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 on Your First Day of Mounjaro
Starting Mounjaro (tirzepatide) is an important health decision, and knowing what to expect on day one can help ease any concerns. Mounjaro is a prescription-only, once-weekly injectable medication licensed in the UK for weight management in adults. Your first injection will typically be a 2.5 mg starter dose, delivered via the KwikPen device. This guide explains the practical steps, what side effects to anticipate, and how to use your UK NHS or private prescription safely. Most people manage their first dose without serious issues, though mild nausea is common.
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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.
Preparing for Your First Mounjaro Injection
Before your first injection day: Check your prescription is from a UK-licensed prescriber and that you have a valid KwikPen device (four pre-filled doses per pen, each containing enough for one week). Store the unopened pen in the fridge at 2–8°C until use. If prescribed through the NHS via a tier 3 specialist weight management service, your prescriber will have confirmed you meet NICE eligibility criteria—typically a BMI of 30 or above, or 27–29.9 with weight-related health conditions including type 2 diabetes or high blood pressure. If prescribed privately following an online clinical assessment, your prescriber will have assessed your suitability.
- Choose an injection site: stomach (abdomen), upper thigh, or upper arm
- Wash your hands and allow the pen to reach room temperature for 15 minutes
- Check the medication is clear (not cloudy or discoloured)
- Use an alcohol wipe to clean the injection site
- Follow the step-by-step injection instructions in your patient information leaflet
- Rotate injection sites each week to reduce irritation
Your GP or prescribing service should have provided you with clear written instructions and contact details for questions on injection day.
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 First-Day Side Effects and How to Manage Them
Nausea is the most frequently reported side effect in the first 1–2 weeks of Mounjaro, though it often improves as your body adjusts. Other common first-day experiences include mild dizziness, indigestion, or a slight headache. These are not dangerous but can feel uncomfortable. Start with small, bland meals (plain rice, toast, chicken broth) on your first day and the days following. Stay hydrated by sipping water throughout the day. Ginger tea or peppermint can help settle nausea naturally. If side effects are severe or you experience severe stomach pain spreading to your back (a rare sign of pancreatitis), contact your GP or NHS 111 immediately.
Use our free GLP-1 weight loss calculator to project your potential weight loss journey—knowing your expected timeline can help you stay motivated through the initial adjustment period.
- 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.
Can I eat normally on my first day of Mounjaro?
Most people can eat normally, but many experience reduced appetite or mild nausea, especially 2–4 hours after injection. Start with smaller portions of bland foods if you feel nauseous. Avoid heavy, fatty, or spicy meals on day one. By day two or three, most people return to regular eating patterns, though appetite tends to be naturally suppressed.
Where is the best place to inject Mounjaro on my first day?
The most common site is the stomach (abdomen), roughly 5cm away from your belly button. Upper thigh and upper arm are also suitable. Choose whichever feels most comfortable and accessible. Clean the site with an alcohol wipe, wait for it to dry, then inject at 90 degrees. Rotate sites each week to prevent irritation or lipohypertrophy (thickened tissue).
What if I feel dizzy or faint after my first injection?
Mild dizziness can occur as your body adjusts, especially if you haven't eaten. Sit down immediately and rest for 5–10 minutes. Drink water or a small sugary drink. If dizziness persists beyond 30 minutes or you feel faint, contact your GP or call NHS 111. Do not drive if you feel unwell.
How long after my first Mounjaro injection will I see weight loss?
Most people begin to notice reduced appetite within 2–3 days of the first injection. Measurable weight loss typically appears within 2–4 weeks as the medication builds in your system and you naturally eat less. Track your progress using our free progress tracker. Remember, you'll be on the 2.5 mg starter dose for four weeks before increasing.
Should I tell my GP about side effects after my first Mounjaro injection?
Yes, contact your GP or prescribing service if you experience severe nausea lasting more than 48 hours, severe stomach pain, vomiting, or any sign of allergic reaction (swelling of lips or throat). Mild nausea and indigestion are expected and usually resolve within days. Your prescriber can offer practical advice or suggest timing adjustments for future doses.
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.