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 take lithium for bipolar disorder or depression and are considering Mounjaro (tirzepatide) for weight management, it's crucial to understand how these medications interact. Mounjaro can affect how your body processes lithium, potentially leading to unsafe lithium levels. This guide explains the interaction, what your GP needs to know, and how to access Mounjaro safely if you're on lithium therapy in the UK.
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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 Lithium Levels
Mounjaro works by slowing gastric emptying—the rate at which food (and medication) leaves your stomach. Because lithium is absorbed in the small intestine, anything that slows stomach emptying can increase how much lithium your body absorbs, potentially raising lithium levels in your bloodstream to dangerous concentrations.
Why this matters: Lithium has a narrow therapeutic window, meaning the difference between an effective dose and a toxic dose is small. Even modest increases in lithium levels can cause:
- Tremors and muscle weakness
- Nausea and diarrhoea
- Confusion or cognitive changes
- Serious kidney or thyroid complications at very high levels
This is why prescribers must carefully monitor anyone taking both medications together. Your GP or prescribing clinician will need to check your lithium levels more frequently if Mounjaro is started, and may adjust your lithium dose.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Can You Take Mounjaro If You're On Lithium?
Yes, you can take Mounjaro and lithium together, but it requires careful medical supervision and is not a decision to make without professional guidance. Your prescriber needs to:
- Know you are on lithium before Mounjaro is prescribed
- Check your baseline lithium levels before starting
- Monitor lithium levels more frequently (typically every 1–2 weeks initially)
- Possibly adjust your lithium dose downward
- Ensure you stay well-hydrated and maintain consistent salt intake
If your GP or specialist determines Mounjaro is suitable, accessing it privately through a UK online prescriber is an option, but you must declare your lithium use during the consultation. The prescriber will assess the risk and may decline if monitoring cannot be arranged.
- 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 interact with my lithium?
Yes, potentially. Mounjaro slows stomach emptying, which can increase lithium absorption and raise blood levels. This is a known interaction that requires careful monitoring. Your GP must know about both medications before Mounjaro is prescribed or started.
Can I take Mounjaro if I'm on lithium?
Yes, but only under close medical supervision with regular lithium level checks. Your prescriber must assess the benefit versus risk. Lithium dose adjustments may be needed. Never start Mounjaro without informing your healthcare provider about lithium use.
How often will my lithium levels be checked?
Initially, every 1–2 weeks after starting Mounjaro to ensure levels remain safe. Once stable, monitoring may continue monthly or as your prescriber recommends. Regular blood tests are essential—do not miss appointments.
What happens if lithium levels get too high?
High lithium levels cause nausea, tremors, confusion, and at severe levels, kidney or thyroid damage. This is why monitoring is critical. If you experience tremors, severe nausea, or confusion, contact your GP immediately—do not wait for your next scheduled test.
Is Mounjaro the best weight loss option if I take lithium?
It depends on your individual situation. Other GLP-1 medications like Wegovy (semaglutide) may have less impact on lithium absorption. Your GP or psychiatrist can discuss whether Mounjaro, Wegovy, or other weight management approaches are safest for you.
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.