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 — What You Need to Know
When taking Mounjaro (tirzepatide) for weight loss, your body's nutritional needs may change. Because Mounjaro slows how quickly food moves through your digestive system and may reduce your appetite, you might eat less overall, which could affect vitamin and mineral intake. This guide explains what nutritional considerations matter when using this prescription-only injection, and when to discuss supplements with your GP. Your healthcare team can assess your individual nutritional status during treatment.
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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 Nutrition and Vitamin Absorption
Mounjaro is a dual GIP and GLP-1 receptor agonist that slows gastric emptying (how quickly food leaves your stomach). This effect typically reduces over time, but while it's active, you may experience:
- Reduced appetite and smaller portion sizes — meaning less food intake overall
- Changes to taste preferences — some people report food seeming less appealing
- Nausea or digestive changes — which may affect nutrient absorption temporarily
- Potential nutrient gaps — if calorie reduction is significant without planning nutritious meals
Unlike some medications, Mounjaro does not directly block vitamin or mineral absorption. However, eating less food naturally means less intake of vitamins and minerals unless you deliberately choose nutrient-dense options. Your GP or a registered dietitian can assess whether supplementation is appropriate based on your individual diet and health status.
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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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Which Vitamins and Minerals to Monitor
When taking Mounjaro, key nutrients to monitor include B vitamins (especially B12, which supports energy and nerve function), iron (crucial for oxygen transport), calcium (bone health), and vitamin D (immune function and bone health). Protein intake is particularly important during weight loss — aim for protein-rich foods at each meal to help preserve muscle mass. Fibre intake should also be gradual and well-hydrated given Mounjaro's digestive effects. Before starting any vitamin supplements, discuss with your GP or prescriber, as some supplements may interact with other medications. They may recommend blood tests to check nutrient levels, particularly if you're on Mounjaro long-term.
- 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.
Should I take vitamin supplements while on Mounjaro?
This depends on your individual diet and nutrient status. Before starting any supplements, discuss with your GP or the prescriber managing your Mounjaro treatment. They may recommend blood tests to check your vitamin and mineral levels, especially B12, iron, and vitamin D. Never self-prescribe high-dose supplements without medical guidance, as some may interact with other medications or excess amounts can be harmful.
Will Mounjaro stop my body absorbing vitamins from food?
Mounjaro does not directly block vitamin absorption. However, it slows gastric emptying and may reduce appetite, so you eat less overall. This can naturally reduce your total vitamin and mineral intake from food. The key is choosing nutrient-dense meals even in smaller portions. Foods like eggs, fish, leafy greens, nuts, and lean meat pack vitamins and minerals into smaller servings.
Can I use my Mounjaro KwikPen while taking vitamin supplements?
Yes, in most cases. Mounjaro itself typically does not directly interact with standard vitamin supplements. However, some medications taken alongside Mounjaro (such as diabetes medicines or oral contraceptives) may interact with certain supplements. Always inform your prescriber of all supplements and medications you take, and check for potential interactions with your pharmacist or GP.
Is protein intake important when taking Mounjaro?
Yes. During weight loss with Mounjaro, adequate protein helps preserve muscle mass and keeps you feeling fuller longer. Aim for protein at each meal — examples include Greek yoghurt, eggs, fish, chicken, lentils, and tofu. A registered dietitian can give personalised protein targets based on your weight and activity level. Using our GLP-1 weight loss calculator can help you plan realistic nutrition goals alongside Mounjaro treatment.
Should I take B12 supplements with Mounjaro?
Not automatically. Only take B12 supplements if your GP recommends it after blood tests or if you have known B12 deficiency. Some people on weight loss medications benefit from B12 support, but individual needs vary. Your prescriber or a registered dietitian can assess whether B12 supplementation is right for you based on your diet, age, and any absorption issues.
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