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 2.5mg — The UK Starter Dose Explained
Mounjaro 2.5mg is the initial dose when you start tirzepatide treatment in the UK. It's designed as a gentle introduction to help your body adjust to this dual GLP-1 and GIP receptor agonist injection. You'll take one 2.5mg injection once weekly for the first four weeks before your prescriber may increase your dose. This starter approach helps minimise side effects whilst your system adapts to the medication. Like all prescription weight loss injections in the UK, Mounjaro requires a clinical assessment and prescription from a qualified healthcare provider.
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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.
How Mounjaro 2.5mg Works and What to Expect
Mounjaro 2.5mg activates two natural hormone pathways — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) — which work together to regulate appetite and blood sugar. This dual mechanism is what distinguishes tirzepatide from other weight loss injections like Wegovy (semaglutide), which works through GLP-1 alone.
During weeks 1–4 on 2.5mg:
- You'll inject once weekly using a pre-filled KwikPen device
- Common side effects include nausea, diarrhoea, and constipation — these typically ease over time
- You may notice reduced appetite or changes in how food tastes
- Your stomach may feel fuller after smaller meals
After four weeks, your prescriber will assess your response and may increase to 5mg. This staged titration approach helps your body adapt gradually whilst achieving safety. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses based on clinical trial data.
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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
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Mounjaro 2.5mg Eligibility and UK Access
In the UK, you may be eligible for a Mounjaro prescription if you have a BMI of 30 or above, or a BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. Adults of South Asian, Chinese, Black African, or Black Caribbean heritage may qualify at lower BMI thresholds due to higher health risks at lower weights.
Access is available through NHS specialist tier 3 weight management services (though waiting lists vary significantly by region) or privately through GPhC-registered online pharmacies following a clinical assessment. A prescriber will review your medical history, current medications, and suitability before issuing a prescription. Use our free BMI calculator to check your personal BMI and understand whether you may meet initial eligibility criteria.
- 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 Mounjaro 2.5mg cause weight loss on its own?
Mounjaro 2.5mg is the starter dose and supports weight loss by reducing appetite and helping you feel fuller, but it works best alongside a reduced-calorie diet and increased physical activity. Clinical trials showed significant weight loss achieved with both medication and lifestyle changes together, not medication alone.
How long do I stay on Mounjaro 2.5mg?
You'll typically remain on 2.5mg for four weeks, then your prescriber may increase your dose based on how you're responding and tolerating it. The dose titration continues in 4-week increments, moving through 5mg, 7.5mg, 10mg, 12.5mg, and up to the maximum 15mg.
What's the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. Mounjaro's dual mechanism may support greater weight loss in some people, though individual results vary significantly and prescriber choice depends on your personal health profile.
Can I get Mounjaro 2.5mg on the NHS in the UK?
Mounjaro is available through NHS specialist weight management services (tier 3), though access and waiting lists vary substantially by region. Your GP can refer you to your local service. Private access is faster, available without NHS referral through online prescribers registered with the GPhC.
What if Mounjaro 2.5mg causes too many side effects?
Common early side effects like nausea often improve within 1–2 weeks as your body adjusts. If side effects are severe or persistent, contact your prescriber — they can adjust your dose, slow titration, or explore alternative options. Never stop injecting without medical guidance.
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.