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 Contraception — What You Need to Know
If you're taking Mounjaro (tirzepatide) and using oral contraceptives, there's an important interaction you need to know about. Mounjaro can affect how your body absorbs oral contraceptive pills, which means your contraception may be less effective. This guide explains the contraception guidance for Mounjaro users in the UK, how it compares to Wegovy, and what to discuss with your GP or prescriber.
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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 slows gastric emptying — the rate at which food and other substances move through your stomach. This can affect how your body absorbs oral medications, including contraceptive pills. The product information states that women taking oral contraceptives should use an additional barrier method (such as condoms) or switch to a non-oral contraceptive for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase.
This means if you're on a standard dose escalation schedule (moving from 2.5 mg to 5 mg to 7.5 mg, etc.), you may need backup contraception for several months while your doses are being adjusted. Once you've reached your stable maintenance dose and haven't increased for 4 weeks, the interaction risk reduces significantly.
- Use condoms or another barrier method as backup
- Consider switching to an implant, coil (IUD), or injection if you prefer not to use barrier methods
- Discuss your specific contraception plan with your GP or prescriber
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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.
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Mounjaro vs Wegovy — Contraception Differences
Wegovy (semaglutide) also slows gastric emptying, but differently from Mounjaro because it works through a single GLP-1 pathway rather than Mounjaro's dual GIP and GLP-1 mechanism. However, Wegovy carries the same contraception guidance as Mounjaro — use backup contraception for 4 weeks after starting and after each dose increase. Neither medication is safer for oral contraceptive users than the other in terms of interaction risk.
The choice between Mounjaro and Wegovy is based on individual suitability, medical history, and prescriber assessment — not on contraception factors. Your prescriber will assess which is appropriate for you following a clinical evaluation. Use our free GLP-1 weight loss calculator to explore potential weight loss projections with either 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.
Can I use my contraceptive pill while taking Mounjaro?
You can take your pill, but it may be less effective because Mounjaro reduces how your body absorbs oral medications. Use backup contraception (condoms) or switch to a non-oral method for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. Once your dose is stable, discuss ongoing contraception with your GP.
What's the best contraception to use if I'm taking Mounjaro?
Non-oral methods are safest: contraceptive implants, coils (IUDs), injections, or patches. If you prefer oral contraceptives, use condoms as backup during dose escalation. Barrier methods (condoms, diaphragms) are also effective when used correctly. Discuss your options with your GP or prescriber to choose what suits you best.
How long do I need backup contraception with Mounjaro?
You need backup contraception for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. If you're escalating doses (2.5 mg → 5 mg → 7.5 mg, etc.) monthly, this could be several months. Once you've reached a stable dose and haven't increased for 4 weeks, the main risk period ends.
Does Wegovy have the same contraception issues as Mounjaro?
Yes. Wegovy (semaglutide) carries the same guidance — use backup contraception for 4 weeks after starting and after each dose increase. Both medications slow gastric emptying and affect oral medication absorption. The choice between them is based on individual health factors, not contraception considerations.
What non-oral contraception options should I consider?
Options include contraceptive implants (Nexplanon), coils (copper IUD or hormonal IUD like Mirena), contraceptive injections (Depo-Provera), or contraceptive patches. Each has different effectiveness and duration. Your GP can advise which is most suitable for your health history and lifestyle. These methods avoid the absorption issue entirely.
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