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 Contraceptive Pills: What You Need to Know
If you're taking a contraceptive pill and considering Mounjaro (tirzepatide), it's important to understand how these medications interact. Mounjaro may reduce how well your body absorbs oral contraceptives, which means your pill might be less effective at preventing pregnancy. The UK prescribing guidance recommends additional contraceptive protection if you start Mounjaro or increase your dose. This guide explains the interaction, how long you need extra protection, and what alternatives exist.
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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 Contraceptive Pill Absorption
Mounjaro (tirzepatide) works by slowing how quickly food moves through your stomach and digestive system. This delayed gastric emptying can also affect how your body absorbs other medications taken by mouth — including contraceptive pills. When a medication is absorbed more slowly or less completely, your body receives a lower dose than intended.
Oral contraceptives rely on consistent, reliable absorption to maintain contraceptive effectiveness. If Mounjaro reduces this absorption, the hormone levels in your blood may drop below the protective threshold, increasing the risk of innovative ovulation and unintended pregnancy.
MHRA and UK prescribing guidance therefore recommends that women taking oral contraceptives should add a barrier method (condom) or switch to a non-oral contraceptive if they start Mounjaro or increase their dose. This precaution applies during treatment and continues for 4 weeks after each dose increase and 4 weeks after stopping Mounjaro.
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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.
Contraceptive Alternatives and Practical Guidance
If you want to use Mounjaro, several non-oral contraceptive options are unaffected by the interaction: the contraceptive implant (Nexplanon), intrauterine device (coil), hormonal coil (Mirena), contraceptive injection (Depo-Provera), and barrier methods like condoms. Discuss these options with your GP or contraception clinic before starting Mounjaro.
If you prefer to continue taking the contraceptive pill, you must use additional barrier contraception (condoms) during the first 4 weeks of Mounjaro treatment, after each dose increase, and for 4 weeks after your final injection. Set a reminder on your phone or calendar — this timing is easy to forget but important.
Always inform your prescriber about any contraceptives you're using when requesting Mounjaro, and tell your contraception provider if you start a new weight loss medication.
If you want to use Mounjaro, several non-oral contraceptive options are unaffected by the interaction: the contraceptive implant (Nexplanon), intrauterine device (coil), hormonal coil (Mirena), contraceptive injection (Depo-Provera), and barrier methods like condoms. Discuss these options with your GP or contraception clinic before starting Mounjaro.
If you prefer to continue taking the contraceptive pill, you must use additional barrier contraception (condoms) during the first 4 weeks of Mounjaro treatment, after each dose increase, and for 4 weeks after your final injection. Set a reminder on your phone or calendar — this timing is easy to forget but important.
Always inform your prescriber about any contraceptives you're using when requesting Mounjaro, and tell your contraception provider if you start a new weight loss medication.
- 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 from working?
Mounjaro may reduce your pill's effectiveness by slowing medication absorption in your gut. This doesn't mean your pill will definitely fail, but pregnancy risk increases. UK guidance recommends additional barrier contraception (condoms) during treatment and after dose increases to stay safe.
How long do I need to use condoms with Mounjaro and the pill?
Use condoms for 4 weeks after you start Mounjaro, for 4 weeks after each dose increase, and for 4 weeks after your final injection. After this period, oral contraceptive effectiveness typically returns to normal.
Can I switch to a different contraceptive method?
Yes. The contraceptive implant, coil, hormonal coil, and contraceptive injection are not affected by Mounjaro. Discuss these options with your GP or contraception clinic. Many women find these methods convenient long-term options.
Do I need to tell my prescriber about my contraceptive?
Yes, absolutely. Inform your prescriber or pharmacist about any contraceptives you use before starting Mounjaro. This ensures they can give you the correct guidance and document the interaction in your medical record.
Does Wegovy have the same contraceptive interaction?
Yes. Wegovy (semaglutide), like Mounjaro, also slows gastric emptying and may reduce oral contraceptive absorption. The same 4-week additional protection rule applies with Wegovy. Always check with your prescriber about any GLP-1 medication and contraception.
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.