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
Can You Take Mounjaro With Metformin Safely?
Yes, Mounjaro (tirzepatide) can generally be taken alongside metformin, as there is no known direct interaction between the two medications. However, because both drugs can affect blood sugar levels and digestion, your prescriber needs to assess your individual situation carefully. If you're already taking metformin for type 2 diabetes or prediabetes, inform your GP or prescriber before starting Mounjaro. They may adjust your metformin dose or monitor you more closely during the dose titration phase. This ensures both medications work safely together for your health.
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.
Mounjaro and Metformin: What You Need to Know
There is no direct pharmacological interaction between Mounjaro (tirzepatide) and metformin. Both are safe to use together, and in fact, many people taking metformin for diabetes are eligible for Mounjaro if they meet NHS weight management criteria. However, because both medications affect blood sugar control and gastric function, your prescriber must review your full medical history before starting Mounjaro.
Key points to discuss with your GP or prescriber:
- Your current metformin dose and how well it controls your blood sugar
- Any history of stomach or digestion problems — Mounjaro slows gastric emptying, which could affect metformin absorption slightly
- Your kidney function (both drugs require adequate kidney function)
- Any other diabetes medications you take, such as sulphonylureas or insulin — these may need dose adjustment when starting Mounjaro
- Your current HbA1c and blood sugar targets
Your prescriber may monitor your blood sugar more closely when you first start the Mounjaro KwikPen to ensure both medications are working effectively together.
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.
NHS Prescribing and Your GP: What to Tell Them
If you're taking metformin and interested in Mounjaro through the NHS, your GP should refer you to a tier 3 specialist weight management service for assessment. Private prescribers will also ask about your metformin use before issuing a prescription. Be clear and honest about all medications you take, including doses and how long you've been taking them.
Remember that starting Mounjaro is not a reason to stop metformin — both can work together. Your prescriber may suggest a temporary review or slight adjustment of either medication during the initial weeks, depending on how you respond. Never stop or change any medication without medical guidance. If you experience side effects like severe nausea or vomiting after starting Mounjaro, contact your GP, as this could briefly affect metformin absorption.
- 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 metformin from working?
No. Mounjaro does not reduce metformin's effectiveness. Both work via different mechanisms to improve blood sugar control. However, because both affect digestion and blood sugar, your prescriber may monitor your levels more closely initially to ensure optimal dosing of each medication.
Do I need to take Mounjaro and metformin at specific times?
Mounjaro is injected once weekly, while metformin is typically taken with food one to three times daily. There is no requirement to time them together. Continue taking metformin as prescribed unless your GP advises otherwise after you start Mounjaro.
Can I get Mounjaro on the NHS if I'm already on metformin?
Yes, potentially. Metformin use does not disqualify you from NHS weight management services. You must meet NHS eligibility criteria (BMI ≥30, or ≥27 with weight-related health conditions like diabetes) and be referred to a tier 3 specialist service by your GP for assessment and prescription.
What if metformin gives me side effects — should I stop it before starting Mounjaro?
No. Do not stop metformin without medical advice. If you experience side effects, discuss them with your GP. They can assess whether adjusting the dose or timing helps, and they will factor this into any decision about starting Mounjaro. Both decisions are made together with your prescriber.
Could metformin and Mounjaro together cause low blood sugar?
Low blood sugar risk is higher if you also take sulphonylureas or insulin alongside both metformin and Mounjaro. Metformin and Mounjaro alone are less likely to cause dangerous low blood sugar, but your prescriber will monitor this. Report any dizziness, sweating, or shakiness to your GP immediately.
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.