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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Do You Need Extra Vitamins While Taking Mounjaro?
If you're taking Mounjaro (tirzepatide) for weight management in the UK, you may wonder whether you need additional vitamin supplements. Mounjaro can affect how your body absorbs nutrients because it slows gastric emptying—the rate at which food moves through your stomach. This is particularly important during the early weeks of treatment or at higher doses. Your GP or the prescriber managing your Mounjaro treatment can assess your individual nutrient needs and recommend supplements if necessary. This guide covers what you need to know about vitamins and Mounjaro within an NHS and UK context.
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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.
How Mounjaro Affects Nutrient Absorption
Mounjaro's mechanism and nutrient impact: Mounjaro works as a dual GIP and GLP-1 receptor agonist, which slows how quickly food passes through your stomach. While this effect typically reduces over time as your body adapts, it can initially affect nutrient absorption, particularly for vitamins and minerals that require adequate stomach acid and normal transit time to be absorbed properly.
During weight loss itself, your body also uses stored nutrients to support metabolism, which can deplete certain micronutrients. Common nutrient deficiencies to monitor while on any weight loss treatment include:
- B vitamins (especially B12, folate, and thiamine) — crucial for energy and nervous system function
- Iron — particularly important for women of reproductive age
- Calcium and vitamin D — essential for bone health, especially if losing weight quickly
- Vitamin C — supports immune function and collagen synthesis
Your NHS GP or prescriber should discuss whether baseline screening is appropriate before starting Mounjaro, and whether supplementation would benefit you based on your diet and health history.
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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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Timing and Safety: Taking Vitamins with Mounjaro
If your prescriber recommends vitamins or mineral supplements, timing is important. Take oral medications and supplements at least 4 hours apart from Mounjaro injections if possible, as Mounjaro may reduce absorption of orally taken substances. Take water-soluble vitamins (B vitamins, vitamin C) with food when you eat, and fat-soluble vitamins (A, D, E, K) with a meal containing fat for optimal absorption.
Never start a new supplement without mentioning it to your GP or prescriber, especially if you're taking diabetes medications or other treatments. Some supplements can interact with medications or affect blood sugar control. Keep a simple record of what you're taking and report any unusual symptoms—fatigue, weakness, hair loss, or neurological changes—to your healthcare team promptly. These could indicate a nutrient deficiency requiring intervention.
- 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 Mounjaro cause vitamin deficiencies?
Mounjaro itself doesn't directly cause deficiencies, but it slows gastric emptying, which may temporarily affect nutrient absorption. Combined with reduced food intake during weight loss, your body uses stored nutrients faster. Your GP can assess whether supplementation is needed based on your individual diet and health status.
Should I take vitamins before or after my Mounjaro injection?
There's no specific requirement to time vitamins around your injection day. However, separate oral medications and supplements from each other by at least 4 hours if possible to maximise absorption. Ask your prescriber for personalised timing guidance based on what you're taking.
Is vitamin B12 supplementation recommended while on Mounjaro?
B12 status depends on your baseline levels and dietary intake. Vegans, people with digestive conditions, or those with low B12 at baseline may benefit from supplementation. Your GP can check your B12 level before starting Mounjaro and recommend supplementation if needed—either oral or intramuscular injections.
Can I take multivitamins with Mounjaro?
Yes, multivitamins are generally safe with Mounjaro. Choose a formulation taken once daily with food. Tell your prescriber what's in it, particularly if it contains iron or calcium, as these may need separate timing. Avoid taking multivitamins at the same time as other medications requiring specific timing.
What should I do if I develop symptoms of nutrient deficiency on Mounjaro?
Report symptoms like persistent fatigue, weakness, hair loss, dizziness, or numbness to your GP immediately. These could indicate deficiencies in B12, iron, calcium, or other nutrients. Your GP can arrange blood tests and recommend appropriate supplementation or dietary changes.
Ready to take control of your weight loss journey?
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