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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Mounjaro 2.5 mg/0.5 ml Pen — Your Starting Dose Explained
The Mounjaro 2.5 mg/0.5 ml pen is the initial starter dose for tirzepatide weight management in the UK. This first injection contains 0.5 ml of solution delivering 2.5 mg of the active ingredient — a dual GIP and GLP-1 receptor agonist. You'll use this dose for your first four weeks before your prescriber considers increasing it. Understanding what this starting dose does, how to use it, and what happens next is essential for anyone beginning Mounjaro treatment 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.
How the 2.5 mg Starter Dose Works
Mounjaro 2.5 mg activates two natural hormone pathways in your body: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism is what distinguishes it from other weight loss injections like Wegovy, which acts on GLP-1 alone. The 2.5 mg starting dose is intentionally lower than maintenance doses to help your body adjust and allow your prescriber to monitor how you respond.
At this starter dose, you may experience reduced appetite, feel fuller after eating smaller portions, and notice slower digestion. Clinical trial data showed that participants eventually reaching higher Mounjaro doses lost an average of 15–21% of their body weight, though results vary significantly between individuals. The 2.5 mg dose is a stepping stone toward your therapeutic dose, typically reached by week 21 (at 15 mg maximum).
Each KwikPen contains enough solution for one injection. You'll store it in your fridge at 2–8°C until use, then inject it once weekly into your stomach, thigh, or upper arm.
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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.
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Getting Mounjaro 2.5 mg in the UK — What You Need to Know
NHS access: Mounjaro is available through NHS specialist weight management services (tier 3/4) for adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes, high blood pressure, or cardiovascular disease. South Asian, Chinese, Black African, and Black Caribbean adults may qualify at lower BMI thresholds. Waiting lists vary significantly by region.
Private access: You can obtain a Mounjaro prescription through a UK-licensed online prescriber following a clinical assessment. Private prescriptions start at around £129–£150 per month for the starter dose pack. A prescriber will confirm your eligibility, review your medical history, and ensure Mounjaro is suitable before issuing your prescription — prescription is essential, as tirzepatide is not available without one.
Track your progress using our free progress tracker to monitor weight changes week by week.
- 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.
How long do I stay on the 2.5 mg Mounjaro dose?
You remain on the 2.5 mg starter dose for four weeks (one KwikPen = four weekly injections). After four weeks, your prescriber will assess your response and may increase you to 5 mg. This gradual dose escalation continues every 4 weeks until you reach your therapeutic dose, typically between 10–15 mg depending on your individual response and tolerance.
What's the difference between Mounjaro and Wegovy at the starter dose?
Mounjaro 2.5 mg uses a dual GIP+GLP-1 mechanism, while Wegovy's starter dose (0.25 mg) uses GLP-1 alone. Clinical trials showed Mounjaro may produce greater weight loss on average — approximately 15–21% versus 15% with Wegovy — though individual results vary. Both are prescription-only and require a clinical assessment.
Can I get Mounjaro 2.5 mg through the NHS?
Yes, through NHS specialist weight management services if you meet eligibility criteria (BMI 30+, or 27+ with weight-related health conditions). Access is through referral and varies by region. Alternatively, private online prescribers in the UK can issue prescriptions following a clinical assessment without an NHS referral.
What side effects might I experience on the 2.5 mg starter dose?
Common side effects include nausea, diarrhoea, constipation, and stomach pain. These typically ease over the first few weeks as your body adjusts. More serious side effects are rare but may include pancreatitis (severe stomach pain) or gallbladder issues. Contact your GP if you experience severe or persistent symptoms.
How do I inject the Mounjaro KwikPen correctly?
Wash your hands, pinch the skin on your stomach, thigh, or upper arm, insert the needle at 90 degrees, and press the injection button until you hear a click. Hold for 3 seconds, then remove. Rotate injection sites weekly. Never inject into areas with bruising or hardened skin. Your prescriber will provide detailed injection training.
Ready to take control of your weight loss journey?
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