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 From Your First Mounjaro Dose
Starting Mounjaro (tirzepatide) is a significant step in your weight management journey, and it's natural to wonder what side effects you might experience. Your first dose is typically a 2.5 mg injection designed to help your body adjust to the medication gradually. Most people experience some gastrointestinal side effects in the initial weeks, but understanding what's normal and when to seek help can ease your concerns. This guide explains what commonly happens after your first Mounjaro injection, how to manage symptoms, and what the cost and access situation looks like across the UK.
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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.
Common First Dose Side Effects and What They Mean
The most common side effects after your first Mounjaro injection include nausea, mild stomach discomfort, constipation, or diarrhoea. These typically occur within 24–48 hours of your first dose and often improve within a few days as your body adjusts. Nausea is the most frequently reported effect and tends to be mild at the starting 2.5 mg dose.
- Nausea and vomiting: Usually peak within the first 48 hours and often ease by day 3–4
- Stomach pain or discomfort: Mild cramping is common; severe pain is not normal
- Constipation or diarrhoea: Can occur as the medication affects how quickly food moves through your digestive system
- Fatigue: Feeling tired is common in the first week
- Injection site reactions: Redness, bruising, or mild soreness at the injection site
Most side effects are temporary and improve as your body adjusts. However, if you experience severe, persistent stomach pain spreading to your back, seek immediate medical attention—this could indicate pancreatitis, though this is rare.
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.
Accessing and Affording Mounjaro in the UK
Mounjaro access in the UK splits into two routes: NHS specialist services and private prescribing. NHS access is through tier 3 weight management services following GP referral, though waiting lists can be substantial in many areas. Private online prescribing has become increasingly available, with GPhC-registered prescribers offering consultations and GPhC-registered pharmacies dispensing prescriptions. Monthly costs for private Mounjaro typically range from £150–£280 depending on your dose, with the 2.5 mg starting dose at the lower end. Eligibility for both routes requires a BMI of 30 or above, or 27 or above with weight-related health conditions such as type 2 diabetes, hypertension, or cardiovascular disease. Lower BMI thresholds apply for adults of South Asian, Chinese, Black African, or Black Caribbean heritage due to higher health risks at lower weights.
- 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 definitely get nausea with my first Mounjaro injection?
No—not everyone experiences nausea, though it is the most common side effect. Many people have mild nausea or none at all on the 2.5 mg starting dose. Individual responses vary significantly. Your prescriber can suggest dietary and lifestyle adjustments to reduce nausea risk, such as eating smaller meals and staying hydrated.
How long do first dose side effects last?
Most side effects peak within 24–48 hours and improve within 3–7 days as your body adjusts. Gastrointestinal symptoms often ease as you progress through the titration schedule. If side effects persist beyond two weeks or worsen, contact your prescriber.
Should I stop taking Mounjaro if I have side effects?
Do not stop without medical guidance. Most side effects are temporary and manageable with practical strategies like eating smaller, bland meals and staying hydrated. Contact your prescriber if symptoms are severe or concerning—they may adjust your dose or offer additional support.
What's the difference between NHS and private Mounjaro in the UK?
Both routes provide the same medication at the same doses. NHS access is through specialist services with referral requirements; private access is through online prescribers without referral. NHS is free but often has waiting lists; private is faster but involves a cost, typically £150–£280 monthly.
Can I use our weight loss calculator before starting Mounjaro?
Yes—our GLP-1 weight loss calculator can help you estimate potential outcomes based on clinical trial data and your starting weight. This provides realistic context about expected results alongside lifestyle changes, though individual outcomes always vary.
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.