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 progestin-only pill (mini pill) and considering Mounjaro for weight management, it's important to understand how these two medicines interact. Mounjaro (tirzepatide) is a prescription-only weight loss injection licensed in the UK that can affect how your body absorbs oral medications, including hormonal contraceptives. This guide explains what the evidence shows and what your prescriber needs to know before you start treatment.
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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 Oral Contraceptive Absorption
Mounjaro slows gastric emptying — the rate at which food and medications leave your stomach. This can reduce how much of an oral medication your body absorbs, including progestin-only pills (the mini pill, brands like Cerazette, Norgeston, and Micronor). The MHRA-approved prescribing information for Mounjaro specifically states that women taking oral contraceptives should add a barrier contraceptive 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 the mini pill as your sole contraceptive, you'll need backup protection during these periods
- The effect typically lessens as your body adjusts to the medication — absorption should normalise after the initial adjustment window
- Your prescriber must know about your contraception before starting tirzepatide
Always inform your GP or prescriber of all medications and contraceptive methods before beginning Mounjaro treatment.
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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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Mini Pill vs Combined Pill — Is There a Difference?
The mini pill (progestin-only pill) is affected by Mounjaro's impact on medication absorption. The combined oral contraceptive pill (containing oestrogen and progestin) may also be affected, though clinical guidance typically focuses on the mini pill specifically. Either way, if you use any oral contraceptive, inform your prescriber before starting Mounjaro. They may recommend switching to a long-acting reversible contraceptive (LARC) such as an implant, IUD, or injection — these are not affected by Mounjaro's gastric effects and remain highly effective. If you prefer to stay on the pill, barrier methods during dose escalation periods provide reliable additional protection. Your GP or prescriber can discuss which option suits your circumstances 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.
Does Mounjaro (tirzepatide) make the mini pill less effective?
Yes, potentially. Mounjaro slows stomach emptying, which can reduce absorption of oral medications including the mini pill. The MHRA advises adding barrier contraception or switching methods for 4 weeks after starting and after each dose increase. After this adjustment period, absorption should normalise. Your prescriber must assess this individually.
What contraceptive methods are NOT affected by Mounjaro?
Long-acting reversible contraceptives (LARCs) like intrauterine devices (IUDs), implants, and injections are not affected by Mounjaro's impact on oral medication absorption. These remain highly effective options alongside tirzepatide. Ask your GP about switching if the mini pill becomes unreliable with Mounjaro.
Can I stay on the mini pill while taking Mounjaro?
You can, but you'll need additional contraceptive protection during the initial 4 weeks and after each dose increase. After your body adjusts (usually beyond 4 weeks), the mini pill may regain effectiveness. Your prescriber will give personalised guidance based on your health and preferences.
What's the difference between the mini pill and combined oral contraceptive with Mounjaro?
Both oral contraceptives may be affected by Mounjaro's gastric effects. The prescribing guidance specifically mentions oral contraceptives requiring barrier backup. Your GP can advise whether the combined pill, mini pill, or a LARC method is best for you on tirzepatide.
When should I discuss my contraception with my prescriber?
Before starting Mounjaro, always mention your current contraceptive method. Your GP or the prescriber issuing your tirzepatide prescription needs this information to assess whether the mini pill remains suitable or if switching to barrier methods, LALRCs, or other contraceptives is advisable for your safety.
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