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 Are Safe To Take With Mounjaro?
If you're prescribed Mounjaro (tirzepatide), you may wonder whether it's safe to take vitamins and supplements alongside it. The answer depends on the specific vitamin, how you take it, and how Mounjaro affects nutrient absorption in your digestive system. This guide explains the key nutritional considerations when using this prescription weight loss medication in the UK, and why discussing supplements with your prescriber is important 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.
Mounjaro and Nutrient Absorption — What You Need To Know
Mounjaro (tirzepatide) works by slowing how quickly food moves through your stomach and intestines — a process called gastric emptying. This mechanism, combined with reduced appetite, means you eat less food overall. The concern with nutrient absorption is real but manageable.
Because Mounjaro slows digestion, oral medications and some supplements may pass through your system more quickly before being fully absorbed. This is particularly important for:
- Oral contraceptives — may be less effective; consider barrier methods for 4 weeks after starting and after each dose increase
- Certain diabetes medications — insulin or sulphonylurea doses may need adjusting
- Fat-soluble vitamins (A, D, E, K) — may be less efficiently absorbed if you're eating significantly less fat
- Vitamin B12 — reduced food intake may lower intake; some users report deficiency
The good news: you can manage these concerns. Taking vitamins at different times from Mounjaro (where possible) and discussing your regimen with your GP or prescriber helps.
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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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Mounjaro vs Wegovy — Nutritional Considerations
Mounjaro (dual GIP+GLP-1 agonist) and Wegovy (GLP-1 agonist only) have similar effects on digestion — both slow gastric emptying and reduce appetite. Nutritional absorption risks are comparable between the two medications. The key difference is their mechanism, not their impact on vitamin intake.
Both medications may affect oral medication absorption equally. Individual nutritional needs vary based on your starting diet, how much weight you lose, and your personal health factors. Your prescriber can review whether you need specific supplementation. Use our free GLP-1 weight loss calculator to understand projected outcomes, and discuss nutritional support with your healthcare team before starting either medication.
- 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 a multivitamin with Mounjaro?
Yes, you can generally take a standard multivitamin with Mounjaro. Take it at a different time of day from your injection if possible (e.g., morning vs evening). Avoid taking it immediately before or with a meal, as absorption may be delayed. Always tell your prescriber which supplements you're taking.
Will Mounjaro cause vitamin deficiencies?
Mounjaro itself doesn't cause deficiencies, but reduced food intake may lower your intake of certain nutrients — especially B12, iron, and calcium if you're eating less. Some users report lower B12 levels after several months. Regular blood tests with your GP can monitor this, and supplementation may be recommended.
Should I take vitamin B12 with Mounjaro?
Many people on long-term weight loss medication benefit from B12 supplementation, especially if they eat less animal protein. B12 is water-soluble and generally safe at higher doses. Discuss B12 testing and supplementation with your GP — injections or sublingual tablets are common options.
Can I take iron supplements with Mounjaro?
Iron supplements can be taken with Mounjaro, but absorption may be affected by slower digestion. Take iron on an empty stomach if tolerated, or at a different time from your injection. Vitamin C enhances iron absorption. Have your iron levels checked regularly with your GP.
Does Mounjaro affect calcium and vitamin D absorption?
Mounjaro may indirectly affect fat-soluble vitamin absorption (including vitamin D) if you're eating significantly less dietary fat. Calcium and vitamin D are especially important for bone health during weight loss. Your GP can check your levels and recommend supplementation if needed, particularly if you're losing weight rapidly. ```
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