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 Supplements Should I Take When on Mounjaro?
When you're taking Mounjaro (tirzepatide) for weight management, you may wonder whether supplements are safe or necessary. Because Mounjaro slows how quickly food moves through your stomach and can affect nutrient absorption, some people benefit from targeted supplementation — but this must be discussed with your GP or prescriber first. This guide covers key nutrients to monitor, safe supplement choices, and critical interactions to avoid.
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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.
Key Nutrients to Monitor on Mounjaro
Mounjaro works partly by slowing gastric emptying, which means food — and nutrients — spend longer in your digestive system. While this effect typically reduces over time, it can affect how your body absorbs certain vitamins and minerals during the early weeks of treatment. Reduced appetite also means you may eat smaller portions, lowering your intake of essential nutrients.
- B vitamins: Particularly B12 — slower digestion can reduce absorption. Many people on GLP-1 medications benefit from B12 monitoring, especially if vegetarian or vegan.
- Iron: Reduced food intake may lower iron consumption; slower gastric emptying can affect absorption.
- Calcium and vitamin D: Necessary for bone health, especially important for long-term weight management.
- Magnesium: May help with constipation, a common side effect of Mounjaro.
- Folate: Essential for women of childbearing age, as Mounjaro is not safe during pregnancy.
Ask your GP to check these nutrient levels before starting Mounjaro and at regular intervals during treatment. Your prescriber can advise whether supplementation is needed based on your individual diet and blood tests.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Supplements to Avoid or Discuss with Your GP
Oral contraceptives: Mounjaro can reduce the absorption of oral medications — including the contraceptive pill. If you take the pill, use a barrier method or switch to a non-oral contraceptive for 4 weeks after starting Mounjaro and 4 weeks after each dose increase. This is crucial for contraceptive reliability.
- Fat-soluble vitamins (A, D, E, K): If taking high-dose supplements, discuss with your GP — slower fat absorption may affect how much you absorb.
- Iron supplements: Can cause stomach irritation; check timing with your prescriber if needed.
- Calcium supplements: Best taken separately from other medications and supplements — ask your pharmacist about timing.
- Herbal supplements: Some interact with medications — always declare all supplements to your prescriber.
Your prescriber should assess any supplements you're taking before you start Mounjaro, as individual interactions depend on your health, other medications, and dose titration schedule.
- 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?
Generally yes, but discuss with your GP first. A standard multivitamin taken with food is usually safe. However, if you develop deficiency (flagged by blood tests), your prescriber may recommend prescription-strength supplements instead. Avoid mega-dose multivitamins without professional guidance.
Will Mounjaro affect my vitamin B12 levels?
Possibly — Mounjaro slows gastric emptying, which can reduce B12 absorption, especially if you're already at lower risk (vegetarian, vegan, older age, or previous gastric surgery). Ask your GP to monitor B12 levels regularly. Supplementation or B12 injections may be recommended if levels drop.
Is it safe to take iron supplements with Mounjaro?
Iron can cause stomach irritation, and Mounjaro often causes nausea and stomach upset — combining them may worsen symptoms. If you need iron supplementation, discuss timing with your pharmacist. Iron should be taken separately from other supplements and medications, typically on an empty stomach if tolerated.
Do I need magnesium supplements for constipation on Mounjaro?
Constipation is common on Mounjaro, but magnesium isn't always necessary first-line. Try increasing water intake, fibre, and physical activity first. If constipation persists, ask your GP before taking magnesium supplements — some forms can interact with medications or worsen stomach symptoms.
Should I take supplements differently when increasing my Mounjaro dose?
Potentially — when you increase your dose, side effects like nausea may worsen, making stomach irritation from supplements more likely. Discuss timing and whether to pause or adjust supplements during dose titration with your prescriber. Blood tests at each escalation can guide what you actually need.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.