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
Free tracker included with every download this week
What Drugs Interact With Mounjaro?
If you're taking Mounjaro (tirzepatide), it's essential to understand which other medications may interact with it. Some drugs can reduce how well Mounjaro works, while others may increase side effects or create safety risks. This guide covers the main interactions UK patients should know about, and why it's crucial to tell your prescriber about all medications you're currently taking before starting this prescription-only weight management injection. Your GP or prescriber will assess your individual medication history to ensure Mounjaro is safe for you.
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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 (birth control pills) are the most important interaction to know about. Mounjaro may slow how quickly oral medications are absorbed in the stomach, which could reduce their effectiveness. If you're taking the contraceptive pill, your prescriber will recommend adding a barrier method (condoms) or switching to a non-oral contraceptive for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. This is a critical safety point, as reduced contraceptive effectiveness could lead to unplanned pregnancy.
- Diabetes medications: If you take sulphonylureas (e.g. gliclazide, glibenclamide) or insulin, Mounjaro may enhance their blood sugar-lowering effect, increasing the risk of low blood sugar (hypoglycaemia). Your prescriber may need to reduce your diabetes medication doses. Never adjust these yourself — always discuss with your GP.
- Blood pressure medications: Weight loss from Mounjaro can lower blood pressure naturally. If you're on antihypertensive drugs, your BP may drop further. Your GP may need to monitor and adjust your dose.
- Other GLP-1 medications: Do not take Mounjaro with other GLP-1 or GIP receptor agonists (e.g. Wegovy, Saxenda, or diabetes GLP-1s). Combined use increases side effect risk significantly.
Always inform your prescriber of all medications, including over-the-counter medicines, supplements, and herbal products.
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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.
What You Must Tell Your Prescriber
Before starting Mounjaro, provide a complete list of every medication you're taking—prescription, over-the-counter, and supplements. This includes painkillers, antacids, vitamins, and herbal remedies. Your prescriber needs this information to assess whether Mounjaro is safe for you and whether any dose adjustments to other medications are needed. If you start any new medications after beginning Mounjaro, inform your prescriber or GP immediately. Certain conditions like pancreatitis history, gallbladder disease, or severe gastroparesis (stomach paralysis) may also affect safety.
Use our free BMI calculator to check your current health status, and discuss your weight management goals with your prescriber as part of your full clinical assessment.
- 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 Mounjaro with paracetamol or ibuprofen?
Yes, paracetamol and ibuprofen are generally safe with Mounjaro, though Mounjaro may slightly slow their absorption. Take them as normal, but if you have concerns about pain relief effectiveness, discuss with your GP or pharmacist.
Will Mounjaro interact with my antidepressant medication?
Most antidepressants are safe with Mounjaro, but some may have slightly reduced absorption due to Mounjaro's effect on gastric emptying. Inform your prescriber of any antidepressants you take. They may monitor you or adjust timing of doses if needed.
What if I'm taking blood thinners like warfarin or apixaban?
Blood thinners are not known to have direct interactions with Mounjaro, but weight loss can affect how these medications work. Your GP should monitor your clotting levels and may adjust your dose. Always inform them before starting Mounjaro.
Can I take Mounjaro if I'm on thyroid medication?
Thyroid medications like levothyroxine are generally safe with Mounjaro. However, Mounjaro may slightly affect absorption. Take your thyroid medication at least 30 minutes before food or Mounjaro injection, as recommended for all oral medications with this injection.
Is tirzepatide safe if I'm taking antibiotics?
Most antibiotics are safe with Mounjaro. However, since Mounjaro slows stomach emptying, oral antibiotics may be absorbed more slowly. If your GP prescribes antibiotics, mention you're on Mounjaro. Continue your antibiotic course as prescribed—absorption delays are usually minor.
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.