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 Mounjaro in Your First Week
Most people do not experience significant weight loss in their first week of Mounjaro — and that is completely normal. Mounjaro (tirzepatide) is a prescription-only weight loss injection that works gradually over several weeks as your body adjusts to the medication. Your first week focuses on tolerating your starting dose (usually 2.5 mg) and establishing a routine, rather than gradual weight management. Understanding what to realistically expect helps you stay motivated and avoid disappointment. This guide explains the first-week timeline, what changes you might notice, and how long it typically takes to see meaningful results.
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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.
How Mounjaro Works in Week One
Mounjaro is a dual GIP and GLP-1 receptor agonist — meaning it activates two natural hormone pathways that regulate appetite and blood sugar. In your first week, the medication is still building up in your system. Most people start on a 2.5 mg dose, which is a starter injection designed to help your body adjust rather than deliver maximum effect.
During week one, you may notice:
- Reduced appetite or interest in foods you normally enjoy
- Mild nausea (very common — usually reduces after a few days)
- A feeling of fullness after smaller meals
- Changes to bowel habits (constipation or diarrhoea)
- Slight changes in taste preferences
Weight loss in week one is typically minimal to none — usually under 0.5 kg. This is expected. Your prescriber will gradually increase your dose every 4 weeks. The real effects on appetite and weight loss typically become more noticeable from week 3 onwards, as the medication reaches therapeutic levels in your bloodstream.
Clinically BackedWeight Loss
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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.
First Week Timeline and Side Effects
Your first injection is given on day one. Most side effects (if any) appear within 24–48 hours and often improve within 3–5 days as your body adjusts. Nausea is the most common first-week side effect — eating smaller, more frequent meals and avoiding fatty or spicy foods can help.
It is important to stay hydrated and eat regular, balanced meals even if your appetite is reduced. Skipping meals or severe undereating can cause dizziness, fatigue, or low blood sugar. If side effects are severe or persistent beyond one week, contact your GP or prescriber — they may adjust your dose or offer support.
Try our free GLP-1 weight loss calculator to see projected results at different doses over coming weeks. Weight loss accelerates significantly between weeks 4 and 12 as your dose increases. Using our progress tracker can help you log changes and stay motivated beyond week one.
- 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 lose weight in my first week on Mounjaro?
Most people do not experience significant weight loss in week one. A 2.5 mg starting dose is designed for tolerance, not maximum effect. Weight loss typically becomes noticeable from week 3 onwards as the medication builds in your system and your dose increases every 4 weeks. Individual results vary — some people notice appetite changes immediately, others take longer.
What is the starting dose of Mounjaro in the UK?
The standard UK starting dose is 2.5 mg, injected once weekly. This continues for weeks 1–4, then increases to 5 mg for weeks 5–8. Doses increase every 4 weeks (2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg maximum). Your prescriber decides the escalation schedule based on your tolerance and response.
Is nausea in the first week normal on Mounjaro?
Yes, nausea is very common in week one and affects up to 1 in 10 people taking Mounjaro. It usually improves within 3–5 days as your body adjusts. Eating smaller meals, avoiding fatty foods, staying hydrated, and eating slowly can help. If nausea persists beyond one week or is severe, contact your prescriber for guidance.
When will I start seeing real weight loss on Mounjaro?
Most people notice meaningful weight loss from week 4 onwards — when your dose increases to 5 mg and the medication reaches therapeutic levels. Significant weight loss typically accelerates between weeks 8 and 12 as doses increase further. Clinical trials showed average weight loss of 15–21% body weight over 72 weeks on higher doses, alongside diet and exercise.
How should I manage my diet in my first week on Mounjaro?
Eat regular, balanced meals even if your appetite is reduced — do not skip meals. Choose smaller portions, eat slowly, and chew thoroughly. Avoid fatty, spicy, or greasy foods if nausea occurs. Stay well hydrated with water throughout the day. Your GP or a dietitian can provide personalised nutrition support during your first weeks on Mounjaro.
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.