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 Is Tirzepatide? UK Health Guide
Tirzepatide is a prescription-only weight management injection licensed in the UK under the brand name Mounjaro. It's a dual GIP and GLP-1 receptor agonist — meaning it activates two natural hormone pathways to help regulate appetite and support feeling fuller after meals. Available as a once-weekly injection, tirzepatide may help adults with obesity or overweight with weight-related health conditions. Clinical trials showed participants lost an average of 15–21% of their body weight, though individual results vary significantly. Access is available through NHS specialist weight management services or private online prescribing following a clinical assessment.
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.
How Tirzepatide Works and UK Eligibility
Mechanism of action: Tirzepatide activates two hormone pathways — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action helps regulate blood sugar, reduce appetite, and slow gastric emptying, so food leaves the stomach more slowly. It is not insulin and works differently from diabetes medications.
- May help reduce food cravings and overeating
- Supports feeling fuller after smaller meals
- Helps regulate fat utilisation and storage
- Effects typically paired with reduced-calorie diet and increased physical activity
UK eligibility: Adults with a BMI of 30 or above, or 27–29.9 with at least one weight-related health condition (type 2 diabetes, high blood pressure, abnormal blood fats, sleep apnoea, or cardiovascular disease history). South Asian, Chinese, Black African, and Black Caribbean adults may have lower thresholds. Always have a clinical assessment before starting.
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.
Tirzepatide Doses, Side Effects, and Access
Dosing: Tirzepatide starts at 2.5 mg once weekly, increasing gradually to a maximum of 15 mg weekly. Each dose is adjusted over 4+ weeks; most people reach maintenance doses of 5–15 mg. Supplied as pre-filled KwikPen devices with 4 doses per pen (4 weeks supply).
Common side effects: Nausea, diarrhoea, constipation, vomiting, and stomach pain are very common, especially at the start. Injection site reactions, fatigue, and hair loss may occur. Serious side effects are rare but seek urgent care for severe stomach pain, gallbladder problems, or allergic reactions.
Access in the UK: Available through NHS tier 3 specialist weight management services (referral from GP) or private online prescribing. Private access is often faster; NHS waiting lists vary by region. Use our free BMI calculator and weight loss calculator to check your eligibility and explore potential outcomes.
- 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 tirzepatide the same as semaglutide?
No. Tirzepatide (Mounjaro) is a dual GIP and GLP-1 agonist, while semaglutide (Wegovy) is a GLP-1-only agonist. Both are GLP-1 medications licensed for weight management in the UK, but tirzepatide activates two hormone pathways. Clinical trials suggest tirzepatide may produce slightly higher average weight loss, though individual results vary significantly.
Can I get tirzepatide on the NHS?
Yes, tirzepatide is available through NHS specialist tier 3 weight management services. You'll need a GP referral and assessment showing you meet eligibility criteria (BMI ≥30 or BMI 27–29.9 with weight-related health conditions). Availability and waiting times vary significantly by region and local NHS commissioning decisions.
What's the difference between Mounjaro and tirzepatide?
Mounjaro is the brand name for tirzepatide. They are the same medication — tirzepatide is the active ingredient. Mounjaro is manufactured by Eli Lilly and supplied as a pre-filled KwikPen injection device in doses from 2.5 mg to 15 mg once weekly.
How much weight can I expect to lose with tirzepatide?
In clinical trials (SURMOUNT-1), average weight loss ranged from 15% at lower doses to 21% at the maximum 15 mg dose over 72 weeks, alongside diet and exercise. However, individual results vary considerably — some people lose more, some less. Your GP or prescriber can give personalised estimates based on your health profile.
Is tirzepatide safe long-term?
Tirzepatide has been studied in clinical trials lasting over 72 weeks with a good safety profile when used as prescribed. Common side effects usually improve over time as your body adjusts. Always report any concerns to your prescriber. Long-term safety beyond 2–3 years is still being researched; ongoing monitoring is recommended.
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.