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?
Taking Mounjaro (tirzepatide) may affect how your body absorbs certain nutrients, making supplementation important for some people. Because Mounjaro slows gastric emptying and can reduce appetite, vitamin and mineral deficiencies are a real concern for long-term users. This guide explains which vitamins matter most whilst on this prescription-only weight management injection, how to discuss supplementation with your GP, and how Mounjaro's nutrient impact compares to Wegovy. Always consult your prescriber before starting any new supplements — they can assess your individual needs based on blood work and your medical history.
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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.
Nutrient Concerns With Mounjaro vs Wegovy
Mounjaro (tirzepatide) and Wegovy (semaglutide) both slow how quickly food moves through your stomach, which can reduce nutrient absorption over time. However, because Mounjaro is a dual GIP and GLP-1 receptor agonist (compared to Wegovy's GLP-1 mechanism only), some users report more pronounced appetite suppression, potentially increasing deficiency risk with long-term use.
Key nutrients at risk include:
- Vitamin B12 — reduced intrinsic factor production; many Mounjaro users benefit from supplementation or injections
- Iron — slower gastric transit reduces absorption; women of reproductive age are at particular risk
- Calcium — decreased stomach acid may impair uptake; important for bone health
- Vitamin D — works synergistically with calcium; many UK adults have insufficient levels already
- Folate and B vitamins — reduced appetite may mean lower intake from food sources
Neither Mounjaro nor Wegovy directly causes deficiencies, but the altered digestion and reduced food intake mean your GP should monitor your nutritional status with blood tests, particularly if you experience nausea or reduced appetite for prolonged periods.
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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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Supplementation Strategy: Personal Assessment Required
The right vitamin strategy depends on your individual baseline health, how long you've been on Mounjaro, your blood work results, and your diet quality. Your prescriber or GP should order baseline bloods (B12, iron, folate, vitamin D, calcium) before or shortly after starting treatment, then repeat annually or if symptoms develop. Some users need supplementation from the start; others maintain adequate levels through diet alone.
If prescribed Mounjaro through the NHS, your specialist service can advise on supplementation. If obtained privately, discuss vitamins with the prescriber during your consultation. Never self-supplement aggressively without blood work — excess intake of certain fat-soluble vitamins (A, D, E, K) can accumulate in the body and cause harm. Our free GLP-1 weight loss calculator can help you project your weight loss journey, but your GP must address the nutritional side.
- 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. Some people maintain adequate nutrition through diet alone, whilst others develop deficiencies. Your GP should order baseline blood tests and repeat them annually to assess your individual needs. Never supplement without medical guidance — over-supplementation can be harmful.
Is vitamin supplementation different between Mounjaro and Wegovy users?
Both medications slow gastric emptying and can reduce nutrient absorption, so similar nutrients are at risk (B12, iron, calcium, vitamin D). Mounjaro's dual mechanism may cause more pronounced appetite suppression for some users, potentially increasing deficiency risk, but individual experience varies widely.
Should I get B12 injections whilst on Mounjaro?
Only if blood tests show low B12 levels. Some Mounjaro users benefit from regular B12 supplementation or injections, particularly after 6–12 months of treatment. Your GP can assess whether injections or oral supplements are appropriate based on your levels and intrinsic factor status.
Can I take my vitamins at the same time as Mounjaro?
Mounjaro may reduce absorption of oral medications and supplements because it slows gastric emptying. Discuss timing with your GP — taking supplements 2–3 hours before or after food intake may improve absorption, but individual advice varies.
What blood tests should I ask my GP for before starting Mounjaro?
Request baseline testing for vitamin B12, folate, iron (ferritin and haemoglobin), vitamin D, and calcium. These cover the nutrients most likely to be affected by reduced food intake and slower digestion. Repeat tests annually or if you experience fatigue, weakness, or other deficiency symptoms.
Ready to take control of your weight loss journey?
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