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 Is the Mounjaro 2.5 mg Pen?
The Mounjaro 2.5 mg pen is the first step in weight management treatment using tirzepatide. It's a prescription-only injection containing 2.5 mg of tirzepatide — a dual GIP and GLP-1 receptor agonist. You'll use this starting dose for 4 weeks before potentially moving to a higher dose. In the UK, Mounjaro is available through NHS specialist weight management services or private online prescribing following clinical assessment. This guide explains how the 2.5 mg starting dose works and what the treatment journey looks like.
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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 the 2.5 mg Starting Dose Works
The 2.5 mg pen is your initial dose of tirzepatide — not a full treatment dose, but an introduction to help your body adjust. Mounjaro works differently from older weight loss medications because it activates two natural hormone pathways simultaneously: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1).
At 2.5 mg, you're receiving a low dose that may help:
- Reduce appetite and cravings
- Increase feelings of fullness after eating
- Support blood sugar regulation
- Allow your digestive system to adjust to the medication
You inject the 2.5 mg KwikPen once per week, into your stomach, thigh, or upper arm. After 4 weeks on this dose, if tolerated well, your prescriber will typically move you to 5 mg for the next 4 weeks, continuing the escalation protocol. Use our free GLP-1 weight loss calculator to see projected results across different doses — though individual results vary 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.
UK Eligibility and What to Expect
You may be eligible for Mounjaro in the UK if you have a BMI of 30 or above, or BMI of 27–29.9 with a weight-related health condition (type 2 diabetes, high blood pressure, sleep apnoea, or cardiovascular disease). People of South Asian, Chinese, Black African, or Black Caribbean descent may qualify at lower BMI thresholds due to increased health risks at the same weight.
Most people starting 2.5 mg experience some nausea, reduced appetite, or mild stomach discomfort in the first 1–2 weeks — this typically improves as your body adjusts. Keep our free BMI calculator handy to track changes as you progress through the dose escalation. Always inform your prescriber of side effects; they can slow the escalation if needed. Access in the UK is through NHS specialist services (with potential waiting lists) or private online prescribing with a UK-licensed doctor.
- 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 Mounjaro 2.5 mg a full treatment dose?
No. The 2.5 mg pen is the starting dose only — a low introduction dose designed to help your body adjust over 4 weeks. After 4 weeks, you'll typically move to 5 mg, then escalate further every 4 weeks up to the maximum of 15 mg. The full treatment effect is usually seen at higher maintenance doses (typically 10 mg or above).
How long do I stay on 2.5 mg Mounjaro?
Exactly 4 weeks. You inject 2.5 mg once weekly for 4 weeks (so 4 injections total from one KwikPen). After 4 weeks, assuming it's been tolerated well, your prescriber will move you to 5 mg. Do not increase the dose yourself — always follow your prescriber's guidance on timing.
What side effects might I get on 2.5 mg?
The most common side effects on the starting dose are nausea, reduced appetite, and mild stomach discomfort. These usually improve within the first 1–2 weeks as your body adjusts. Constipation or diarrhoea may also occur. If side effects are severe or persistent, contact your prescriber — they may slow the dose escalation.
Can I access Mounjaro 2.5 mg on the NHS in the UK?
Possibly, through NHS tier 3 specialist weight management services — but availability varies significantly by region and waiting lists can be long. Alternatively, you can access Mounjaro privately through UK-licensed online prescribers following a clinical assessment. Your GP can discuss both routes with you.
How is Mounjaro 2.5 mg different from Wegovy?
Mounjaro is a dual GIP and GLP-1 agonist (tirzepatide), while Wegovy is a GLP-1-only agonist (semaglutide). Both are once-weekly injections for weight management, but Mounjaro's dual mechanism may produce different results. Clinical trials showed Mounjaro users lost approximately 15–21% of body weight, though individual results vary — discuss with your prescriber which is right for you.
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.