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
Mounjaro 2.5 mg/0.5 ml Pen — What UK Adults Need to Know
The Mounjaro 2.5 mg/0.5 ml pen is the standard starting dose for adults beginning weight management treatment with tirzepatide in the UK. This first injection contains enough medication for your initial four-week adjustment period, after which your dose may be increased based on your response and tolerance. Understanding how this starting dose works, who can access it, and what to expect is essential for anyone considering this prescription weight management option through either the NHS or private prescribing routes.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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.
NHS Access and UK Eligibility for Mounjaro
Mounjaro is a prescription-only medicine in the UK approved for weight management in adults meeting specific eligibility criteria. The 2.5 mg starting dose is designed to be well-tolerated while allowing your body to adjust to the dual GIP and GLP-1 receptor agonist effects of tirzepatide.
UK eligibility for Mounjaro typically includes:
- Adults with a BMI of 30 kg/m² or above (obesity)
- Adults with a BMI of 27–29.9 kg/m² (overweight) with weight-related health conditions such as type 2 diabetes, hypertension, or cardiovascular disease
- Lower thresholds for South Asian, Chinese, Black African, or Black Caribbean adults due to higher health risks at lower BMI levels
Access through the NHS is via specialist tier 3 or tier 4 weight management services following GP referral, though waiting lists vary significantly across UK regions. Private prescribing through a UK-licensed prescriber is also available without NHS referral. Your GP or prescriber will assess whether the 2.5 mg starting dose of this tirzepatide KwikPen is appropriate for you based on your individual health profile.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Dosing Schedule and How the Starting Pen Works
Each Mounjaro KwikPen contains four pre-filled doses of 2.5 mg, giving you a four-week supply when taking one injection weekly. You'll self-inject the 0.5 ml dose once weekly on the same day each week into your stomach, thigh, or upper arm. The 2.5 mg starting dose allows your body to adjust to tirzepatide's effects—common adjustments include changes to appetite, food preferences, and digestion—before you progress to the 5 mg dose at week five. Clinical trials showed that participants on Mounjaro experienced average weight loss of around 15% at the 5 mg maintenance dose, though individual results vary. Track your progress using our free progress tracker to monitor changes alongside your dose escalation schedule.
- 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 2.5 mg Mounjaro pen available through the NHS?
Mounjaro is available through NHS specialist weight management services (tier 3/4), though availability and waiting times vary by region. Access requires GP referral and assessment through your local integrated care board. Private prescribing is also available following an online clinical assessment with a UK-licensed prescriber, offering faster access if you meet eligibility criteria.
How long do I stay on the 2.5 mg dose?
You remain on the 2.5 mg starting dose for four weeks (one pen). At week five, if tolerated well, your prescriber will typically escalate you to 5 mg. Some people may stay longer on 2.5 mg if experiencing side effects, but the goal is gradual dose titration toward maintenance doses of 5 mg, 7.5 mg, or higher based on response.
What are the main side effects of the 2.5 mg starting dose?
Common side effects at the starting dose include nausea, diarrhoea, constipation, and reduced appetite. These are usually mild and often improve within the first few weeks as your body adjusts. Severe side effects are rare but warrant immediate medical attention. Always report persistent or concerning symptoms to your GP or prescriber.
How does Mounjaro 2.5 mg compare to Wegovy for weight management?
Both are prescription GLP-1 medications, but Mounjaro is a dual GIP/GLP-1 agonist while Wegovy (semaglutide) activates only the GLP-1 pathway. Clinical trials showed Mounjaro may produce slightly higher average weight loss, though individual results vary significantly. Your prescriber will assess which is more suitable based on your health profile and medical history.
Can I use our BMI calculator to check my eligibility for Mounjaro?
Yes, use our free BMI calculator above to determine your BMI instantly. If your BMI is 27 or above, or 30 or above, you may meet eligibility criteria. However, a clinical assessment by your GP or prescriber is essential to confirm whether Mounjaro is appropriate for you, as other health factors influence suitability beyond BMI alone.
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.