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 Happens in Your First 24 Hours on Mounjaro
Starting Mounjaro (tirzepatide) is an important step, and knowing what to expect in the first 24 hours can help ease any concerns. Your first dose is typically 2.5 mg — a starting dose designed to let your body adjust gradually. Most people experience minimal side effects immediately after injection, though nausea and stomach discomfort can develop over the following hours or days. This guide covers what happens after your first Mounjaro KwikPen injection, how to store your pen, and when to contact your GP if you have concerns.
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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.
Immediate After Your First Injection
The injection itself takes only a few seconds. You'll inject 2.5 mg of tirzepatide subcutaneously (under the skin) into your stomach, thigh, or upper arm. Immediately after injection, you may experience:
- Mild redness or bruising at the injection site — this is normal and usually fades within hours
- A slight stinging sensation that resolves within moments
- No systemic effects (body-wide symptoms) within the first few minutes
GLP-1 receptor agonists like tirzepatide work gradually — they don't produce instant appetite suppression or weight loss. The medication begins binding to GLP-1 receptors in your gut and brain, but noticeable effects typically take hours to days. Many people report that the first 24 hours feel completely normal, whilst others experience early nausea or mild stomach discomfort starting 2–6 hours after injection. This variability is completely normal — individual responses differ significantly.
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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-Day Side Effects and Management
Nausea is the most frequently reported side effect in the first 24 hours, affecting roughly 1 in 3 users. It's usually mild but can be managed by eating smaller, bland meals and staying hydrated. Practical tips for the first day:
- Eat light, plain foods — avoid fatty, spicy, or rich meals
- Drink plenty of water throughout the day
- Take your injection in the evening if you prefer — this allows you to sleep through the adjustment period
- Avoid strenuous exercise for 24 hours post-injection
- Store your KwikPen in the fridge at 2–8°C if unused, or at room temperature below 30°C after first use
Most side effects settle within days as your body adapts. If you experience severe stomach pain, persistent vomiting, or signs of allergic reaction (swelling of lips or throat), contact your GP immediately.
- 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 feel Mounjaro working in the first 24 hours?
Most people don't notice strong appetite suppression or weight loss in the first 24 hours. The medication works gradually — noticeable effects typically develop over days to weeks as your body adjusts to the tirzepatide. Some users report mild nausea or stomach changes within hours, whilst others feel no difference initially.
What should I eat after my first Mounjaro injection?
Eat light, bland foods in the first 24 hours — plain toast, rice, soup, or fruit. Avoid fatty, spicy, or heavy meals that may trigger nausea. Stay well hydrated and eat smaller portions than usual. Your appetite may naturally decrease, so listen to your body's hunger cues.
Is nausea after the first injection normal?
Yes, nausea affects roughly 1 in 3 people in the first 24–48 hours on Mounjaro. It's usually mild and temporary. If nausea persists beyond a few days, worsens, or you cannot keep food down, contact your GP. Never stop your injection without medical guidance.
Can I exercise in the first 24 hours after Mounjaro?
Light activity is fine, but avoid strenuous exercise, heavy lifting, or intense workouts in the first 24 hours. Give your body time to adjust to the injection. Resume normal exercise gradually over the following days, and stay well hydrated during any activity.
How should I store my Mounjaro KwikPen after the first injection?
Store your pen in the fridge at 2–8°C if it's unopened. After first use, you can keep it at room temperature (below 30°C) for up to 30 days. Keep it away from heat and direct sunlight. Dispose of the pen safely 30 days after first use, 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.