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 Mirtazapine — What You Need To Know
If you're taking mirtazapine and considering Mounjaro (tirzepatide) for weight management, it's essential to understand how these medications may interact. Mirtazapine is an antidepressant that can affect appetite and weight, while Mounjaro is a prescription-only GLP-1 receptor agonist approved in the UK for weight management. This guide explains what happens when combining these medications and why informing your prescriber is crucial before starting any new 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.
Mounjaro And Mirtazapine — Key Interaction Considerations
Mirtazapine is an antidepressant that commonly causes weight gain as a side effect, which is why some people taking it seek weight management support. Mounjaro works by reducing appetite and supporting weight loss through a different mechanism (GLP-1 receptor agonism), so theoretically the medications work in opposing directions regarding weight.
Currently, there are no reported direct pharmacological interactions between tirzepatide (Mounjaro) and mirtazapine in clinical literature. However, this does not mean they are automatically safe to use together — your prescriber must assess your individual situation.
- Both medications affect appetite and gut function, though through different pathways
- Mirtazapine may increase appetite; Mounjaro typically decreases appetite
- Gastrointestinal side effects from Mounjaro (nausea, constipation, diarrhoea) may be amplified if you're also taking mirtazapine
- Your mental health stability must be maintained — weight loss medication should not compromise psychiatric treatment
- Any dose changes to either medication should involve your GP or specialist
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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 To Tell Your Prescriber — NHS and Private Routes
Before starting Mounjaro, you must disclose all current medications — including mirtazapine — to your prescriber. This is true whether you're accessing Mounjaro through NHS specialist tier 3 weight management services or via a private online clinical assessment. Your prescriber will review your medical history, current antidepressant use, and mental health status to determine if Mounjaro is suitable.
If you're currently on mirtazapine and your weight is a concern, discuss this with your GP first. They can assess whether your weight is a side effect of the medication itself, explore whether an alternative antidepressant with less weight gain risk might be appropriate, or refer you to specialist services if weight management medication is clinically justified. Never stop or change mirtazapine without medical guidance.
- 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 mirtazapine?
Possibly, but only under professional assessment. There are no known direct drug interactions, but both medications affect appetite and digestion differently. Your GP or prescriber must review your individual situation, mental health stability, and reason for taking mirtazapine before approving combined use. Never self-prescribe or start either medication without medical guidance.
Will Mounjaro interfere with my mirtazapine antidepressant?
Mounjaro is not expected to reduce mirtazapine's antidepressant effectiveness. However, gastrointestinal side effects from Mounjaro (nausea, constipation, diarrhoea) may be more noticeable if combined with mirtazapine's appetite effects. Your prescriber should monitor you closely and may adjust doses if needed.
Why does mirtazapine cause weight gain?
Mirtazapine increases appetite as a side effect of how it affects neurotransmitters in the brain. It also slows metabolism slightly in some people. This is one of the most common causes of weight gain among antidepressants. If weight gain is a significant concern, discuss alternative antidepressants with your GP — switching medications may be more appropriate than adding weight loss treatment.
How do I access Mounjaro on the NHS if I'm on mirtazapine?
Request a referral from your GP to a tier 3 specialist weight management service. Be transparent about your mirtazapine use. NHS eligibility requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions. Availability varies by region (ICB). Private online prescribing is also available in the UK if NHS waiting times are long.
What should I do if I want to lose weight while taking mirtazapine?
Start by discussing options with your GP. They may suggest lifestyle changes (reduced-calorie diet, increased physical activity), exploring alternative antidepressants, or referral to specialist weight management services. Use our free BMI calculator and weight loss calculator to understand your current situation. Weight loss medication like Mounjaro is only one option and should be considered alongside mental health stability.
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.