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
Progesterone and Mounjaro — What UK Women Need to Know
If you're taking progesterone-based contraception and considering Mounjaro (tirzepatide) for weight management, it's important to understand how these medications interact. The UK prescribing guidance for Mounjaro includes specific recommendations about hormonal contraception. This guide explains what you need to know about using both safely, the NHS eligibility criteria for Mounjaro in the UK, and how to discuss this with your GP or prescriber.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Progesterone-Based Contraception
Mounjaro (tirzepatide) can reduce how well oral contraceptives are absorbed in your stomach, potentially lowering their effectiveness. The UK prescribing information states that if you're taking progesterone-based contraception — including the combined oral pill, progestogen-only pill, or any hormone-based method — you may need to take additional precautions when starting Mounjaro or when increasing your dose.
The recommendation is to use a barrier contraceptive method (condoms) or switch to a non-hormonal contraceptive for:
- 4 weeks after starting Mounjaro
- 4 weeks after each dose increase
This precaution is necessary because Mounjaro slows gastric emptying (how quickly food and medicines move through your stomach), which can reduce absorption of oral medications. If you rely on your contraceptive pill for pregnancy prevention, this is an important safety consideration.
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.
Discussing Mounjaro with Your NHS GP or Prescriber
When you discuss Mounjaro eligibility with your NHS GP or specialist weight management service, always mention that you're taking progesterone-based contraception. Your prescriber will assess whether Mounjaro is suitable for you and will give you clear guidance on managing both medications safely.
If you're accessing Mounjaro through a private prescriber following an online consultation, provide the same information during your clinical assessment. Your prescriber will review your contraception method and advise whether to use barrier methods during the titration period or whether switching to an alternative contraceptive might be appropriate for you. Never adjust your contraception or Mounjaro dose without consulting your healthcare provider.
- 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.
Does Mounjaro make the contraceptive pill less effective?
Mounjaro can reduce absorption of oral contraceptives, potentially lowering effectiveness. The UK guidance recommends using barrier methods or switching to non-hormonal contraception for 4 weeks after starting and after each dose increase. Discuss alternative contraception options with your prescriber.
Can I take Mounjaro with the progestogen-only pill?
The same precautions apply to progestogen-only pills as to combined pills — additional contraception is recommended for 4 weeks after starting and during dose increases. Your GP or prescriber can advise on the best approach for your individual circumstances.
What contraceptive methods don't interact with Mounjaro?
Non-hormonal methods including condoms, copper IUDs, diaphragms, and fertility awareness methods are not affected by Mounjaro. Hormonal IUDs (like mirena) may also be suitable — ask your prescriber for guidance on your specific method.
Am I eligible for Mounjaro on the NHS in the UK?
NHS eligibility for Mounjaro requires a BMI of 30 kg/m² or above, or BMI 27–29.9 kg/m² with weight-related health conditions. You'll need referral to a specialist tier 3 weight management service from your GP. Eligibility criteria are the same regardless of contraception method.
When can I stop using barrier contraception if I'm on Mounjaro?
Continue barrier methods for 4 weeks after each dose increase on your titration schedule. Once you've reached your stable maintenance dose with no further increases planned, you can usually return to your regular contraception method — but confirm with your prescriber.
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.