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 The Mini Pill: What You Need To Know
If you're taking the mini pill (progestogen-only pill) and considering Mounjaro for weight management, it's important to understand how these medications interact. Mounjaro (tirzepatide) can reduce the absorption of oral medications, including contraceptives. This means the mini pill may be less effective at preventing pregnancy whilst you're on Mounjaro. Your GP or prescriber will discuss contraceptive adjustments with you before starting Mounjaro, and you may need to use additional protection or switch methods.
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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.
How Mounjaro Affects Oral Contraceptives
Mounjaro is a prescription-only GIP and GLP-1 receptor agonist that slows how quickly food leaves your stomach. This delayed gastric emptying also affects how quickly oral medications — including the mini pill — are absorbed into your bloodstream. Research and clinical guidance indicate that oral contraceptives may have reduced effectiveness when taken alongside Mounjaro or other weight loss injections.
Key points:
- The mini pill relies on consistent hormone levels to prevent pregnancy
- Reduced absorption means lower hormone levels in your body
- This increases the risk of contraceptive failure
- The effect is temporary — it typically reduces after a few months as your body adjusts to Mounjaro
- You will be advised to use backup contraception during treatment
Your prescriber will discuss this openly and help you choose the best approach for your individual circumstances.
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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 Your Prescriber Will Recommend
Before starting Mounjaro, you must tell your prescriber that you're taking the mini pill. They will likely recommend one of the following approaches:
- Add a barrier method: Use condoms alongside the mini pill for added protection during the first 4 weeks and for 4 weeks after each Mounjaro dose increase
- Switch contraception: Change to an alternative method such as the combined pill, implant, coil, or injection — these may be less affected by Mounjaro's impact on absorption
- Continue with additional precautions: Stay on the mini pill but use consistent backup contraception throughout your Mounjaro treatment
Individual recommendations vary based on your health history and contraceptive preferences. Your GP can discuss which option suits you 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.
Can I take Mounjaro while on the mini pill?
Yes, you can take both, but your mini pill may be less effective because Mounjaro slows stomach emptying and reduces absorption of oral medications. You'll need backup contraception or an alternative contraceptive method. Always inform your prescriber before starting Mounjaro if you're on the mini pill.
How long do I need backup contraception with Mounjaro?
You should use additional contraception for 4 weeks after starting Mounjaro, and for 4 weeks after each dose increase. This covers the period when gastric effects are strongest. After 4 weeks, absorption typically improves, though backup protection is still recommended throughout treatment.
What are the best contraceptive alternatives to the mini pill while on Mounjaro?
Long-acting reversible contraceptives (LARCs) such as the coil, implant, or injection are good alternatives because they don't rely on oral absorption. If you prefer oral contraception, the combined pill may work, though your GP may still recommend backup. Discuss options with your prescriber.
Does Mounjaro affect other oral medications besides contraceptives?
Yes. Mounjaro can reduce absorption of any oral medication taken at the same time. Tell your prescriber about all medications you take — they may adjust timing, dosing, or suggest alternatives to ensure your medicines work effectively.
Will my mini pill effectiveness return to normal after stopping Mounjaro?
Yes. Once you stop Mounjaro, gastric function returns to normal and oral medication absorption improves. However, you should continue contraception during your Mounjaro treatment and discuss a plan with your GP for switching back if you decide to change contraceptive methods.
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.