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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Can You Take Antibiotics While Using Mounjaro?
If you're taking Mounjaro (tirzepatide) and need antibiotics, it's important to understand how these medications interact. Mounjaro may affect how your body absorbs oral medications, including some antibiotics. This means certain antibiotics may be less effective, or their absorption could be delayed. Always tell your GP or prescriber about all medications you're currently taking, including Mounjaro, before starting antibiotics. Your healthcare team can assess whether your specific antibiotic is safe to take alongside tirzepatide or whether an alternative is needed.
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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.
How Mounjaro Affects Antibiotic Absorption
Mounjaro works by slowing how quickly food moves through your stomach and digestive system—a process called delayed gastric emptying. This effect can also slow the absorption of oral medications taken at the same time, including certain antibiotics. Not all antibiotics are equally affected: some are absorbed before reaching the areas where Mounjaro has the most impact, whilst others may be significantly delayed. The concern is not that antibiotics become toxic, but that they may reach lower effective levels in your bloodstream, potentially reducing their ability to treat infection. This is particularly important for time-sensitive infections where consistent antibiotic levels matter. Your prescriber can recommend antibiotics least affected by Mounjaro, or suggest taking your antibiotic at a different time of day—for example, several hours before your weekly Mounjaro injection.
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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.
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What to Do If You Need Antibiotics
Always inform your prescriber that you're taking Mounjaro before starting any antibiotic course. Your GP or pharmacist can review your specific antibiotic and tirzepatide together and advise whether timing adjustments are needed. In many cases, the risk is minimal and no changes are required—but your healthcare team needs the full picture of your medications. If you experience signs of infection not improving after a few days of antibiotics, contact your GP promptly. For serious infections, your prescriber may recommend intravenous antibiotics, which bypass absorption issues entirely. Keep a current medication list to share with any healthcare provider, including both your Mounjaro dose and any other treatments you're receiving.
- 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 amoxicillin with Mounjaro?
Amoxicillin absorption may be delayed when taking Mounjaro, but it's generally not considered a major interaction. Always inform your GP or pharmacist that you're on tirzepatide. They can assess your specific situation and advise whether timing changes are needed or if an alternative antibiotic is preferable. Monitor your infection closely for improvement.
Will Mounjaro stop my antibiotics from working?
Mounjaro may slow antibiotic absorption, but this doesn't mean antibiotics won't work—it means levels in your bloodstream may build up more slowly. Most people still respond well to antibiotics whilst on tirzepatide. Your prescriber can choose antibiotics least affected by Mounjaro or adjust timing to minimise any impact on effectiveness.
Should I take my antibiotic at a different time from Mounjaro?
Yes, spacing out medications can help. Since Mounjaro is injected once weekly, you could take oral antibiotics several hours before or after your injection day if your prescriber recommends it. However, antibiotic dosing schedules are fixed (e.g., three times daily), so timing flexibility may be limited. Your GP will advise what's practical for your situation.
What antibiotics are safe with Mounjaro?
Most antibiotics can be used safely with tirzepatide, but some are affected more than others. Intravenous antibiotics bypass absorption issues entirely. Your prescriber and pharmacist will recommend the safest option for you. Always disclose your Mounjaro use before starting any antibiotic to ensure safe co-prescription.
What if my infection isn't improving whilst on Mounjaro and antibiotics?
Contact your GP promptly if your infection symptoms persist beyond 3–4 days of antibiotics. This could indicate the antibiotic isn't effective, a resistant infection, or potentially an absorption issue related to Mounjaro. Your GP may recommend an alternative antibiotic, intravenous treatment, or further investigation to ensure proper diagnosis and treatment.
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