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 Injections and B12 — What You Need to Know
B12 injections are vitamin supplements, not weight loss medications. If you've seen "B12 weight loss injections" advertised online, it's important to understand the difference between vitamin injections and prescription weight management drugs like Wegovy (semaglutide) and Mounjaro (tirzepatide). This guide clarifies what B12 actually does, how it differs from licensed weight loss injections in the UK, and what evidence says about their effectiveness for losing weight.
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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.
B12 Injections vs. Prescription Weight Loss Injections
B12 injections are vitamin B12 supplements used to treat B12 deficiency — they are not weight loss medications. B12 is essential for energy, red blood cell formation, and nerve function. Some private clinics in the UK promote B12 injections alongside diet and exercise, claiming they boost metabolism and energy. However, there is no clinical evidence that B12 injections alone cause weight loss in people with normal B12 levels.
Prescription weight loss injections — such as Wegovy (semaglutide) and Mounjaro (tirzepatide) — are GLP-1 and GIP/GLP-1 receptor agonists. These are licensed weight management medications that work by reducing appetite, helping you feel fuller after smaller meals, and slowing gastric emptying. Clinical trials show these may help participants lose an average of 15–21% of their body weight. Unlike B12, these require a prescription following clinical assessment by a UK-licensed prescriber.
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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.
Should You Consider B12 if You're Deficient, and What About Weight Loss?
If your GP identifies a genuine B12 deficiency (via blood test), B12 injections can improve energy, mood, and neurological function — which may indirectly support weight management by reducing fatigue. However, B12 alone does not cause weight loss.
For prescription weight management in the UK, adults with a BMI of 30 or above — or 27 or above with weight-related health conditions — may be eligible for licensed weight loss medication. Use our GLP-1 weight loss calculator to estimate potential weight loss with prescription options. Your GP or a licensed prescriber can assess whether weight loss medication is appropriate for you following a proper clinical assessment.
Why B12 Weight Loss Claims Are MisleadingSome private clinics bundle B12 injections with diet plans or combine them with other substances (like amino acids or lipotropics), then attribute weight loss to the injections. In reality, any weight loss is almost always due to calorie deficit and lifestyle changes, not the B12 itself. The NHS recommends losing weight at a rate of 0.5–1 kg per week through sustainable dietary changes and increased physical activity. If you're considering weight loss treatment, check your BMI using our free BMI calculator and speak to your GP about evidence-based options rather than unproven supplement combinations.
- 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 B12 a weight loss injection?
No. B12 is a vitamin supplement used to treat B12 deficiency. It is not a weight loss medication. While B12 may improve energy if you're deficient, there is no clinical evidence it causes weight loss in people with normal B12 levels. Licensed weight loss injections like Wegovy and Mounjaro are prescription medications with clinical trial evidence.
Can B12 boost my metabolism for weight loss?
B12 supports energy production and nerve function, but it does not significantly boost metabolism or cause weight loss. If you have low B12, correcting the deficiency may help you feel more energetic, which could indirectly support activity. However, weight loss results from calorie deficit and lifestyle changes, not from B12 alone.
What's the difference between B12 and Wegovy or Mounjaro?
B12 is a vitamin; Wegovy and Mounjaro are prescription weight management medications. Wegovy (semaglutide) and Mounjaro (tirzepatide) are GLP-1/GIP receptor agonists with clinical trial evidence showing an average weight loss of 15–21%. They require prescription and clinical assessment. B12 is an over-the-counter supplement with no weight loss mechanism.
Should I get B12 injections if I want to lose weight?
Only if your GP confirms you have a B12 deficiency. If your B12 levels are normal, injections won't help with weight loss. For prescription weight loss treatment, your BMI, health conditions, and individual circumstances will be assessed by a prescriber. Use our free BMI calculator to check your starting point.
Where can I get legitimate weight loss injections in the UK?
Licensed weight loss injections (Wegovy, Mounjaro, Saxenda) are available through NHS specialist weight management services or private UK-licensed prescribers following clinical assessment. Always ensure any prescriber is UK-licensed and that the medication is prescribed legitimately — be cautious of clinics promoting unproven supplement combinations.
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.