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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What Vitamins Should You Take With Mounjaro?
If you're taking Mounjaro (tirzepatide) as a prescription-only weight management injection in the UK, you may be wondering whether vitamin supplements are necessary. Mounjaro can affect how your body absorbs nutrients and may increase the risk of certain deficiencies, particularly vitamin B12. This guide explains what you need to know about vitamin support whilst using Mounjaro, based on NHS and clinical guidance. Always discuss any supplements with your prescriber before starting them.
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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.
Nutrient Deficiency Risks With Mounjaro
Mounjaro slows gastric emptying—how quickly food moves through your stomach—which may affect nutrient absorption over time. Several vitamins and minerals are at potential risk of deficiency, particularly when following a reduced-calorie diet alongside the injection.
- Vitamin B12: Most commonly affected. B12 is absorbed in the stomach, and slower gastric transit may reduce absorption. Long-term deficiency can cause fatigue, nerve problems, and anaemia.
- Iron: Reduced stomach acid from slower digestion may lower iron absorption, particularly important for women of reproductive age.
- Calcium: Slower transit through the small intestine may reduce calcium uptake, affecting bone health long-term.
- Vitamin D: Fat-soluble vitamin absorption may be reduced; deficiency increases fracture risk.
Your GP or prescriber can arrange blood tests to check your baseline nutrient levels and monitor them during treatment.
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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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Recommended Vitamin Support While Taking Mounjaro
The NHS does not currently have specific mandatory vitamin supplementation guidance for Mounjaro users, but many prescribers recommend baseline blood tests to check B12, iron, folate, and vitamin D levels before starting tirzepatide. If deficiencies are found, supplementation is advised. A daily vitamin B12 supplement (1000 mcg sublingual or cyanocobalamin) may be offered as a precautionary measure. Discuss with your prescriber which supplements suit your individual situation. Use our GLP-1 weight loss calculator to understand your treatment timeline—longer-term use increases deficiency risk. Combine supplementation with a nutrient-rich diet including protein-rich foods, leafy greens, and fortified products to support your health during weight loss.
- 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.
Do I need to take vitamins while on Mounjaro?
Not necessarily mandatory, but blood tests are recommended before starting Mounjaro to check your baseline B12, iron, calcium, and vitamin D levels. If deficiencies are detected, your prescriber will recommend specific supplements. Many clinicians offer B12 supplementation as a precaution, particularly for long-term use.
Should I take B12 supplements with Mounjaro?
B12 absorption is most commonly affected by Mounjaro due to slower gastric emptying. Your prescriber may recommend a daily B12 supplement (typically 1000 mcg sublingual form) or monitor your levels with regular blood tests. This is especially important if you're vegetarian, vegan, or have existing risk factors for deficiency.
Can I take supplements with my Mounjaro injection?
Most oral supplements can be taken with Mounjaro, but timing matters. Some medications and supplements are less well absorbed during slower gastric transit. Take supplements at different times from your weekly injection and with food where possible. Always inform your prescriber of any supplements you're using.
What blood tests does the NHS recommend before Mounjaro?
The NHS typically recommends baseline tests for vitamin B12, folate, iron, and vitamin D levels before starting tirzepatide. Your GP can arrange these through NHS pathology services. Repeat testing at 6–12 months may be offered to monitor for emerging deficiencies, depending on your prescriber's assessment.
Are there foods I should eat more of on Mounjaro to avoid deficiencies?
Yes. Focus on nutrient-dense, protein-rich foods: eggs, fish, lean meats, legumes, fortified cereals, leafy greens, nuts, and seeds. B12-fortified plant-based products are important if you're vegetarian or vegan. Adequate protein intake also helps preserve muscle mass during weight loss. Your prescriber or a dietitian can provide personalised nutrition guidance.
Ready to take control of your weight loss journey?
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