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
Free tracker included with every download this week
Which Vitamins Should You Take While on Mounjaro?
When taking Mounjaro (tirzepatide), your body's nutritional needs may change due to reduced food intake and altered digestion. While Mounjaro can help with weight management, it's important to ensure you're still getting essential vitamins and minerals. This guide covers what you should know about supplementation whilst on this prescription medication in the UK, and when to speak to your GP or prescriber for personalised advice.
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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.
Nutritional Concerns with Mounjaro Treatment
Mounjaro works by reducing appetite and slowing how food moves through your stomach, which means you're likely eating significantly less. Whilst this supports weight loss, it can increase your risk of nutritional deficiencies if you're not eating a balanced diet. Key nutrients of concern include:
- B vitamins (especially B12 and folate) — needed for energy and nerve function
- Iron — critical for oxygen transport, particularly important for women
- Calcium and vitamin D — essential for bone health, especially during weight loss
- Protein — vital to preserve muscle mass during weight loss
- Vitamin C and zinc — support immune function when eating less
Your prescriber or GP can arrange blood tests to check your nutrient levels, particularly if you experience fatigue, weakness, or other symptoms. This is standard NHS practice for patients on weight loss medication.
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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
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
When and How to Supplement Safely
Before starting any vitamin supplements, always discuss with your GP or the prescriber who issued your Mounjaro KwikPen. Some supplements can interact with other medications or may not be necessary if your diet is well-balanced. Many people find a standard multivitamin covers basic nutritional gaps, but individual needs vary.
Focus first on eating nutrient-dense foods: lean proteins, leafy greens, colourful vegetables, whole grains, and healthy fats. If you struggle to eat enough protein or feel constantly fatigued, supplementation may help. Keep a food diary to show your GP — this helps identify specific deficiencies and guides personalised recommendations based on your health and current Mounjaro dose titration progress.
- 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 definitely need vitamin supplements while taking Mounjaro?
Not necessarily. If you eat a balanced, nutrient-dense diet and feel well, you may not need supplements. However, blood tests ordered by your GP can identify specific deficiencies. Many people benefit from a basic multivitamin as insurance, but your prescriber should advise based on your individual situation and symptoms.
Is B12 supplementation essential on Mounjaro?
B12 is important but not automatically essential for everyone. Some people absorb B12 poorly or have dietary restrictions (vegetarian/vegan diets are naturally lower in B12). Your GP can test your B12 levels and recommend supplementation if needed. Injections may be suggested if absorption is poor.
Can vitamin supplements interact with Mounjaro or other medications?
Some supplements can interact with other medicines you're taking. Always inform your GP or prescriber of all supplements and medications. Certain supplements may also affect how quickly food moves through your stomach, which Mounjaro already slows — your prescriber needs to know about these to manage them safely.
What should I do if I feel fatigued or weak while on Mounjaro?
Fatigue can indicate nutritional deficiency, insufficient protein intake, or dehydration. Contact your GP — they can arrange blood tests to check iron, B12, thyroid function, and other markers. Many patients find symptoms improve with better nutrition, adequate hydration, and sometimes targeted supplementation based on test results.
Is it safe to take supplements bought from shops whilst on prescription Mounjaro?
Over-the-counter supplements are generally safe, but quality and strength vary between brands. Discuss any supplements with your GP before starting, especially if you take other medications. The NHS recommends using products from reputable manufacturers and avoiding excessive doses of any nutrient.
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.