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 on Your Mounjaro First Day
Starting Mounjaro (tirzepatide) is a significant step in your weight management journey. Your first day involves a 2.5 mg injection — the starting dose — which you'll administer yourself using a pre-filled KwikPen. This is a prescription-only medicine, so you'll have received clear written instructions and a clinical assessment before starting. Most people find the injection straightforward, though some experience mild side effects. Understanding what to expect helps you feel more confident and prepared.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
Your First Mounjaro Injection — How It Works
Your first Mounjaro dose is 2.5 mg, administered once weekly. You'll inject it under the skin (subcutaneously) using the KwikPen device provided. The injection itself takes just a few seconds — most people report it's less painful than expected. Injection sites include:
- Stomach (abdomen) — the most common choice
- Upper thigh — convenient and easy to reach
- Upper arm — requires assistance from another person
After your first injection, you'll stay on this 2.5 mg dose for 4 weeks before your prescriber considers increasing to 5 mg. This gradual approach — called dose titration — allows your body to adjust and helps minimise side effects. Keep your KwikPen in the fridge at 2–8°C before use, and store it at room temperature (below 30°C) after your first injection for up to 30 days. Use our GLP-1 weight loss calculator to explore how Mounjaro may support your weight management goals across different dose levels.
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 What to Do
Nausea is the most frequently reported side effect on the first day or within the first few days. It's usually mild and often improves within a week as your body adjusts. Mild stomach discomfort, reduced appetite, or slight dizziness may also occur. These are not reasons to stop — they're expected as your GLP-1 receptor system activates. To manage early side effects:
- Eat small, frequent meals rather than large portions
- Stay well hydrated throughout the day
- Avoid fatty or greasy foods temporarily
- Rest and avoid strenuous exercise on injection day if you feel unwell
Serious side effects are rare, but seek immediate medical attention if you experience severe stomach pain, difficulty breathing, or signs of an allergic reaction. Contact your prescriber if side effects persist beyond one week or feel unbearable — they can provide tailored guidance.
- 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.
Should I eat before my first Mounjaro injection?
You can eat normally before your injection — there's no need to fast. However, start with smaller meals on your first day if you feel anxious about the injection, as nausea may be more noticeable if you've eaten a large meal shortly before. Staying hydrated is more important than eating beforehand.
How do I know if I've injected Mounjaro correctly?
The KwikPen will click or signal when the injection is complete — this usually happens after about 3–5 seconds. You may see a small amount of blood or clear fluid at the injection site, which is normal. Your prescriber should have shown you the correct technique during your consultation or provided instructional materials.
Can I exercise on my first day of Mounjaro?
Light activity is fine, but avoid strenuous exercise or the gym on your first injection day. Your body is adjusting to the medication, and intense activity may worsen nausea or dizziness. Resume normal exercise within 24–48 hours if you feel well, alongside a reduced-calorie diet for best results.
What if I feel dizzy or unwell after my first Mounjaro injection?
Dizziness can occur as your body adjusts. Sit or lie down for 10–15 minutes, drink water, and avoid standing up too quickly. If dizziness persists for several hours or you feel faint, contact your GP or prescriber. Serious side effects are rare, but your healthcare provider should always be informed of significant symptoms.
How do I store my Mounjaro KwikPen after the first injection?
After your first use, store the pen at room temperature (below 30°C) away from heat and direct sunlight. You can use it for up to 30 days after opening. Keep it in a cool, dark place — not necessarily the fridge — and always dispose of the pen safely in a sharps bin after 30 days, even if medication remains.
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.