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 Contraceptive Pills — What You Need to Know
If you take an oral contraceptive pill and are considering Mounjaro (tirzepatide), it's important to understand how this weight loss injection may affect your contraception. Mounjaro can reduce how quickly your body absorbs oral medications, including contraceptive pills, which means your pill may be less effective during treatment. This guidance applies whether you're starting Mounjaro or increasing your dose. Your prescriber will discuss this with you during your clinical assessment, but here's what the evidence shows about managing both safely.
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 Contraceptive Pill Effectiveness
Mounjaro works by slowing gastric emptying — the rate at which food (and medications) move through your stomach. This is beneficial for appetite control but means oral medications, including contraceptive pills, may not be absorbed as efficiently. The clinical data shows that this reduced absorption could lower contraceptive effectiveness, increasing the risk of unintended pregnancy.
The 4-Week Rule: UK prescribers recommend that women taking oral contraceptives should use an additional barrier method of contraception (such as condoms) for 4 weeks after starting Mounjaro AND for 4 weeks after each dose increase. This precaution covers the period when absorption may be reduced.
After the 4-week period ends, most women can rely on their pill again — absorption typically stabilises. However, this timeline is a precaution based on how tirzepatide works, not a assures. Your prescriber may give you individualised guidance based on your specific pill and health profile.
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.
Alternative Contraception Options and NHS Support
If managing the 4-week barrier method timing seems impractical, you may want to discuss alternative contraception methods with your GP or sexual health clinic. Long-acting reversible contraception (LARC) methods like intrauterine devices (IUDs), implants, or injections are not affected by Mounjaro's impact on medication absorption — they work independently of your digestive system.
These alternatives avoid the need for repeated 4-week precaution periods and may be more convenient during tirzepatide treatment. Your GP can discuss which option suits your health needs and preferences. Be transparent with your prescriber about your contraception when starting Mounjaro to ensure you get personalised guidance that keeps you safe and protected.
- 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 stop my contraceptive pill working completely?
No — Mounjaro reduces how well your pill is absorbed, not eliminates absorption. This is why the 4-week additional contraception rule exists. Your pill will still work, but absorption may be lower during this period, so a backup method is recommended.
How long do I need to use extra contraception when starting Mounjaro?
UK prescribers recommend using a barrier method (condoms) or alternative contraception for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. After this period, absorption typically stabilises.
If I switch contraception to an IUD or implant, do I still need the 4-week precaution?
No — IUDs, implants, and other long-acting reversible contraception (LARC) methods work through local placement rather than absorption, so they are not affected by Mounjaro. You would not need the 4-week additional precaution.
Should I tell my GP I'm taking Mounjaro if I'm on the pill?
Yes, absolutely. Always inform your prescriber about all medications you're taking, including contraception. This allows them to give you personalised guidance and ensure you remain protected throughout your treatment.
Does this affect other types of hormonal contraception like the patch or ring?
The same absorption concern applies to other forms of hormonal contraception (patch, ring, implant hormones). Discuss your specific contraception with your prescriber — they may recommend the 4-week additional precaution or suggest an alternative method like an IUD.
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.