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 the Progesterone Only Pill — What You Need to Know
If you're taking the progesterone only pill (POP) and considering Mounjaro for weight management, it's important to understand how these medications interact. Mounjaro (tirzepatide) may affect how your body absorbs oral medications, including hormonal contraception. The UK regulatory guidance recommends additional contraceptive precautions when starting Mounjaro or increasing your dose. This guide explains the interaction, what the evidence shows, and how to protect yourself while using both treatments. Always discuss this with your GP or prescriber before starting any new medication.
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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 Oral Contraceptive Absorption
Mounjaro is a GIP and GLP-1 receptor agonist — a once-weekly injection licensed for weight management in UK adults. One of its mechanisms of action is slowing gastric emptying (how quickly food leaves the stomach). This delay in stomach emptying can reduce how quickly oral medications are absorbed into the bloodstream, including the active ingredients in the progesterone only pill.
The progesterone only pill (POP) — also called the mini-pill — works by thickening cervical mucus and thinning the womb lining. Its effectiveness depends on consistent blood levels of the hormone. If Mounjaro reduces absorption, this could theoretically lower hormone levels and reduce contraceptive protection. The UK product information for Mounjaro (tirzepatide) specifically states: "Women of childbearing potential should consider using an additional method of contraception when starting Mounjaro and for 4 weeks after each dose increase." This is a precautionary measure based on the known effect on oral medication absorption.
- Mounjaro may slow absorption of oral medications
- This includes hormonal contraceptives
- The effect is typically temporary and reduces over time as your body adjusts
- Your prescriber should be informed if you use the POP
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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.
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What Precautions Should You Take?
The recommended approach is to use an additional contraceptive method alongside your progesterone only pill for the first 4 weeks after starting Mounjaro, and for 4 weeks after each dose increase (when taking Mounjaro from 2.5 mg through to 15 mg via your Mounjaro KwikPen). Additional methods include condoms, which also offer protection against sexually transmitted infections. Some women choose to switch to a different form of contraception entirely during Mounjaro treatment — for example, the combined oral contraceptive pill, the contraceptive patch, or a long-acting reversible contraceptive (LARC) like an implant or coil, which are not affected by gastrointestinal absorption.
Your prescriber will review your individual circumstances and can advise on the best approach. After 4 weeks on a stable Mounjaro dose, absorption of the POP typically normalises and additional contraception is generally not needed, though this should be confirmed with your healthcare provider. Keep open communication with both your GP and your Mounjaro prescriber about your contraception choices.
- 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 definitely make the progesterone only pill less effective?
Not necessarily. Mounjaro slows gastric emptying, which may reduce absorption of oral medications. However, individual absorption varies, and effectiveness may not be significantly reduced for everyone. This is why additional contraception is recommended as a precaution, especially during dose titration. Your GP can discuss your individual risk.
How long do I need to use additional contraception with Mounjaro?
The UK guidance recommends additional contraception (like condoms) for 4 weeks after starting Mounjaro AND for 4 weeks after each dose increase. After 4 weeks on a stable dose, absorption usually normalises. Always confirm this timeline with your prescriber.
Can I switch to a different contraceptive while taking Mounjaro?
Yes. Many women switch to a LARC (implant, coil, or injection) or the combined pill during Mounjaro treatment. These are not affected by gastrointestinal absorption. Discuss your options with your GP — they can advise on what's best for you.
What if I become pregnant while taking Mounjaro?
Mounjaro is not recommended during pregnancy. If you become pregnant, stop Mounjaro immediately and contact your GP. Effective contraception is essential while using Mounjaro. This is another reason why backup contraception during dose changes is important.
Does Mounjaro affect other medications the same way?
Yes. Mounjaro may reduce absorption of any oral medication. If you take other tablets (including antibiotics, pain relief, or other medications), mention this to your prescriber. Some medications may need timing adjustments or dose reviews.
Ready to take control of your weight loss journey?
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