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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Weight Loss and Low Iron — What You Need to Know
Weight loss and low iron are connected health concerns that many UK adults face together. If you're experiencing low iron levels (anaemia) and considering weight management through prescription medication, it's important to understand how these factors interact and what support is available. This guide explains prescription weight loss options available in the UK, how they may affect iron absorption, and when to speak with your GP about managing both conditions safely.
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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.
UK Prescription Weight Loss Medication Options
Three prescription-only weight management medications are currently licensed in the UK: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All are GLP-1 receptor agonists (or dual GIP/GLP-1 in Mounjaro's case) that work by helping regulate appetite and supporting feeling fuller after meals.
Eligibility criteria: Adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions, may be eligible. These conditions include type 2 diabetes, high blood pressure, or cardiovascular disease. Clinical trials showed average weight loss of 15–21% of body weight alongside a reduced-calorie diet and increased physical activity — though individual results vary significantly.
Important: GLP-1 medications can affect how your stomach processes food, which may impact iron absorption. If you have low iron or anaemia, your GP must assess whether prescription weight loss treatment is appropriate for you before starting.
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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.
NHS vs Private Access and Next Steps
NHS access: Available through specialist tier 3 weight management services, usually following a GP referral. Availability varies significantly by region, and waiting lists can be lengthy. Your GP can discuss whether you're eligible and make a referral if appropriate.
Private access: Available through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber — no NHS referral needed. Costs vary by dose and provider.
Before starting any weight loss medication, tell your prescriber about low iron, anaemia, or any iron supplements you're taking. They'll assess whether prescription medication is safe for you and may recommend monitoring your iron levels during treatment. Use our free BMI calculator and GLP-1 weight loss calculator to understand your current health metrics and explore potential outcomes — then discuss results with your GP.
- 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.
Can I take weight loss injections if I have low iron?
Individual circumstances vary. GLP-1 medications can affect stomach acid and nutrient absorption, which may impact iron levels. Your prescriber must know about your low iron status before prescribing. They'll assess whether it's safe and may recommend iron monitoring. Always inform your GP or prescriber about anaemia or iron deficiency before starting treatment.
How does weight loss medication affect iron absorption?
GLP-1 medications slow stomach emptying and reduce stomach acid production, both of which can affect how your body absorbs iron from food and supplements. This is especially important if you have existing low iron. Your prescriber may adjust your iron supplementation or recommend timing it separately from the medication. Discuss this with your GP.
What are the UK weight loss medication options?
Three prescription-only options are licensed in the UK: Mounjaro (once weekly, 2.5–15mg), Wegovy (once weekly, 0.25–2.4mg), and Saxenda (daily, 0.6–3mg). All require a prescription from a UK-licensed prescriber and clinical assessment. Access is through NHS specialist services or private online prescribing services registered with the GPhC.
How do I find out if I'm eligible for NHS weight loss medication?
Speak with your GP about weight management referral to a specialist tier 3 service. You're potentially eligible if your BMI is 30 or above, or 27–29.9 with weight-related health conditions like diabetes or high blood pressure. Be sure to mention your low iron — this won't necessarily exclude you but your GP needs to know to make a safe referral.
Can I use the NHS Weight Loss Plan while managing low iron?
Yes. The NHS Weight Loss Plan and self-help tools support calorie-controlled eating, which is appropriate for managing both weight and overall health. A reduced-calorie diet rich in iron-containing foods (red meat, leafy greens, pulses) can support weight management and iron levels simultaneously. Your GP or dietitian can advise on balancing weight loss with iron nutrition.
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.