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 With 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 some oral medications, including certain antibiotics. This UK health guide explains what you need to know about combining these treatments safely, when to inform your GP or prescriber, and practical steps to take if you're prescribed antibiotics while using Mounjaro.
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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 Medication Absorption
Mounjaro slows gastric emptying — the rate at which food and medications move through your stomach into your small intestine. This means oral medications, including antibiotics taken by mouth, may be absorbed more slowly or less completely than usual. The effect tends to be most noticeable in the first weeks of starting Mounjaro or after a dose increase, and typically reduces over time as your body adjusts.
This doesn't mean you cannot take antibiotics with Mounjaro. Rather, it means the timing and type of antibiotic matter. Oral antibiotics that are typically affected include certain penicillins and fluoroquinolones. Intravenous (IV) antibiotics bypass the stomach entirely and are not affected by Mounjaro's effect on gastric emptying. Your GP or prescriber will assess which antibiotic is most appropriate for your infection and your current use of tirzepatide.
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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 or GP that you are taking Mounjaro before they prescribe antibiotics. Mention both the medication name (tirzepatide or Mounjaro) and your current dose. Your GP can then choose an antibiotic that is less affected by reduced gastric absorption, or recommend timing adjustments — for example, taking your antibiotic at a different time of day away from other medications.
Do not stop taking Mounjaro to take antibiotics unless your prescriber specifically advises this, which is rare. Instead, follow your prescriber's guidance on antibiotic selection and timing. If you experience unusual symptoms while taking both medications — such as severe nausea, vomiting, or your infection not improving — 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 stop my antibiotics from working?
Not necessarily. Mounjaro may reduce how quickly your body absorbs some oral antibiotics, but this doesn't mean they won't work. Your GP can choose an antibiotic less affected by this effect, or adjust timing. Intravenous antibiotics are unaffected by Mounjaro's gastric effects. Always tell your prescriber you're taking Mounjaro.
Which antibiotics are safe to take with Mounjaro?
Most antibiotics are safe with Mounjaro, but absorption of some oral antibiotics (like certain penicillins and fluoroquinolones) may be reduced. Your GP will select the most appropriate antibiotic for your infection and current Mounjaro dose. Never self-prescribe or change your antibiotic without medical advice.
Should I stop Mounjaro if I'm prescribed antibiotics?
No. You should continue taking Mounjaro unless your prescriber specifically advises otherwise, which is very rare. Instead, inform your GP that you're taking tirzepatide so they can prescribe an appropriate antibiotic and adjust timing if needed.
Can I take antibiotics on the same day as my Mounjaro injection?
Yes, you can take antibiotics on your Mounjaro injection day. However, timing matters — your GP or pharmacist may recommend spacing them out (for example, taking your antibiotic several hours before or after your injection) to optimise absorption of the antibiotic.
What should I do if my infection doesn't improve while taking Mounjaro?
Contact your GP promptly. Do not stop your Mounjaro or antibiotic without medical advice. Your prescriber may switch your antibiotic to a different type or adjust the dose. Keep your GP informed of all medications you're taking, including your Mounjaro KwikPen dose, to help them make the best decision.
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