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 I Take While on Mounjaro?
If you're taking Mounjaro (tirzepatide) for weight management, vitamin and mineral supplementation may become important because this GLP-1 receptor agonist can affect how your body absorbs nutrients from food. Since Mounjaro slows gastric emptying and reduces overall food intake, your nutritional needs don't disappear — they become more critical. This UK guide explains what you should consider, when to speak to your GP, and how to support your health while using this prescription weight loss injection.
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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 Nutrient Absorption
Mounjaro works by slowing how quickly food moves through your stomach, which can initially improve nutrient absorption. However, because the medication also significantly reduces appetite and food intake, you're consuming fewer calories and potentially fewer nutrients overall. This matters particularly for vitamin B12, iron, calcium, and folate — nutrients often obtained from foods that may become less appealing or be eaten in smaller quantities while taking tirzepatide.
Research on GLP-1 receptor agonists has raised questions about potential effects on nutrient absorption, particularly concerning vitamin B12 levels over extended use. Some people experience deficiency symptoms including fatigue, tingling in extremities, or anaemia. Additionally, reduced food intake can lower intake of essential minerals like calcium and magnesium, which is particularly important if you have risk factors for osteoporosis.
- B12 deficiency is most commonly reported with long-term GLP-1 use
- Reduced food volume means fewer nutrients from whole foods
- Certain medications taken alongside Mounjaro (like metformin for diabetes) can also lower B12
- Your individual risk depends on baseline nutrition and diet quality
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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
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What to Do: Nutrition Planning and Your GP
Before starting Mounjaro and during treatment, speak to your GP or a registered dietitian about your nutritional needs. They may recommend baseline blood tests to check B12, folate, iron, and calcium levels — particularly important if you have risk factors for deficiency. Your GP can then advise whether supplementation is appropriate for you personally.
Generally, a high-quality daily multivitamin may be sensible as nutritional insurance, but this must be discussed with your healthcare provider. Focus on eating nutrient-dense foods when appetite allows — lean proteins, green vegetables, eggs, and fortified dairy support key nutrients. If you're taking oral medications alongside Mounjaro, check with your GP about timing: take supplements at least 2–4 hours away from your Mounjaro injection to avoid any potential absorption interference. Remember that individual nutrition needs vary significantly based on your health history, diet, and how your body responds to tirzepatide.
- 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 a multivitamin while on Mounjaro?
Not necessarily — it depends on your individual diet, health status, and baseline nutritional levels. Speak to your GP or dietitian. They may recommend baseline blood tests to assess your B12, folate, iron, and calcium levels. If you're eating well-balanced meals despite reduced appetite, you may not need supplements. If your diet is limited or you have risk factors for deficiency, a daily multivitamin may be recommended.
Can Mounjaro cause vitamin B12 deficiency?
B12 deficiency has been reported in some people taking GLP-1 receptor agonists over extended periods. This may relate to reduced food intake, potential effects on stomach acid production, or the medication's mechanism. Risk is higher if you're also taking metformin. Your GP can monitor your B12 levels with blood tests and recommend B12 supplementation or injections if needed. Symptoms include fatigue, tingling, or neurological changes.
When should I take vitamin supplements around my Mounjaro injection?
Take supplements at least 2–4 hours away from your Mounjaro injection to avoid potential interference with medication absorption. If you're taking oral medications alongside Mounjaro (such as diabetes medications or oral contraceptives), spacing is particularly important. Your GP can advise on specific timing for your personal medication schedule.
Which vitamins and minerals are most important while taking Mounjaro?
The key nutrients to monitor are vitamin B12, folate, iron, calcium, and magnesium. B12 is most commonly flagged in research on GLP-1 medications. Calcium and magnesium support bone health, particularly important given reduced food intake. Iron is important if you're at risk of anaemia. Your GP can advise on which are most relevant for your individual health profile.
Should I take calcium and vitamin D supplements on Mounjaro?
This depends on your dietary calcium intake and baseline vitamin D levels. Because Mounjaro reduces overall food intake, including calcium-rich foods, some people benefit from supplementation. Vitamin D is essential for calcium absorption. Ask your GP to check your baseline levels with a blood test. They may recommend supplements if you have risk factors for osteoporosis or low vitamin D, particularly if you're postmenopausal or have limited sun exposure.
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