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 Drugs Interact With Mounjaro?
If you're taking Mounjaro (tirzepatide) in the UK, it's important to know which medications may interact with it. Mounjaro can affect how your body absorbs other medicines, particularly oral medications, and some drugs may affect how well Mounjaro works. Your GP or prescriber must know about all medications you're taking before you start treatment. This guide explains the key drug interactions with Mounjaro, what to watch for, and why disclosure matters for your safety.
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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 Drug Interactions With Mounjaro
Oral contraceptives are the most significant interaction with Mounjaro. When you start taking tirzepatide or increase your dose, the medication may reduce how well oral contraceptives are absorbed, potentially reducing their effectiveness. The MHRA recommends that women taking oral contraceptives should add a barrier contraceptive method (such as condoms) or switch to a non-hormonal method for 4 weeks after starting Mounjaro AND 4 weeks after each dose increase. This is important because Mounjaro slows gastric emptying, affecting medication absorption through the stomach.
Diabetes medications may need adjustment if you're already taking them. If you use sulphonylureas or insulin, Mounjaro's effects on blood sugar could increase your risk of low blood sugar (hypoglycaemia). Your prescriber will review and adjust doses as needed—never stop or change diabetes medications without guidance.
- Inform your GP of all medications before starting
- Include over-the-counter medicines and supplements
- Report any new prescriptions while on Mounjaro
Clinically BackedWeight Loss
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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.
Managing Medications Safely With Mounjaro
Mounjaro may affect the absorption of other oral medications taken at the same time because it slows how quickly food and medication move through your stomach. This doesn't mean you can't take other medicines, but timing and spacing can matter. Always discuss any new prescriptions with your GP or pharmacist, and mention that you're on Mounjaro. Keep a complete list of all medications, including painkillers, antihistamines, and any supplements. If you experience unusual symptoms after starting a new medicine alongside Mounjaro—such as reduced effectiveness or new side effects—contact your GP promptly.
- 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 blood pressure medication?
Blood pressure medications are not typically considered major interactors with Mounjaro. However, as Mounjaro supports weight loss, your blood pressure may improve over time, and your prescriber might need to adjust your dose downwards. Always inform your GP that you're starting Mounjaro so they can monitor this.
Can I take paracetamol or ibuprofen with Mounjaro?
Yes, occasional use of paracetamol or ibuprofen is generally considered safe alongside Mounjaro. However, because Mounjaro slows stomach emptying, absorption may be slightly delayed. If you need regular pain relief, discuss this with your GP to ensure it's appropriate for your situation.
What should I do if I'm prescribed a new medicine while on Mounjaro?
Always tell your new prescriber or pharmacist that you're taking Mounjaro (tirzepatide). Give them a complete list of all medications and supplements you're taking. They can assess whether there are any interactions and advise on timing or dose adjustments if needed.
Does Mounjaro affect the contraceptive implant or coil?
The MHRA guidance specifically applies to oral contraceptives. Long-acting reversible contraception like implants or intrauterine devices (IUDs) are not affected by Mounjaro's absorption effects. However, always mention you're on Mounjaro when discussing contraception with your GP.
Can I take supplements or vitamins with Mounjaro?
Most supplements are safe, but timing matters. Since Mounjaro affects stomach emptying, space supplements a few hours away from your injection if possible. Discuss any regular supplements with your GP or pharmacist, especially if you're taking iron, calcium, or vitamin B12 supplements.
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.