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 and Vitamins: What You Need to Know
When taking Mounjaro (tirzepatide), your body's ability to absorb certain vitamins and minerals can be affected. Because Mounjaro slows gastric emptying—how quickly food moves through your stomach—it may impact nutrient absorption and increase the risk of deficiencies over time. This is particularly important to discuss with your GP or prescriber before starting Mounjaro, as the NHS recommends monitoring and supplementation for some patients on weight loss medications.
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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 Mounjaro Affects Vitamin and Mineral Absorption
Mounjaro (tirzepatide), a dual GIP and GLP-1 receptor agonist, works by slowing gastric emptying—the rate at which food leaves your stomach. This mechanism, which helps support weight management, can also reduce how efficiently your body absorbs certain nutrients from food.
Vitamins and minerals most commonly affected include:
- Vitamin B12 — absorption requires intrinsic factor and stomach acid; delayed gastric transit may reduce uptake
- Iron — absorption is optimised in the stomach and upper intestine; slower transit may decrease absorption efficiency
- Calcium — similarly dependent on stomach acidity and transit time
- Folate — may be less efficiently absorbed with prolonged gastric emptying
Your prescriber should assess your baseline nutrient status before starting the UK prescription for Mounjaro dose titration, and monitor you regularly throughout treatment. If you experience fatigue, shortness of breath, or other symptoms of deficiency, inform your GP immediately.
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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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NHS Guidance and Vitamin Supplementation Recommendations
The NHS advises that adults on weight loss medications, including Mounjaro and other GLP-1 therapies, should have baseline blood tests to check B12, iron, folate, and calcium levels before starting treatment. Many patients may require supplementation, particularly with B12, which is often given as injections or high-dose oral supplements to bypass absorption issues.
Your GP or specialist prescriber (typically at a tier 3 weight management service) should discuss supplementation needs with you individually. Regular monitoring—typically every 6–12 months—is recommended to catch deficiencies early. Use our free GLP-1 weight loss calculator to understand your treatment pathway, and discuss supplement needs openly with your healthcare team before starting any new weight loss injection.
- 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 I definitely develop vitamin deficiencies on Mounjaro?
Not necessarily. Risk varies by individual based on diet quality, baseline nutrient status, and how long you take Mounjaro. However, the NHS recommends baseline testing and ongoing monitoring to catch deficiencies early and prevent symptoms like fatigue or nerve damage.
Should I start taking vitamins before or after starting Mounjaro?
Have a blood test and discuss with your GP or prescriber first. They'll advise whether you need supplementation based on your baseline levels. Starting unnecessary supplements wastes money; targeted supplementation based on test results is more effective.
Is vitamin B12 supplementation essential for everyone taking Mounjaro?
Not everyone requires it, but B12 deficiency is common on long-term weight loss medications. Your prescriber will assess your risk based on your diet (B12 is mainly in animal products) and recommend B12 injections or high-dose oral supplements if needed.
How often should I have blood tests checked while on Mounjaro?
The NHS typically recommends baseline testing before starting, then repeat testing at 6 months and annually thereafter. More frequent monitoring may be needed if you have risk factors like pernicious anaemia or dietary restrictions.
Can I take vitamins at the same time as my Mounjaro injection?
Oral medications and supplements may not be absorbed as well while on Mounjaro due to delayed gastric emptying. Discuss timing with your GP—taking supplements several hours apart from Mounjaro may improve absorption.
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.