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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Best Vitamins While On Mounjaro
If you're taking Mounjaro (tirzepatide) for weight management, you may be wondering whether vitamin supplements are necessary. Mounjaro can affect how your body absorbs certain nutrients, and reduced food intake means you may consume fewer vitamins and minerals from food. This guide covers which vitamins may be important while on this GLP-1 medication, how to take them safely alongside Mounjaro, and when to speak to your GP about supplementation. Always consult your healthcare professional before starting any new supplements.
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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.
Nutrient Deficiencies With GLP-1 Medications
Mounjaro and other GLP-1 agonist medications can increase the risk of certain nutrient deficiencies for several reasons. First, reduced food intake means you consume fewer calories and less food overall, which directly lowers your intake of vitamins and minerals. Second, slowed gastric emptying (how quickly food moves through your stomach) may affect nutrient absorption in some cases. Third, Mounjaro may increase certain metabolic demands.
The vitamins most commonly affected include:
- Vitamin B12 — essential for energy and nerve function; deficiency can cause fatigue and neuropathy
- Iron — critical for oxygen transport; deficiency leads to anaemia
- Calcium — vital for bone health; long-term deficiency increases fracture risk
- Vitamin D — supports calcium absorption and immune function; deficiency is common in the UK
- Folate (B9) — important for cell function and DNA synthesis
- Magnesium — supports muscle and nerve function
Your GP may recommend blood tests to check your baseline nutrient levels before starting Mounjaro, particularly if you have a history of anaemia, osteoporosis, or pernicious anaemia.
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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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Vitamin Supplementation While On Mounjaro
If your GP identifies nutrient deficiencies or recommends preventative supplementation, timing and formulation matter. Take most vitamin supplements with food to improve absorption and reduce stomach upset — though Mounjaro may slow digestion, food-based absorption still works. Avoid taking supplements immediately before or after your weekly Mounjaro injection, as your stomach may be more sensitive.
B12 is particularly important: if you develop deficiency, your GP may recommend either oral B12 supplements with intrinsic factor, B12 injections (intramuscular), or sublingual B12 tablets that bypass stomach absorption. Iron supplements should be taken separately from calcium, magnesium, and caffeine — these reduce iron absorption. Always inform your GP about all supplements you're taking, as some may interact with other medications you use alongside Mounjaro.
- 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 a multivitamin while on Mounjaro?
Not necessarily. A daily multivitamin may be helpful if your diet is very restricted, but it's not automatically needed for everyone on Mounjaro. Your GP can order blood tests to check your nutrient levels and recommend specific supplements only if deficiencies are found. Over-supplementation of some vitamins (like fat-soluble vitamins A, D, E, K) can be harmful.
Can I take vitamin supplements with my weekly Mounjaro injection?
Yes, but ideally not on the same day. Take your injected Mounjaro on your scheduled day, then resume normal supplementation the following day. This reduces stomach irritation and allows your digestive system to stabilise. If you take daily supplements, continue them as normal on non-injection days.
Is B12 deficiency common with Mounjaro?
B12 deficiency is possible with any GLP-1 medication due to reduced food intake, particularly if you reduce animal product consumption. It's not assured, but your GP may recommend B12 screening after 6–12 months on Mounjaro, especially if you experience fatigue or tingling in your fingers and toes.
How much Vitamin D should I take on Mounjaro in the UK?
The NHS recommends that UK adults consider a daily supplement of 10 micrograms (400 IU) of Vitamin D during autumn and winter, or year-round if you have limited sun exposure. While on Mounjaro, this recommendation remains the same — discuss your specific needs with your GP based on your calcium intake and bone health history.
Can supplements interfere with Mounjaro's effectiveness?
Standard vitamin supplements do not reduce Mounjaro's weight loss effect. However, some medications and supplements can interact with other drugs you're taking. Always tell your GP and prescriber about all supplements, herbs, and over-the-counter products you use — they'll advise whether any adjustments are needed.
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