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 The Contraceptive Implant?
Yes, Mounjaro (tirzepatide) may reduce the effectiveness of hormonal contraceptive implants through slowed absorption. If you use a contraceptive implant and are considering Mounjaro as a weight management medication, it's important to understand how this prescription-only injection interacts with hormonal birth control. This guide explains the interaction, what prescribers recommend, and what steps to take to maintain contraceptive protection whilst using tirzepatide.
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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 Hormonal Contraceptives
Mounjaro works by slowing gastric emptying — the rate at which food and medications leave your stomach. This delayed absorption can affect how quickly your body absorbs oral medications, including hormonal contraceptives taken by mouth. However, the contraceptive implant delivers hormones directly into your bloodstream through the arm, bypassing the stomach entirely.
Despite this, clinical guidance suggests Mounjaro may still reduce implant effectiveness because tirzepatide can affect overall absorption and metabolism of hormonal compounds. The mechanism is not fully understood, but prescribers apply a precautionary approach based on evidence from GLP-1 medications like semaglutide (Wegovy).
UK prescribing guidance: If you use a contraceptive implant and start Mounjaro, you should add a barrier method (condom) or consider switching to a non-hormonal contraceptive for the duration of treatment to reduce the small but real risk of contraceptive failure.
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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 Should Do Before Starting Mounjaro
Before beginning Mounjaro treatment, inform your prescriber that you use a contraceptive implant. They will discuss whether to continue the implant, switch contraceptive methods, or add additional protection. Some patients choose to use condoms alongside their implant for added safety.
If you are taking oral contraceptive pills (the combined pill or progestin-only pill), Mounjaro is more likely to reduce effectiveness — in this case, barrier methods are strongly recommended during treatment and for 4 weeks after each dose increase. Document this conversation with your healthcare provider and keep records of your contraceptive plan.
Always discuss your personal health circumstances with your GP or prescriber before making any changes to contraception.
- 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 my contraceptive implant stop working if I use Mounjaro?
Mounjaro may reduce implant effectiveness slightly, though the exact risk is not fully known. The implant releases hormones directly into your bloodstream, which is more reliable than oral contraceptives. However, prescribers recommend adding a barrier method (condom) or switching to non-hormonal contraception as a precaution whilst taking tirzepatide.
What contraceptive methods are safest with Mounjaro?
Non-hormonal methods are safest: condoms, copper IUD, or barrier methods. If continuing a hormonal implant, use condoms as well. Oral contraceptive pills are less reliable with Mounjaro because they are absorbed through the stomach, where Mounjaro slows absorption. Discuss options with your GP or prescriber.
Do I need to use condoms if I have the implant and take Mounjaro?
Yes, prescribers recommend using condoms alongside a hormonal implant whilst taking Mounjaro as a precautionary measure. This applies for the entire duration of treatment and for 4 weeks after each dose increase, to minimise the risk of contraceptive failure.
Can I switch from the implant to another contraceptive method?
Yes. Many patients switch to a copper IUD (non-hormonal) before starting Mounjaro. This is a conversation to have with your GP or sexual health clinic. They can discuss the best option based on your health history and preferences. Changes should be made before starting tirzepatide if possible.
Will my prescriber know about this interaction?
A UK-licensed prescriber should be aware of Mounjaro and contraceptive interactions. Always disclose all medications and contraceptive methods during your consultation. If you're unsure about the guidance you've been given, ask your prescriber or GP to clarify the recommended approach for your specific situation.
Ready to take control of your weight loss journey?
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