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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Which Vitamins Should You Take While On Mounjaro?
Mounjaro (tirzepatide) can affect how your body absorbs nutrients because it slows gastric emptying and may reduce appetite. This means you might eat smaller portions and absorb fewer nutrients from food. In the UK, your GP or prescribing service should assess your nutritional status before and during treatment. Certain vitamins and minerals—particularly B12, iron, calcium, and vitamin D—require attention. This guide covers what the evidence suggests about vitamin supplementation while taking Mounjaro, and when to speak with your healthcare provider about your individual needs.
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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 Risks and Vitamin Deficiencies on Mounjaro
Mounjaro works by reducing appetite and slowing how quickly food moves through your stomach. This can lead to reduced food intake and, over time, potential nutrient gaps. Key vitamins and minerals at risk of deficiency include:
- Vitamin B12: Absorption happens mainly in the stomach; reduced food intake and slower digestion may lower B12 levels, causing fatigue and neuropathy in severe cases
- Iron: Low stomach acid from reduced appetite may impair iron absorption, particularly important if you're menstruating
- Calcium and Vitamin D: Essential for bone health; reduced food intake may lower intake, especially critical as you lose weight
- Folate: Important for cell division and energy; deficiency is possible with significantly reduced food consumption
Your GP can arrange blood tests to check your levels before starting Mounjaro and periodically during treatment. The NHS may recommend supplementation based on your individual results, not routine supplementation for everyone.
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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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NHS Guidance on Supplementation With Mounjaro
The UK's NICE guidance and NHS specialist tier 3 weight management services do not recommend routine vitamin supplementation for all Mounjaro users. Instead, the approach is personalised: your prescriber will typically arrange baseline blood tests (including B12, iron, folate, and vitamin D) before you start, then repeat testing at intervals during treatment. If deficiencies are found, your GP can prescribe appropriate supplementation. Many people eating a varied diet with adequate portions may not need supplements, but those with significantly reduced intake, existing deficiencies, or specific health conditions may benefit. Always discuss any supplements you're considering—including over-the-counter multivitamins—with your GP or prescriber, as some supplements can interact with other medications or affect the absorption of tirzepatide itself.
- 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 automatically when starting Mounjaro in the UK?
No. The NHS approach is personalised testing, not routine supplementation. Your GP will arrange blood tests before and during treatment to identify any deficiencies. Vitamins are prescribed only if testing shows a need. Many people maintain adequate nutrition with smaller, balanced meals and don't require supplements.
Which vitamin is most important to monitor on Mounjaro?
Vitamin B12 is particularly important because reduced food intake and slower gastric movement can impair absorption. Low B12 causes fatigue, neuropathy, and cognitive issues. Your GP should check B12 levels before starting and during treatment, especially if you're on Mounjaro long-term.
Can I take over-the-counter multivitamins while on Mounjaro?
Yes, but inform your GP or prescriber first. Some supplements may interact with Mounjaro or other medications, and some may not be absorbed as effectively due to slower gastric emptying. Your prescriber can advise whether over-the-counter supplements are necessary based on your test results.
How often should I have blood tests to check vitamin levels on Mounjaro?
Baseline testing should occur before starting Mounjaro. Follow-up testing depends on your individual situation and prescriber's recommendations, typically at 3–6 months, then annually. If you have symptoms of deficiency (fatigue, numbness, or weakness), contact your GP sooner.
Does eating less on Mounjaro automatically mean I'll develop vitamin deficiencies?
Not necessarily. If you eat smaller portions of nutrient-rich foods—including protein, vegetables, whole grains, and dairy—you can maintain adequate nutrition. Deficiency risk increases with very low intake or poor food quality. Blood tests are the only accurate way to know your status.
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