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
Free tracker included with every download this week
Mounjaro And Progesterone Only Pill — What You Need To Know
If you're taking a progesterone only pill (POP) and considering Mounjaro, you may be wondering whether this weight management injection will affect your contraception. The short answer is: yes, there's an important interaction you need to discuss with your prescriber. Mounjaro can reduce how well oral medications—including the progesterone only pill—are absorbed in your gut. This means your contraceptive protection may be compromised. Your GP or prescriber will need to discuss alternative contraception options with you before starting treatment.
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Calculate Your BMI Instantly
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 Contraceptives
Mounjaro (tirzepatide) slows gastric emptying—the rate at which food and medication move through your stomach. This can reduce the absorption of oral medications taken at the same time, including hormonal contraceptives like the progesterone only pill.
According to the UK Product Information for Mounjaro, women taking oral contraceptives should add a barrier method (condoms) or switch to a non-oral contraceptive for four weeks after starting the medication AND four weeks after each dose increase. This precautionary period ensures reliable contraceptive cover whilst your body adjusts to tirzepatide.
- The progesterone only pill must be taken at the same time each day (within a 3-hour window)
- Mounjaro may interfere with this precise absorption timing
- Barrier methods or alternative contraception are safer options during treatment
- Your prescriber will discuss the best contraceptive choice for you
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
NHS Guidance And What To Do Next
Before starting Mounjaro, inform your GP or prescriber that you're using the progesterone only pill. They will assess your individual situation and discuss contraceptive alternatives with you. Non-oral options like the contraceptive implant, coil (IUD), or injection are unaffected by Mounjaro and may be recommended whilst you're on tirzepatide treatment.
If you continue using the progesterone only pill alongside Mounjaro, you must use additional barrier contraception (condoms) during the adjustment periods mentioned above. Never stop your contraception without medical guidance. Your NHS GP can refer you to sexual health services if you need support changing contraception methods.
- 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 stop my progesterone only pill from working?
Mounjaro may reduce absorption of oral contraceptives, but it won't completely prevent it. The risk is significant enough that UK guidance recommends additional barrier contraception or switching to a non-oral method. Your prescriber will assess your individual risk and discuss the safest option for you.
Can I use condoms alongside my progesterone only pill when taking Mounjaro?
Yes. Adding condoms as a barrier method is one recommended approach during the adjustment periods after starting Mounjaro and after each dose increase (four weeks each time). However, switching to a non-oral contraceptive is often considered more reliable. Discuss this with your GP.
What contraceptive options work better with Mounjaro?
Non-oral methods are unaffected by Mounjaro's effect on stomach emptying. These include: contraceptive implant, coil (IUD), contraceptive injection, and copper coil. Your GP can discuss which option suits your health needs and preferences. Your local sexual health clinic can fit these quickly.
Should I stop my progesterone only pill when I start Mounjaro?
No—never stop contraception without medical guidance. Talk to your prescriber or GP before starting Mounjaro so you can plan a contraceptive change if needed. Stopping suddenly risks unplanned pregnancy. Your GP can arrange a faster appointment if this is urgent.
Do other weight loss medications like Wegovy have the same interaction?
Yes. Wegovy (semaglutide) is also a GLP-1 injection that affects stomach emptying and oral medication absorption. The same contraceptive precautions apply to Wegovy. Any GLP-1 medication requires discussion with your prescriber about contraception safety.
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.