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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Progesterone and Mounjaro — What You Need to Know
If you're taking progesterone-based contraception or hormone therapy and considering Mounjaro (tirzepatide), you may have questions about whether they can be used together safely. Mounjaro is a prescription-only GLP-1 and GIP receptor agonist licensed for weight management in UK adults, and it does interact with how your body absorbs certain medications, including hormonal contraceptives. This guide explains what you need to know about combining progesterone and Mounjaro, and why your prescriber's guidance is essential.
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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 Progesterone and Hormone Absorption
Mounjaro slows how quickly food and medications move through your stomach and digestive system — a process called delayed gastric emptying. This delay can reduce how well your body absorbs oral medications, including progesterone-based contraceptives and hormone replacement therapy (HRT).
The official prescribing information for tirzepatide states that women using oral contraceptives should consider adding a barrier contraception method or switching to a non-oral form during treatment with Mounjaro. Specifically, you should use backup contraception for 4 weeks after starting Mounjaro AND 4 weeks after each dose increase. This precaution applies to all oral hormonal methods, including combined oral contraceptives, progesterone-only pills, and oestrogen-progestin combinations.
If you're taking progesterone for hormone replacement therapy rather than contraception, the same absorption concern applies — your HRT may not be fully effective while on Mounjaro without additional monitoring.
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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 This Means for Your Treatment Plan
Before starting Mounjaro, always inform your prescriber if you're taking any progesterone-based medication — whether for contraception or HRT. Your prescriber may recommend switching to a non-oral method, such as a contraceptive implant, intrauterine device (IUD), injection, or patch. If staying on oral progesterone, use condoms or another barrier method as backup.
If you're already on Mounjaro and using oral progesterone contraception, don't stop your medication — contact your GP or the prescriber who issued your Mounjaro prescription to discuss your options. They can help you understand the actual risk level based on your individual circumstances and suggest the safest approach for your weight management and contraceptive needs.
- 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 oral progesterone contraception while on Mounjaro?
Oral progesterone contraceptives may be less effective on Mounjaro because the medication delays how your body absorbs medications from the gut. You should use a backup contraception method (such as condoms) or switch to a non-oral form. Always discuss with your GP or prescriber before making changes.
How long do I need backup contraception when taking Mounjaro?
You need backup contraception for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. This covers the period during which absorption may be compromised. Your prescriber will advise you on timing based on your specific dose escalation schedule.
What non-oral contraception options are available?
Non-oral options include contraceptive implants (Nexplanon), intrauterine devices (IUDs), contraceptive injections (Depo-Provera), contraceptive patches, and vaginal rings. These bypass the digestive system, so Mounjaro doesn't affect their effectiveness. Your GP can discuss which suits you best.
Does Mounjaro affect HRT progesterone or just contraceptive progesterone?
Mounjaro can affect absorption of any oral progesterone medication, whether for contraception or hormone replacement therapy. If you're on HRT, discuss with your GP whether switching to a patch, gel, or spray form might be safer while taking tirzepatide.
What should I do if I'm already on both progesterone and Mounjaro?
Contact your GP or prescriber immediately — don't stop either medication. They can assess your individual risk, review whether your current contraception is still effective, and discuss alternatives such as switching to a non-oral method or adjusting your Mounjaro schedule if needed.
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.