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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Vitamins and Nutrient Absorption on Mounjaro
Taking Mounjaro (tirzepatide) can affect how your body absorbs certain vitamins and nutrients, especially as the medication slows gastric emptying and changes how quickly food moves through your digestive system. This means some people may benefit from vitamin supplementation or more frequent nutritional monitoring whilst on treatment. This guide explains what you need to know about vitamins on Mounjaro, when supplementation might be needed, and how to discuss this with your GP or prescriber.
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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 Vitamin and Nutrient Absorption
Mounjaro works partly by slowing gastric emptying — the rate at which food moves from your stomach into your small intestine. This effect typically reduces over time, but in the early weeks and at higher doses, it can significantly slow digestion. A slower digestive process means nutrients may have less time to be absorbed, and reduced appetite means you may consume fewer calories and a narrower range of foods overall.
Key nutrients at risk of deficiency on GLP-1 and dual GIP/GLP-1 medications include:
- Vitamin B12 — required for nerve function and energy; absorption occurs in the terminal ileum, and rapid transit can reduce uptake
- Iron — especially important for women of reproductive age; reduced food intake and stomach acid changes may impair absorption
- Calcium — crucial for bone health; may be less absorbed with reduced dairy or food intake
- Folate — essential during reproductive years; reduced fruit and vegetable intake increases deficiency risk
- Vitamin D — fat-soluble vitamin; reduced fat intake or reduced sun exposure during weight loss phase may lower levels
The UK Medicines and Healthcare products Regulatory Agency (MHRA) recommends that people on Mounjaro have their nutritional status monitored, particularly during the first year of treatment.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Do You Need Vitamins Whilst Taking Mounjaro?
Whether you need vitamin supplementation depends on your individual diet, baseline nutritional status, and how your body responds to Mounjaro. Your GP or prescriber should assess you before recommending supplements. Some people maintain adequate nutrition through a diverse, balanced diet; others benefit from targeted supplementation.
Your prescriber should:
- Review your baseline vitamin and mineral levels if possible
- Assess your current diet and eating patterns
- Monitor you regularly (at least annually, or more frequently if deficiency risk is identified)
- Recommend specific supplements only if there is evidence of deficiency or high risk
- Ensure any supplements do not interact with Mounjaro or other medications
Do not start high-dose supplements without guidance — excessive intake of some vitamins (especially fat-soluble vitamins A, D, E, K) can be harmful. A balanced multivitamin may be appropriate for some, but this should be discussed with your prescriber or a registered dietitian.
- 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.
Does Mounjaro cause vitamin deficiency?
Mounjaro can increase the risk of deficiency in certain nutrients, particularly B12, iron, calcium, folate, and vitamin D, due to slower digestion and reduced food intake. However, deficiency is not assured — it depends on your individual diet and baseline nutritional status. Regular monitoring by your GP or prescriber is recommended.
Should I take a multivitamin whilst on Mounjaro?
A multivitamin may be appropriate, but only if recommended by your prescriber or a registered dietitian. Taking supplements without medical guidance can lead to excessive intake of some vitamins, which carries health risks. Your prescriber will assess your individual need based on your diet and blood results.
How does Mounjaro compare to Wegovy when it comes to vitamin absorption?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are GLP-1 type medications that slow gastric emptying and reduce appetite, so both carry similar risks of nutrient malabsorption. Mounjaro is a dual GIP/GLP-1 agonist whilst Wegovy is GLP-1 only, but regarding vitamin absorption, the mechanisms are similar. Your prescriber will monitor you on either medication.
Can I take vitamin supplements at the same time as Mounjaro?
Most oral supplements can be taken alongside Mounjaro, but timing matters. Some vitamins (especially iron and calcium) are better absorbed on an empty stomach, whilst others are fat-soluble and require food. Ask your prescriber or pharmacist about the best timing for your specific supplements to maximise absorption.
What should I do if blood tests show low vitamin levels on Mounjaro?
Inform your GP or prescriber immediately. They may recommend targeted supplementation, dietary changes (such as increasing specific food sources), or dose adjustment of Mounjaro. B12 deficiency, for example, may be a UK pharmacy with injections rather than oral supplements, as absorption via the digestive tract is compromised.
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