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: Key Nutrition Considerations
When taking Mounjaro (tirzepatide) for weight management, vitamin and nutrient absorption can be affected due to how the medication slows digestion. Understanding these potential impacts is important for maintaining good health during your weight loss journey. This guide covers what you need to know about vitamins whilst taking Mounjaro, which nutrients may be at risk, and how to work with your GP to protect your nutritional status.
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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 Absorption
Mounjaro works by slowing gastric emptying—the rate at which food moves from your stomach into your small intestine. This is how it helps you feel fuller for longer. However, this slower digestion can affect how your body absorbs certain vitamins and minerals, particularly in the early weeks of treatment.
Nutrients most commonly affected include:
- Vitamin B12 — essential for nerve function and energy
- Iron — critical for oxygen transport and preventing anaemia
- Calcium — vital for bone health
- Vitamin D — supports calcium absorption and immune function
- Folate (B9) — important for cell division and DNA synthesis
The degree of absorption change varies between individuals. Most people experience reduced absorption gradually as the medication continues, though this effect often lessens over time as your body adapts. Your GP can assess your individual risk based on your baseline nutrition status and existing health conditions.
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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.
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Protecting Your Nutrition While on Mounjaro
Work with your GP to monitor your vitamin levels through regular blood tests, particularly if you have existing risk factors for deficiency (previous anaemia, vegetarian/vegan diet, or limited sun exposure). Taking supplements may be recommended—your GP can advise on timing and dosage.
Practical steps you can take:
- Eat nutrient-dense foods: lean proteins, leafy greens, fortified cereals
- Space oral medications and supplements at least 2–4 hours apart from Mounjaro doses
- Stay well hydrated to support digestive function
- Report persistent fatigue, weakness, or unusual symptoms to your GP
If you experience symptoms of deficiency—such as persistent tiredness, shortness of breath, or tingling—contact your GP promptly. They may recommend B12 injections, iron supplementation, or other interventions tailored to your needs.
- 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. Many people maintain adequate levels, especially if they eat a balanced diet. However, slower digestion can reduce absorption, so monitoring through blood tests is wise. Your GP can assess your individual risk and recommend preventative measures if needed.
Should I take vitamin supplements while on Mounjaro?
This depends on your baseline nutrition status and blood test results. Your GP may recommend supplementation for B12, iron, or vitamin D, particularly if you have existing risk factors. Never start high-dose supplements without professional guidance.
Can I take supplements at the same time as Mounjaro?
No. Mounjaro may reduce the absorption of oral medications and supplements. Space them at least 2–4 hours apart. Your GP can advise on the best timing for your specific supplements.
How often should I have blood tests while taking Mounjaro?
Your GP will recommend a schedule based on your individual health. Initial baseline tests are common before starting, then follow-up tests at 3–6 months and annually thereafter, depending on your risk factors and results.
What B12 symptoms should I watch for?
Persistent fatigue despite adequate sleep, shortness of breath during normal activity, tingling in hands or feet, difficulty concentrating, or unusual weakness. Report any of these to your GP, who can check your B12 level and discuss supplementation options.
Ready to take control of your weight loss journey?
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