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
Does Mounjaro Affect Birth Control?
Yes, Mounjaro (tirzepatide) can potentially affect how well oral contraceptive pills work. If you're taking the contraceptive pill and considering Mounjaro as a prescription weight management option, it's important to understand this interaction and what precautions your GP or prescriber will recommend. This guide explains the evidence, what you need to do, and how to discuss this with your healthcare team.
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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 May Affect Oral Contraceptives
Mounjaro slows gastric emptying — the rate at which food and medications move from your stomach into your intestines. This means oral contraceptive pills may not be absorbed as effectively as they should be, potentially reducing their contraceptive protection.
The prescribing information for Mounjaro states that women of childbearing age taking oral contraceptives should use additional contraceptive methods (such as barrier methods like condoms) or consider switching to a non-oral contraceptive during specific periods:
- For 4 weeks after starting Mounjaro
- For 4 weeks after each dose increase
This precaution applies because Mounjaro's effect on gastric emptying is most pronounced when you first start the medication or increase the dose. As your body adjusts, gastric motility may normalise, though individual variation is significant.
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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.
What You Need to Do If You're on the Pill
If you're currently taking an oral contraceptive and want to start Mounjaro, inform your prescriber before your prescription is issued. They will:
- Review your contraceptive method
- Advise whether to use additional contraception during the adjustment periods
- Discuss alternatives such as the contraceptive implant, IUD, or injection if you prefer
- Explain what additional protection (such as condoms) you need during the first 4 weeks and after each dose increase
Non-oral contraceptives like the coil, implant, or injection are not affected by Mounjaro because they do not rely on gastric absorption. If you're considering switching contraceptive methods, your GP can discuss which options might suit your circumstances best.
- 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 the pill definitely stop working if I take Mounjaro?
Not necessarily. The risk of reduced effectiveness is highest when you first start Mounjaro or increase your dose, because gastric emptying is most affected during these periods. Using additional contraception (such as condoms) during the first 4 weeks and 4 weeks after each dose increase significantly reduces the risk. After your body adjusts, absorption typically improves, though individual responses vary. Always follow your prescriber's guidance.
Can I switch to a different birth control method while taking Mounjaro?
Yes. Non-oral contraceptives such as the coil (IUD/IUS), contraceptive implant, or injection are not affected by Mounjaro because they don't rely on stomach absorption. If you're concerned about the pill's effectiveness with Mounjaro, your GP can discuss alternative methods. The choice depends on your medical history, preferences, and circumstances. This conversation should happen before you start Mounjaro.
How long do I need to use backup contraception with Mounjaro?
The prescribing information recommends additional contraceptive methods (such as condoms) for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. This is when gastric emptying is most affected. After these periods, gastric function usually stabilises. However, your prescriber or GP may give you different guidance based on your individual situation, so follow their specific advice.
Does Mounjaro affect contraceptive injections or implants?
No. Mounjaro does not affect contraceptive methods that bypass the gastrointestinal system, such as the contraceptive implant (Nexplanon), injection (Depo-Provera), coil (IUD/IUS), or patches. These methods work independently of stomach absorption and are unaffected by Mounjaro's impact on gastric emptying. They remain reliable throughout your tirzepatide treatment.
Should I tell my prescriber about my contraceptive method?
Yes, absolutely. Always inform your GP or the prescriber conducting your clinical assessment that you're taking an oral contraceptive before starting Mounjaro. This ensures they can give you accurate guidance on whether you need additional contraception, discuss alternative methods, or make any other adjustments to your treatment plan. Open communication about all medications and contraception is essential for safe, personalised care.
Ready to take control of your weight loss journey?
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