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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Mounjaro and Lithium — What You Need to Know
If you're taking lithium for mood or mental health conditions and considering Mounjaro (tirzepatide) for weight management, it's important to understand how these medications interact. Lithium is a mood-stabilising medication commonly prescribed in the UK, and Mounjaro is a dual GIP and GLP-1 receptor agonist injection for weight loss. This guide explains what prescribers need to know, potential interactions, and why medical assessment is essential before combining these treatments. Always inform your healthcare provider of all medications you're taking.
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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.
Mounjaro, Lithium, and Medication Interactions
Mounjaro (tirzepatide) and lithium do not have a direct pharmacological interaction in the traditional sense — they do not compete for the same metabolic pathways or significantly alter each other's absorption. However, there are important clinical considerations when combining these medications.
How lithium works: Lithium is a mood stabiliser that works by altering sodium transport in nerve and muscle cells. It requires stable kidney function and careful monitoring because the therapeutic dose is close to the toxic dose.
Mounjaro's effect on kidneys: Mounjaro can affect kidney function indirectly through weight loss and changes in fluid balance. Additionally, some GLP-1 medications have been associated with dehydration in clinical practice, which could theoretically affect lithium levels and clearance.
Key concern: If Mounjaro causes dehydration or if you lose significant weight while taking lithium, your lithium levels could change. Lithium is monitored through regular blood tests, and your prescriber may need to check your lithium levels more frequently or adjust your dose if you start Mounjaro.
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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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How to Use Mounjaro Safely with Lithium
If you take lithium and are eligible for Mounjaro, this combination is not necessarily contraindicated — but it requires careful prescriber management. Your GP or specialist should:
- Know about your lithium prescription before starting Mounjaro
- Monitor your lithium blood levels at baseline and at regular intervals after starting Mounjaro
- Advise you to stay well hydrated throughout treatment
- Adjust your lithium dose if necessary based on blood test results
Wegovy (semaglutide), another GLP-1 injection, has similar considerations with lithium. Neither medication is absolutely contraindicated with lithium, but both require prescriber oversight. Use our free GLP-1 weight loss calculator to estimate potential outcomes, and discuss all options with your healthcare provider.
- 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.
Is it safe to take Mounjaro if I'm on lithium?
Mounjaro and lithium do not have a direct drug interaction, but combining them requires prescriber oversight. Mounjaro can affect kidney function and fluid balance, which may alter lithium levels. Your GP or specialist should monitor your lithium blood levels regularly and adjust your dose if needed. Always inform your prescriber before starting any new medication.
Will Mounjaro affect my lithium levels?
Potentially, yes. Mounjaro-induced weight loss and possible dehydration could affect how your kidneys clear lithium, changing your blood lithium concentration. This is why baseline and regular lithium monitoring is essential when starting Mounjaro. Your prescriber will arrange appropriate blood tests to track this.
How often will I need lithium blood tests if I start Mounjaro?
Your prescriber will decide based on your individual circumstances. You'll likely have a baseline lithium level checked before starting Mounjaro, then repeat tests at regular intervals — typically every 4–8 weeks initially, or more frequently if your dose changes. Stay in regular contact with your GP.
Can I take Wegovy instead of Mounjaro if I'm on lithium?
Wegovy (semaglutide) has similar considerations to Mounjaro when combined with lithium. Both GLP-1 and dual GIP/GLP-1 medications can affect kidney function and hydration. Either medication requires prescriber oversight and lithium monitoring. Your prescriber will advise which option is most suitable for you.
What should I tell my prescriber before starting Mounjaro?
Tell your prescriber you take lithium, the dose, and how long you've been taking it. Mention any recent lithium blood test results if you have them. Discuss your weight management goals and any other medications or supplements. Your prescriber needs this full picture to assess whether Mounjaro is suitable and how to monitor you safely.
Ready to take control of your weight loss journey?
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