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 Progesterone Only Pill — What You Need to Know
If you're taking a progesterone only pill (POP) and considering Mounjaro (tirzepatide), it's important to understand how this weight management injection may affect your contraception. Mounjaro can slow stomach emptying and may reduce how quickly oral medications — including hormonal contraceptives — are absorbed. This means your progesterone only pill might be less effective at preventing pregnancy. Your GP or prescriber needs to know about both medications before starting Mounjaro, and you may need to use additional contraceptive protection or switch to a different method.
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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 dual GIP and GLP-1 receptor agonist — a once-weekly injection licensed in the UK for weight management in eligible adults. It works partly by slowing gastric emptying, meaning food (and medications taken orally) move through your digestive system more slowly. This delayed absorption can reduce the effectiveness of the progesterone only pill.
The official product information for Mounjaro states that women taking oral contraceptives should add a barrier contraceptive method (such as condoms) or switch to a non-oral contraceptive for 4 weeks after starting Mounjaro AND 4 weeks after each dose increase. This guidance applies to all oral hormonal contraceptives, including progesterone only pills, which rely entirely on hormonal absorption for effectiveness.
This is different from some other medications — for example, Wegovy (semaglutide) also affects oral absorption but may have a lower impact due to its single GLP-1 mechanism compared to Mounjaro's dual mechanism.
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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.
Practical Steps if You're on a Progesterone Only Pill
Before starting Mounjaro, inform your prescriber that you're using a progesterone only pill as contraception. They will discuss your options, which may include:
- Using condoms or another barrier method alongside your POP during the adjustment period (first 4 weeks, then 4 weeks after each dose increase)
- Switching to a long-acting reversible contraceptive (LARC) such as an intrauterine device (IUD/coil) or implant — these are unaffected by Mounjaro
- Switching to a combined oral contraceptive, though this still requires barrier protection during titration periods
The key is that you should never rely solely on a progesterone only pill for contraception while starting or escalating Mounjaro. Use our free BMI calculator to determine your eligibility for weight management treatment, and discuss contraceptive options with your GP or licensed prescriber.
- 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 my progesterone only pill not work?
Not necessarily — it may reduce absorption, but the level of risk isn't fully quantified. That's why the guidance recommends additional contraception during the first 4 weeks and after each dose increase. Your prescriber can advise based on your individual circumstances. Never assume it's still fully effective without asking.
How long do I need to use backup contraception with Mounjaro?
According to the product information, use barrier contraception (such as condoms) or switch methods for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. This covers the period when absorption changes are most likely. After this window, your POP may return to normal effectiveness, but confirm with your GP.
Can I switch from a progesterone only pill to something else while on Mounjaro?
Yes — many women switch to long-acting reversible contraception (LARC) like an IUD or implant while using Mounjaro because these methods aren't affected by oral absorption. Discuss your options with your GP. A hormonal coil is a good option as it combines hormonal contraception with non-oral delivery.
Is the interaction between Mounjaro and POPs the same as with Wegovy?
Mounjaro and Wegovy both affect oral medication absorption by slowing gastric emptying. However, Mounjaro uses a dual GIP+GLP-1 mechanism while Wegovy is GLP-1 only. The clinical impact may differ slightly, but both carry official guidance recommending barrier contraception during the first 4 weeks and after dose escalations.
What if I want to stay on a progesterone only pill — can I just use condoms?
Yes — using condoms consistently alongside your POP during the first 4 weeks of Mounjaro and after each dose increase is an approved approach. This provides dual protection during the high-risk absorption window. After this period, your POP should return to normal effectiveness, but confirm timing with your GP.
Ready to take control of your weight loss journey?
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