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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What Vitamins Should Not Be Taken With Mounjaro?
Mounjaro (tirzepatide) is a prescription-only weight management injection that may affect how your body absorbs certain oral medications and supplements, including vitamins. Because Mounjaro slows gastric emptying—how quickly food and substances move through your stomach—it can reduce the absorption of some vitamins taken by mouth. This means certain vitamin supplements may be less effective if taken at the same time as your Mounjaro dose. Always discuss your current vitamin routine with your GP or prescriber before starting tirzepatide to ensure safe supplementation alongside your treatment.
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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 Vitamin Absorption
Mounjaro works as a dual GIP and GLP-1 receptor agonist, which helps regulate appetite and blood sugar. However, this mechanism slows down gastric emptying—the process by which food and substances leave your stomach and move into your small intestine where absorption occurs. This delayed emptying can reduce how much of an oral vitamin or supplement your body actually absorbs into the bloodstream.
- Fat-soluble vitamins (A, D, E, K) may be particularly affected because they require healthy fat digestion and adequate stomach residence time for optimal absorption
- Oral contraceptives and other oral medications are also affected—women taking birth control should add a barrier method or switch to non-oral contraception for 4 weeks after starting Mounjaro and after each dose increase
- The degree of absorption reduction varies between individuals and may improve as your body adapts over weeks
- Taking vitamins at a different time from your Mounjaro injection may help—discuss timing with your GP
Your prescriber should review all supplements and medications you take before starting tirzepatide to identify potential interactions.
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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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Which Vitamins Require Caution With Mounjaro?
Whilst no vitamins are absolutely "contraindicated" with Mounjaro, certain supplements warrant particular caution. B vitamins (especially B12), iron supplements, and fat-soluble vitamins (A, D, E, K) may have reduced effectiveness if taken simultaneously with your injection. If you're taking prescription vitamin supplements for diagnosed deficiencies, your GP should monitor your levels with blood tests during treatment with tirzepatide.
- Space vitamin intake at least 2–4 hours away from your Mounjaro injection day if possible
- Request blood work to check B12, vitamin D, and iron levels if you're on long-term supplementation—Mounjaro may require dose adjustments to your vitamins
- Inform your prescriber about all over-the-counter vitamins, minerals, and herbal supplements
Never stop prescribed vitamin supplements without medical guidance, especially if treating a diagnosed deficiency.
- 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 multivitamins while on Mounjaro?
Multivitamins can be taken during Mounjaro treatment, but absorption may be reduced due to delayed gastric emptying. Take your multivitamin at a different time from your weekly injection (ideally several hours apart) to maximise absorption. Ask your GP to monitor your nutrient levels with blood tests, as you may need a higher-dose supplement or more frequent dosing to maintain adequate levels.
Should I take vitamin B12 supplements with Mounjaro?
B12 supplements may have reduced absorption with Mounjaro. If you're taking B12 for diagnosed deficiency, inform your prescriber—they may recommend injected B12 instead of oral tablets, which bypasses the stomach entirely. Blood tests should check your B12 levels regularly during tirzepatide treatment.
Is it safe to take vitamin D with Mounjaro?
Vitamin D can be taken with Mounjaro, but absorption is likely reduced. Fat-soluble vitamins like D require adequate stomach time and fat digestion. Take your vitamin D supplement several hours away from your injection day. Ask your GP to test your vitamin D levels annually—you may need a higher dose than usual.
What about iron supplements—can I take them with Mounjaro?
Iron supplements may have reduced absorption with Mounjaro. If you're treating iron deficiency anaemia, discuss this with your GP before starting tirzepatide. They may recommend taking iron at a different time of day from your injection, or monitoring your iron levels more frequently with blood tests.
Do I need to stop my vitamins before starting Mounjaro?
No—do not stop prescribed or recommended supplements without consulting your GP. Instead, inform your prescriber about all vitamins and supplements you're currently taking. They'll assess whether to continue, adjust timing, increase doses, or switch to alternative forms (like injected vitamins) to ensure you maintain adequate nutrition during tirzepatide treatment.
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