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 During Your First Month on Mounjaro
Your first month on Mounjaro is an adjustment period where your body adapts to tirzepatide, a prescription-only GIP and GLP-1 receptor agonist. You'll start on the lowest dose (2.5 mg) injected once weekly, with most people experiencing changes in appetite and digestion within the first few weeks. This guide explains what typically happens during this initial phase, common side effects, and practical tips for managing your first month successfully.
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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.
What Happens During Week One to Four on Mounjaro
Your first Mounjaro injection is the 2.5 mg starting dose, administered once weekly as a subcutaneous injection (into your stomach, thigh, or upper arm). Most people notice changes within 3–5 days, though individual responses vary. Common early experiences include reduced appetite, changes in food preferences, and mild nausea. You'll continue this 2.5 mg dose for exactly four weeks before any dose adjustment is considered. During this period, your prescriber may ask you to keep notes on how you feel, any side effects, and changes in hunger or eating patterns. It's important to stick to your reduced-calorie diet and increase physical activity as advised — Mounjaro works alongside lifestyle changes, not instead of them. Use our free GLP-1 weight loss calculator to understand potential outcomes as you progress through your treatment plan.
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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.
Managing Side Effects and Staying Comfortable
Nausea, constipation, and diarrhoea are the most common side effects in the first month — affecting more than 1 in 10 people. These typically ease within the first 2–3 weeks as your body adjusts. Practical strategies include:
- Eating smaller, more frequent meals (especially helpful if nausea is present)
- Staying well hydrated — aim for at least 6–8 glasses of water daily
- Eating slowly and chewing thoroughly to aid digestion
- Adjusting your diet to include easier-to-digest foods if experiencing constipation or diarrhoea
- Avoiding high-fat or greasy foods, which may worsen nausea
- Contacting your prescriber if side effects are severe or persistent beyond week three
Many people find that keeping a progress tracker helps them monitor side effects, appetite changes, and any weight loss alongside their lifestyle efforts.
- 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 month on Mounjaro?
Weight loss in month one is typically modest — usually 1–2 kg (2–4 lbs) for most people. Significant weight loss develops over weeks 5–12 as your dose increases and your body fully adjusts. Results vary based on how strictly you follow your reduced-calorie diet and exercise plan. Use our free GLP-1 weight loss calculator to see projected timelines.
What should I do if I feel very sick on the 2.5 mg starting dose?
Mild to moderate nausea is normal and usually settles within 2–3 weeks. However, if you're unable to eat, drink, or function, contact your prescriber immediately. They may suggest dietary adjustments, anti-nausea strategies, or in rare cases, temporarily pausing treatment. Never stop your injection without guidance.
Can I stop Mounjaro after the first month if I don't like it?
Yes — Mounjaro is not mandatory to continue. If you decide it's not right for you, discuss this with your prescriber. They'll advise on safe stopping (usually you can stop at any time without tapering). However, most people benefit from staying on the 2.5 mg dose for the full four weeks to allow proper adjustment.
How do I inject Mounjaro correctly in my first month?
Your prescriber will demonstrate the KwikPen injection technique at your initial appointment. Rotate injection sites weekly between your stomach, thigh, and upper arm to prevent irritation. Leave at least 2 cm between injection sites. Store unopened pens in the fridge (2–8°C); after first use, you can store at room temperature for up to 30 days.
What's the difference between starting Mounjaro and Wegovy?
Mounjaro (tirzepatide) starts at 2.5 mg weekly, while Wegovy (semaglutide) starts at 0.25 mg weekly. Mounjaro has a dual GIP and GLP-1 mechanism versus Wegovy's single GLP-1 mechanism. Both require prescription and clinical assessment. Clinical trials show Mounjaro may produce slightly greater average weight loss, though individual results vary significantly.
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.