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 Supplements When Taking Mounjaro
When taking Mounjaro (tirzepatide) or Wegovy (semaglutide) for weight management, questions about vitamins and supplements are common. Both medications slow gastric emptying — how quickly food moves through your stomach — which can affect how your body absorbs certain nutrients. This guide explains what you need to know about supplementation whilst on GLP-1 or GIP/GLP-1 medications, and why discussing vitamins with your prescriber is essential before starting treatment.
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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 and Wegovy Affect Nutrient Absorption
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist that slows gastric emptying — the rate at which food leaves your stomach. This effect typically reduces over time as your body adapts. Slowed stomach emptying can affect how efficiently your digestive system absorbs oral medications and some nutrients, particularly B vitamins, calcium, and iron.
Wegovy (semaglutide) works similarly as a GLP-1 agonist alone, producing the same gastric slowing effect. Both medications may reduce appetite significantly, which can mean eating smaller meals or less varied foods — potentially increasing your risk of nutritional gaps even without malabsorption concerns.
- Vitamin B12 absorption may be affected (especially if you have pernicious anaemia risk factors)
- Iron absorption can be reduced if taken orally with these medications
- Calcium absorption may be compromised, affecting bone health
- Folate and other B vitamins require adequate food intake for proper absorption
The timing of vitamin supplements relative to your Mounjaro or Wegovy dose matters — spacing them apart can help maximise absorption.
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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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Vitamin Supplementation: Mounjaro vs Wegovy Considerations
Unlike specific medications where direct comparison between Mounjaro and Wegovy shows clear differences in mechanism (tirzepatide's dual action versus semaglutide's single GLP-1 pathway), vitamin needs are largely similar on both medications. Individual requirements depend on your diet quality, the dose you're taking, how long you've been on treatment, and any pre-existing nutritional deficiencies.
Your prescriber should assess your nutritional status before starting either medication and monitor you during treatment. Some patients benefit from a baseline blood test checking B12, folate, iron, and vitamin D levels. If you're eating significantly less on Mounjaro or Wegovy, a broad multivitamin may help fill gaps, but this should be discussed with your GP or prescriber first. Always inform them of any supplements you're already taking — some vitamins can interact with other medications you may be on.
- 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.
Can I take vitamins while on Mounjaro?
Yes, you can take vitamins whilst on Mounjaro, but timing matters. Space vitamin supplements at least 2–4 hours away from your Mounjaro injection day if possible. Because Mounjaro slows gastric emptying, oral vitamins may be absorbed less efficiently. Discuss your current supplements with your prescriber before starting treatment — they can advise on appropriate supplementation based on your diet and individual needs.
Should I take B12 supplements on Mounjaro?
B12 absorption may be affected by Mounjaro's effects on gastric emptying. Some patients on long-term GLP-1 or GIP/GLP-1 therapy benefit from B12 supplementation, whilst others maintain adequate levels through diet. Ask your prescriber about baseline B12 testing before starting Mounjaro. If you're vegetarian, vegan, or have existing B12 deficiency risk, supplementation may be recommended. Sublingual or injected B12 bypasses gastric absorption issues.
Is vitamin supplementation different on Mounjaro vs Wegovy?
Both medications slow stomach emptying, so vitamin absorption concerns are similar. However, individual nutrient needs vary based on dose, treatment duration, and diet quality. Mounjaro's higher dose range (up to 15 mg) may have more pronounced effects on appetite and food intake than Wegovy's maximum (2.4 mg), potentially affecting nutritional intake. Your prescriber can assess whether your specific situation requires different supplementation on either medication.
What vitamins should I prioritise on GLP-1 medication?
Prioritise vitamins most at risk of reduced absorption: B12, folate, iron, and calcium. Vitamin D is also important, especially if you're eating less. However, don't self-supplement without guidance — excess iron or fat-soluble vitamins (A, D, E, K) can accumulate. A baseline blood test can reveal any pre-existing deficiencies, helping your prescriber recommend targeted supplementation rather than a scatter-gun approach.
Can I use the MyWeightCheck calculator to track nutritional changes?
Our progress tracker helps you monitor weight loss and health markers over time on Mounjaro or Wegovy. Tracking not only weight but also how you feel, energy levels, and any nutritional symptoms can help you and your prescriber identify whether supplementation is needed. Combine this with blood tests your GP recommends — together they give a complete picture of your nutritional status during treatment.
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