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
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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 the first dose might cause. Most people experience mild to moderate side effects initially, particularly nausea and digestive changes. This guide covers what's typical with the 2.5mg starting dose in the UK context, how long side effects usually last, and when you should contact your GP. Remember, individual experiences vary significantly — what one person experiences may differ from another.
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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 Dose Side Effects and Timeline
The 2.5mg starting dose of Mounjaro is specifically designed to be gentle on your system. Most people who experience side effects notice them within the first few days after injection, though some don't experience any at all.
Very common side effects (may affect more than 1 in 10 people):
- Nausea — the most reported side effect, often mild and improving after a few days
- Diarrhoea or constipation — digestive adjustments are typical
- Stomach discomfort or reduced appetite — which may actually support your weight loss goals
- Mild fatigue or dizziness — usually temporary
These effects typically peak within 24–48 hours of your injection and improve significantly by day 3–4. Your body is adapting to the medication's effects on how your stomach empties and how your brain signals fullness. If side effects persist beyond 7 days, contact your GP for guidance.
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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.
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Managing Side Effects and When to Seek Help
Simple strategies can help ease early side effects. Eating smaller, more frequent meals, staying well hydrated, and avoiding greasy foods can reduce nausea. Try ginger tea, peppermint, or antiemetic medication (your pharmacist can advise). Many people find side effects are much milder on subsequent doses as their body adjusts.
Seek urgent medical attention if you experience: severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face or throat), or severe vomiting that prevents you from keeping fluids down. Contact your GP if side effects last more than a week, or if you're concerned about how the medication is affecting you. Your prescriber may adjust your approach or dose timing. Always follow NHS guidance and never stop medication without medical advice.
- 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.
Is nausea on the first Mounjaro dose normal?
Yes, nausea is very common with the first dose and affects many people. It's typically mild to moderate and improves within 3–4 days as your body adjusts. Eating smaller meals and staying hydrated can help. If nausea persists beyond a week or is severe, contact your GP.
How long do first dose side effects usually last?
Most side effects peak within 24–48 hours and improve significantly by days 3–7. Individual timelines vary — some people feel fine within a day, others take up to a week. Each subsequent dose typically causes fewer side effects as your body adapts to the medication.
Should I take my first Mounjaro dose on a specific day of the week?
Your prescriber will advise you on the best timing. Many people choose to inject on a day when they can rest at home if needed, though this isn't essential. You can take it any day of the week as long as you space doses 7 days apart. Consistency is more important than the specific day.
Can I prevent side effects from my first Mounjaro injection?
While you cannot prevent side effects entirely, starting with the low 2.5mg dose helps. Eating smaller meals before and after injection, staying hydrated, and avoiding alcohol can reduce their intensity. Some people take anti-nausea medication as a precaution — discuss this with your GP or pharmacist.
When should I contact my GP about first dose side effects?
Contact your GP if side effects persist beyond 7 days, are severe, or if you experience severe stomach pain, allergic reactions, or vomiting that prevents fluid intake. Minor nausea and appetite changes are expected and usually resolve quickly. Your GP can provide reassurance or suggest adjustments to help you manage the early phase.
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.