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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Vitamins and Nutrient Support on Mounjaro
If you're taking Mounjaro (tirzepatide) for weight management, you may wonder whether vitamin and mineral supplementation is necessary. Mounjaro can affect how your body absorbs certain nutrients because it slows gastric emptying — the rate at which food moves through your stomach. This guide explains what you need to know about vitamins on Mounjaro, potential deficiency risks, and when to speak with your GP about supplementation. Individual needs vary, so personalised advice from your healthcare provider is essential.
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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.
Nutrient Absorption and Mounjaro Treatment
Mounjaro works partly by slowing how quickly food passes through your digestive system. While this helps with appetite control, it can also affect nutrient absorption over time, particularly if you're following a reduced-calorie diet alongside the injection. Key nutrients at potential risk include:
- Vitamin B12 — essential for energy and nerve function
- Iron — critical for oxygen transport in blood
- Calcium — important for bone health
- Vitamin D — supports calcium absorption and immunity
- Folate — needed for cell division and DNA synthesis
The extent of absorption changes varies between individuals. Some people experience minimal impact, whilst others on lower-calorie diets may develop deficiencies over months. Your GP can assess your individual risk through blood tests and recommend supplementation if needed. Never self-diagnose deficiencies — symptoms like fatigue or hair loss can have multiple causes.
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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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When to Consider Vitamin Supplementation
Not everyone on Mounjaro needs supplements, but certain situations increase the risk of deficiency. If you're following a very low-calorie diet alongside tirzepatide, eating a restricted range of foods, or have a history of nutrient absorption problems, supplementation may be appropriate. Speak with your GP about supplementing if:
- You've been on Mounjaro for several months and notice fatigue or weakness
- You're following a calorie-restricted diet (under 1200 kcal/day)
- You have dietary restrictions (vegan, gluten-free, or limited food intake)
- You're experiencing hair loss or skin changes
- Blood tests show low nutrient levels
Your prescriber may recommend a basic multivitamin, B12 supplementation, or iron depending on your diet and symptoms. Take supplements as directed — timing and dosage matter for optimal absorption.
- 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 vitamins while on Mounjaro?
Yes, vitamin supplementation is generally safe with Mounjaro. However, timing matters — take oral supplements at least 4 hours apart from other medications if possible, as Mounjaro may reduce absorption of some oral medicines. Always inform your GP about any supplements you're taking.
Should I take B12 supplements on Mounjaro?
B12 deficiency is a potential risk with GLP-1 and tirzepatide medications, particularly on extended use or low-calorie diets. Your GP can check B12 levels through a blood test and recommend supplementation if needed. Some people benefit from B12 injections or sublingual tablets for better absorption.
Does Mounjaro cause vitamin deficiencies?
Mounjaro doesn't directly cause deficiencies, but it slows gastric emptying and encourages eating less food, which together can reduce nutrient intake and absorption over time. This risk is higher with very low-calorie diets. Regular monitoring by your GP helps catch deficiencies early.
What about iron supplements with Mounjaro?
Iron can be taken with Mounjaro, though absorption may be reduced due to slower digestion. Iron supplements work best on an empty stomach but may cause stomach upset. Discuss timing and dosage with your GP. Some people find that taking iron with vitamin C improves absorption.
Will my GP test for nutrient deficiencies?
Your prescriber should monitor your health regularly whilst on Mounjaro. If you report symptoms like persistent fatigue, weakness, or hair loss, ask for blood tests to check B12, iron, calcium, and vitamin D levels. This helps identify deficiencies early and guide supplementation decisions.
Ready to take control of your weight loss journey?
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