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 Medication Can You Not Take With Mounjaro?
Mounjaro (tirzepatide) can interact with certain medications, which is why it's essential to inform your prescriber of everything you're currently taking. The main concern is how Mounjaro affects absorption of oral medications, particularly oral contraceptives. This informational guide explains the key medication interactions, what to watch for, and how to discuss your full medication list with your GP or prescriber before starting tirzepatide 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.
Key Medication Interactions With Mounjaro
Oral contraceptives are the most important interaction to know about. Mounjaro slows how quickly food (and oral medications) move through your stomach, which may reduce how well your contraceptive pill is absorbed. When starting Mounjaro, women using oral contraceptives should either add a barrier contraception method (such as condoms) or switch to a non-oral method for 4 weeks after starting tirzepatide, AND for 4 weeks after each dose increase. This precaution ensures contraceptive reliability during the period when absorption may be affected.
- Diabetes medications: If you take sulphonylureas or insulin, your prescriber may need to adjust your doses when you start Mounjaro, as tirzepatide affects how your body regulates blood sugar. Never adjust these doses yourself — always discuss changes with your diabetes care team.
- Other oral medications: Because Mounjaro slows gastric emptying, any oral medication taken at the same time may have reduced absorption. This is less of a concern with medications taken at different times of day, but always mention your complete medication list to your prescriber.
Your UK prescriber will assess your individual medication profile during your consultation and provide specific guidance tailored to your situation.
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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.
Medications You Can Generally Take With Mounjaro
Most medications are safe to take with Mounjaro, provided your prescriber is aware of them. Medications taken several hours apart from food (such as evening doses taken before bed) are unlikely to be significantly affected by Mounjaro's gastric effects. The key is transparency: always provide your prescriber or GP with a complete, up-to-date list of every medication you're taking, including over-the-counter medicines, supplements, and herbal remedies. Your prescriber can then assess your individual risk and provide personalised guidance. If you have questions about your specific medications, ask your pharmacist or GP before starting treatment.
- 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.
Will Mounjaro stop my oral contraceptive from working?
Mounjaro may reduce contraceptive pill absorption due to slowed stomach emptying. Adding barrier contraception or switching to a non-oral method for 4 weeks after starting and after each dose increase reduces this risk. Discuss contraception options with your GP or prescriber before starting treatment.
Can I take insulin with Mounjaro?
Yes, but your insulin doses may need adjustment. Mounjaro affects blood sugar regulation, so your diabetes care team should review and potentially lower your insulin dose to avoid low blood sugar. Never adjust insulin yourself — always work with your healthcare team.
What should I tell my GP about my medications?
Provide a complete list including prescription medications, over-the-counter drugs, vitamins, supplements, and herbal remedies. Include doses and when you take them. This helps your prescriber identify any potential interactions and give you personalised guidance before starting Mounjaro.
Does Mounjaro interact with blood pressure medications?
Most blood pressure medications are safe with Mounjaro, though absorption of some oral forms may be slightly reduced. Your GP can assess your specific medication and may monitor your blood pressure more closely when starting treatment.
Can I take my other medications at the same time as Mounjaro injections?
Mounjaro injections themselves don't directly interact with other medications. However, since Mounjaro slows stomach emptying, oral medications taken with food may have reduced absorption. Taking oral medicines at different times (especially several hours before meals) can minimise this effect. Ask your pharmacist for specific advice.
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.