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 on Your First Mounjaro Dose
Starting Mounjaro (tirzepatide) can feel uncertain. Your first dose is always 2.5 mg — a low starting amount designed to allow your body to adjust gradually. Most people experience minimal side effects at this initial dose, though some nausea or mild stomach changes are common. Understanding what typically happens in the first week, first month, and beyond can help you feel more prepared. This guide covers the realistic timeline, common experiences, and practical tips for your Mounjaro journey in 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.
Your First Dose Timeline — What Happens Week by Week
Your first Mounjaro injection delivers 2.5 mg via the KwikPen auto-injection device. The injection itself takes seconds and is given under the skin (subcutaneously) in your stomach, thigh, or upper arm. Most people report the injection is relatively painless — the needle is very fine.
First 24–48 hours: Some people feel no different at all. Others notice mild nausea, which is the most common early effect. This typically subsides within a few days.
Week 1–2: Any nausea usually peaks around day 2–3, then improves. Appetite may begin to reduce slightly. You might feel fuller after eating less. Some people experience loose stools or mild stomach discomfort.
Week 3–4: By the end of week 4, your body is adjusting to the 2.5 mg dose. Side effects often settle. You'll remain on this dose for a full 4 weeks before the prescriber considers increasing to 5 mg. You may notice modest appetite reduction and early signs of eating differently — eating less without consciously trying.
Key point: Weight loss is NOT expected at 2.5 mg. This dose is purely about tolerance and adjustment. Your prescriber will increase your dose only if you tolerate the 2.5 mg well.
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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 Your First Dose — Practical Tips
Eating and drinking: Eat smaller, frequent meals. Avoid greasy, spicy, or heavy foods that may trigger nausea. Stay hydrated with water and sugar-free drinks. Ginger tea may help with nausea.
Injection timing: Take your injection at the same day and time each week (e.g. every Monday morning). This builds a routine and makes it easier to remember.
Storage: Keep your KwikPen in the fridge at 2–8°C until first use. Once opened, it can stay at room temperature (below 30°C) for up to 30 days. Never freeze it.
Injection sites: Rotate injection sites each week to avoid irritation. Most people find the stomach easiest to self-inject.
When to contact your GP: Seek advice if nausea is severe and unmanageable, you cannot keep food or fluids down, or you experience severe stomach pain (rare but needs checking for pancreatitis).
- 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 the first Mounjaro dose painful?
No. The KwikPen uses a very fine needle and most people find the injection quick and relatively painless. Any discomfort is minimal and over in seconds. Injection site redness or slight bruising can occur but resolves within hours.
Will I lose weight on my first 2.5 mg dose?
Weight loss is not expected at 2.5 mg. This starting dose is purely for tolerance and body adjustment. The real appetite-reducing effects appear at higher doses (5 mg and above). Weight loss typically becomes noticeable after 4–8 weeks once you've reached a therapeutic dose.
What's the most common side effect of the first dose?
Nausea is the most common side effect, affecting roughly 1 in 3 people in the first week. It's usually mild to moderate and improves by day 4–7 as your body adjusts. Taking smaller meals and staying hydrated helps. Diarrhoea or constipation can also occur but are less frequent initially.
How long do I stay on 2.5 mg before increasing?
You remain on 2.5 mg for exactly 4 weeks. Your prescriber will not increase your dose before the 4-week mark. If you tolerate it well, you'll move to 5 mg in week 5. If side effects are significant, your prescriber may extend the 2.5 mg phase or adjust the plan.
Can I take Mounjaro if I'm also on other medications?
Possibly, but it depends on what medications you take. Mounjaro can affect how your body absorbs some oral medications — if you take oral contraceptives, you may need additional contraception for 4 weeks. Always tell your prescriber about all medications, supplements, and herbal products before starting. They'll assess compatibility and adjust if needed.
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.