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 Implant?
Yes, Mounjaro (tirzepatide) may affect contraceptive implants and other hormonal contraception. Like other GLP-1 and GIP medications, Mounjaro can slow how quickly your stomach empties, which may reduce the absorption of oral medications—and potentially affect how implants work. This is an important consideration when deciding between weight loss injections like Mounjaro and Wegovy. Your GP or prescriber should assess your contraceptive method before starting either medication to ensure you're protected against unintended pregnancy whilst using weight loss treatment.
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.
Mounjaro, Wegovy, And Contraceptive Implants
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist that slows gastric emptying—how quickly food moves through your stomach. This same mechanism can affect the absorption of other substances, including hormonal contraceptives. Whilst the implant releases hormones directly into the bloodstream (not orally), Mounjaro's effects on your digestive system may theoretically impact how well hormonal contraception works. The product information advises that women using oral contraceptives should add a barrier method or switch to a non-oral contraceptive for 4 weeks after starting Mounjaro and 4 weeks after each dose increase.
Wegovy (semaglutide), a GLP-1-only medication, has similar gastrointestinal effects. Both medications carry the same contraceptive guidance: women on hormonal contraception should discuss alternative protection methods with their prescriber. The implant, being non-oral, may be less affected than pills, but individual factors vary. Your prescriber will assess your specific contraceptive method and may recommend additional precautions to ensure continued protection.
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.
Which Medication Is Safer With An Implant?
Neither Mounjaro nor Wegovy can be definitively said to be "safer" with an implant—both affect gastric emptying and both carry contraceptive guidance. The choice depends on your individual health circumstances, which your GP or licensed prescriber will assess during a clinical consultation. If you have an implant, inform your prescriber before starting either medication. They may recommend using a backup barrier method (like condoms) during the first 4 weeks of treatment and after each dose increase, to maximise protection. Our GLP-1 weight loss calculator can help you understand what different weight loss medications might achieve for you, but your prescriber will assess which option is most suitable given your contraceptive method and health history.
- 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 Mounjaro with a contraceptive implant?
Yes, you can use Mounjaro with an implant, but inform your prescriber first. Mounjaro may affect how well hormonal contraception works by slowing gastric emptying. Your GP may recommend backup contraception (like condoms) for 4 weeks after starting and after each dose increase. Individual risk varies, so prescriber guidance is essential.
Is Wegovy safer than Mounjaro for implant users?
No—both Wegovy and Mounjaro have similar effects on digestion and carry the same contraceptive guidance. Both may affect hormonal contraception absorption. The choice between them depends on your overall health, not contraceptive safety alone. Your prescriber will assess which is more suitable for you.
Do I need backup contraception on Mounjaro?
Yes, the product information recommends that women on oral contraceptives use backup contraception (like condoms) for 4 weeks after starting Mounjaro and 4 weeks after each dose increase. If you have an implant, discuss with your GP whether backup contraception is recommended in your case.
How do GLP-1 medications affect contraceptive implants specifically?
GLP-1 and dual GIP/GLP-1 medications slow stomach emptying, which primarily affects oral medication absorption. Implants release hormones directly into the bloodstream, so the effect may be less direct—but individual variation exists. Your prescriber will assess your specific risk and advise accordingly.
Should I switch contraception before starting Mounjaro or Wegovy?
Not necessarily, but discuss your options with your GP or prescriber before starting. They may recommend sticking with your implant and using backup contraception during treatment, or switching to a non-hormonal method like an IUD. The decision depends on your individual circumstances and risk tolerance.
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.