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 essential to understand how these medications interact. Lithium requires careful monitoring because tirzepatide may affect how your body handles lithium levels. This guide explains the interaction, why it matters, and what your GP or prescriber needs to know before you start Mounjaro. Always inform your prescriber of all medications you're taking before beginning any weight management treatment.
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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 May Affect Lithium Levels
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist that slows gastric emptying and affects fluid balance in the body. Lithium is absorbed in the small intestine and is entirely dependent on kidney function for elimination. Because Mounjaro slows how quickly food moves through your stomach, it may alter lithium absorption and potentially increase lithium levels in your blood.
Additionally, GLP-1 and GIP agonists can affect fluid and electrolyte balance, which may influence how your kidneys handle lithium excretion. This doesn't mean you cannot take both medications together, but it does mean your lithium levels require more frequent monitoring than usual. Your prescriber and GP need to work together to ensure your lithium dose remains safe and effective.
The UK Medicines and Healthcare products Regulatory Agency (MHRA) and NICE do not contraindicate the combination, but caution and monitoring are essential.
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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.
What Your GP and Prescriber Need to Do
Before starting Mounjaro, inform your prescriber that you take lithium. Your GP should:
- Check your current lithium level — a baseline blood test is essential
- Arrange more frequent lithium level checks — typically every 2–4 weeks during Mounjaro titration, then every 4–12 weeks once stable
- Monitor kidney function — lithium depends entirely on kidney filtration for safety
- Review your lithium dose — it may need adjustment downward if levels rise
- Monitor for lithium toxicity signs — tremor, confusion, nausea, or diarrhoea warrant immediate review
Communication between your mental health prescriber and weight management provider is crucial. If you're accessing Mounjaro through an NHS specialist weight management service or private prescriber, ensure they know about your lithium and arrange shared care with your GP. Use our free BMI calculator to understand your baseline weight health before your appointment.
Dehydration increases lithium toxicity risk. Because Mounjaro commonly causes nausea, vomiting, and diarrhoea — especially during dose escalation — maintain hydration and report prolonged GI symptoms to your GP. Never stop lithium suddenly, even if side effects occur; abrupt withdrawal can cause relapse of bipolar symptoms. If Mounjaro side effects make it difficult to tolerate lithium safely, your prescriber can discuss alternatives or dose adjustments. Weight management is important for overall health, but mental health stability is paramount. Your GP can advise whether Mounjaro is the right choice for you, or whether another weight loss option might be safer given your lithium therapy.
- 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 if I'm on lithium?
Potentially yes, but it requires careful monitoring. Mounjaro may affect lithium absorption and kidney handling of lithium, so your GP must arrange more frequent lithium level checks. Never start Mounjaro without informing your prescriber of your lithium use. Shared care between your mental health and weight management providers is essential for safety.
How often will I need lithium level checks if I take Mounjaro?
Baseline checks before starting, then typically every 2–4 weeks during Mounjaro dose titration (weeks 1–20), then every 4–12 weeks once on a stable Mounjaro dose. Your GP will advise the exact schedule based on your lithium level and kidney function. More frequent monitoring reduces toxicity risk significantly.
What signs of lithium toxicity should I watch for?
Tremor, confusion, memory problems, slurred speech, nausea, vomiting, diarrhoea, or loss of coordination. Because Mounjaro causes GI symptoms, report any that don't settle within a few days. Dehydration worsens lithium toxicity, so drink plenty of water. Contact your GP immediately if toxicity symptoms appear.
What if Mounjaro side effects make my lithium harder to manage?
Discuss with your GP or prescriber. You may need to adjust your lithium dose, slow the Mounjaro escalation schedule, or explore alternative weight management options. Mental health stability is a priority. Your prescriber can help find a safe combination or recommend a different approach if needed.
Does NICE or the NHS mention Mounjaro and lithium together?
NICE guidance on tirzepatide does not contraindicate concurrent lithium use, but emphasises the need for careful monitoring and communication between prescribers. The UK Medicines and Healthcare products Regulatory Agency (MHRA) lists lithium as a medication requiring caution with tirzepatide. Always follow your GP's advice on individual suitability and monitoring.
Ready to take control of your weight loss journey?
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