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
When Should You Stop Mounjaro Before Surgery?
If you take Mounjaro (tirzepatide) and have upcoming surgery, you'll need to discuss the timing of your injection with your GP or surgical team. There is no single fixed rule for when to stop Mounjaro before surgery — guidance depends on the type of surgery, your individual health, and your prescriber's assessment. This guide explains what you need to know and the key questions to ask your healthcare team before your procedure.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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.
Why Timing Matters: Mounjaro and Surgical Risk
Mounjaro slows gastric emptying — how quickly food leaves your stomach — which is why nausea and digestive changes are common side effects. Before surgery, particularly procedures requiring anaesthesia, an empty stomach is essential to reduce the risk of aspiration (stomach contents entering the airways). The longer Mounjaro remains in your system, the slower your stomach empties, which can complicate pre-operative fasting requirements.
Additionally, Mounjaro is a GIP and GLP-1 receptor agonist that affects blood sugar regulation and appetite. If you have type 2 diabetes or prediabetes, stopping before surgery may require adjustments to other diabetes medications. Your surgical team needs to know you are taking Mounjaro so they can plan anaesthesia and post-operative care appropriately.
- General guidance: Most prescribers recommend stopping Mounjaro 4 weeks before elective surgery to allow the medication to clear your system
- Emergency surgery: Different rules apply — inform the surgical team immediately that you take Mounjaro
- Individual variation: Your specific timeline depends on surgery type, dose, and health factors
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 To Do: Your Pre-Surgery Action Plan
Contact your GP or prescriber as soon as you know surgery is planned — do not wait until pre-operative assessment. Provide them with the exact date of your procedure and the type of surgery. Ask explicitly: "When should I stop my Mounjaro injections?" and "Do I need to adjust any other medications?" If you take other diabetes medications, discuss whether doses need adjusting after you stop Mounjaro.
Write down the date you stop and any alternative medications you're given. Inform your anaesthetist and surgical team at pre-operative assessment that you take or have recently taken Mounjaro — include this on all pre-surgery forms. After surgery, ask when you can restart Mounjaro and whether any adjustments are needed. Consider using our free GLP-1 weight loss calculator to understand your current dose and discuss restart timing with your prescriber.
- 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 the week before surgery?
No — most guidance recommends stopping Mounjaro 4 weeks before elective surgery to allow gastric emptying to normalise and reduce aspiration risk during anaesthesia. Emergency surgery requires immediate disclosure to your surgical team, but you should not take a dose the week of surgery without explicit guidance from your prescriber and anaesthetist.
Will stopping Mounjaro affect my weight before surgery?
You may notice some weight return during the 4 weeks off Mounjaro, as appetite regulation changes. This is temporary and normal. Discuss any concerns about timing with your GP — they may adjust your post-operative restart schedule based on individual circumstances.
What if I've just had a Mounjaro injection before learning about surgery?
Contact your GP or prescriber immediately — do not attempt to remove or reverse the injection. They will advise on the exact timing of your surgery relative to when the injection was given and whether any adjustments are needed for anaesthesia or post-operative care.
Do I need to restart Mounjaro on the same schedule after surgery?
Not necessarily. Discuss restart timing with your prescriber before surgery. Some people restart at their previous dose, others may need a gradual re-escalation. Recovery, pain management, and post-operative eating ability all influence when it's safe to restart.
Should I tell my anaesthetist I take Mounjaro if I stopped weeks ago?
Yes — always disclose that you recently took Mounjaro, even if you stopped weeks earlier. This information helps your anaesthetist plan safer anaesthesia and manage gastric emptying risk. Include it in all pre-operative paperwork and mention it at every pre-operative appointment.
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.