI calculator to establish your baseline, and check your progress at week 4 when your dose increases. Individual responses vary, so discuss your experience with your prescriber.
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.