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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Mounjaro Interactions — What You Need to Know
Mounjaro (tirzepatide) can interact with other medications you may be taking, which is important to discuss with your prescriber before starting treatment. Understanding potential interactions helps ensure the medication works safely and effectively alongside any other health treatments. This guide covers the key interactions you should be aware of as a UK patient considering or using Mounjaro, and why your prescriber needs a full list of your current medications.
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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 Mounjaro Interactions and Considerations
Oral contraceptives (the pill): This is the most important interaction. Mounjaro can slow how quickly oral medications are absorbed in your digestive system. Women taking the contraceptive pill should add a barrier method (such as condoms) or switch to a non-oral contraceptive for 4 weeks after starting Mounjaro AND 4 weeks after each dose increase. This is because reduced absorption may lower the pill's effectiveness. Your prescriber will discuss contraception options if this applies to you.
- Discuss alternative contraception with your GP or prescriber before starting
- Barrier methods are a safe additional precaution
- The interaction is temporary — absorption returns to normal after the 4-week window
Diabetes medications: If you take insulin or sulphonylureas (such as gliclazide or glibenclamide), your prescriber may need to adjust your doses when starting tirzepatide. This is because Mounjaro affects how your body regulates blood sugar. Your GP or specialist will monitor your blood sugar levels and adjust doses as needed. Never stop or change diabetes medication without professional guidance.
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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.
Other Medications and General Interactions
Beyond oral contraceptives and diabetes medications, Mounjaro may affect how your stomach absorbs other oral medications due to slowed gastric emptying (the rate food and medications move through your stomach). Always inform your prescriber of all medications you take — including over-the-counter medicines, supplements, and herbal products. Your prescriber will assess whether any adjustments are needed. If you take multiple oral medications, spacing them out (taking them at different times of day) may help reduce interaction risk. People with severe gastroparesis or other significant digestive conditions may not be suitable for Mounjaro, so full disclosure of your medical history is essential.
- Keep your prescriber updated if you start new medications
- Report any unexpected changes in how existing medications work
- Discuss timing of medication doses with your GP
- 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 interact with my type 2 diabetes medication?
Mounjaro can interact with insulin and sulphonylureas because it affects blood sugar regulation. Your prescriber may adjust your diabetes medication doses. Never change your diabetes medication without professional guidance — your GP or specialist will monitor your blood sugar levels and make adjustments as needed.
Do I need to use extra contraception with Mounjaro?
Yes. If you take the contraceptive pill, add a barrier method or switch to non-oral contraception for 4 weeks after starting Mounjaro and 4 weeks after each dose increase. Mounjaro slows medication absorption in your gut, which may reduce the pill's effectiveness during these periods.
Can Mounjaro slow down absorption of my other oral medications?
Mounjaro can slow gastric emptying, potentially affecting how quickly other oral medications are absorbed. Inform your prescriber of all medications you take. Spacing doses at different times of day may help. Your prescriber will assess whether any adjustments are needed for your specific situation.
What should I tell my prescriber about my medications before starting Mounjaro?
Provide a complete list of all medicines — including over-the-counter drugs, supplements, and herbal products. Mention any diabetes medications, contraceptives, and any conditions affecting your digestive system. This allows your prescriber to assess interactions and make appropriate dose adjustments if needed.
Can I take Mounjaro with high blood pressure or cholesterol medication?
Most blood pressure and cholesterol medications are not known to have major interactions with tirzepatide. However, always inform your prescriber of these medications. If you have concerns, your GP can clarify whether any adjustments are needed based on your individual health profile and current treatment plan.
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.