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 Desogestrel — What You Need To Know
If you're taking desogestrel (a progestin-only contraceptive pill, sometimes called the "mini-pill") and considering Mounjaro (tirzepatide) for weight management, you'll want to understand how these medications interact. Mounjaro may affect how your body absorbs oral contraceptives, which means desogestrel effectiveness could potentially be reduced. This is an important consideration that requires discussion with your GP or prescriber before starting Mounjaro. Understanding this interaction helps you make informed decisions about your weight management and contraception options.
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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 is a GIP and GLP-1 receptor agonist that slows gastric emptying — the speed at which food (and other substances) leaves your stomach and enters your small intestine. Because desogestrel is an oral medication, it relies on being absorbed through your digestive system. When Mounjaro slows this process, it may reduce how effectively desogestrel is absorbed, potentially lowering its contraceptive effectiveness.
The product information for Mounjaro specifically notes that it may reduce absorption of orally taken medicines. Desogestrel, like all oral contraceptives, falls into this category. This is not unique to Mounjaro — GLP-1 medications (such as Wegovy, which contains semaglutide) and other GIP/GLP-1 agonists have similar effects on oral medication absorption.
This does not mean you cannot use both medications together, but it does mean your prescriber needs to be aware of both medications so they can give you personalised guidance. Some patients successfully take both, but contraceptive cover may require additional support.
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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
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What Your GP Or Prescriber Should Recommend
When you disclose that you're taking desogestrel, your prescriber should recommend one of these approaches:
- Add a barrier method: Use condoms alongside desogestrel while taking Mounjaro, to provide additional contraceptive cover
- Switch contraception method: Consider moving to a non-oral contraceptive — such as an implant, coil (IUD/IUS), injection, or patch — which bypasses the digestive system entirely
- Timing considerations: Some prescribers may recommend spacing doses, though this is less commonly advised than the above options
The UK prescribing guidance suggests adding a barrier method or switching to non-oral contraception for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. Always confirm the current guidance with your prescriber, as recommendations may evolve.
- 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 definitely make desogestrel less effective?
Mounjaro may reduce desogestrel absorption due to slowed gastric emptying, but effectiveness isn't assured to be lost. However, contraceptive cover cannot be assumed to be reliable. Your prescriber will recommend additional protection (barrier method or contraception switch) to ensure reliable contraception while taking both medications.
Can I switch to a different contraceptive instead?
Yes — switching to a non-oral contraceptive that doesn't rely on stomach absorption is often recommended. Options include the contraceptive implant, IUD/IUS coil, injection (Depo-Provera), or patch. These methods bypass the digestive system entirely, avoiding any interaction with Mounjaro.
How long do I need additional contraceptive protection?
UK prescribing guidance recommends additional barrier protection (or contraception switch) for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. After this period, if you remain on a stable dose, your prescriber will advise whether the recommendation continues.
Should I tell my prescriber about desogestrel before starting Mounjaro?
Yes, absolutely. Always inform your prescriber of all medications, including oral contraceptives, before starting Mounjaro. This allows them to assess the interaction and recommend the safest approach for your individual situation — whether that's a barrier method, contraception switch, or other guidance.
Is this interaction the same with Wegovy or other weight loss injections?
Yes — Wegovy (semaglutide) and other GLP-1 and GIP/GLP-1 medications have similar effects on oral medication absorption. Any oral contraceptive may be affected. Always disclose your contraception method to your prescriber when considering any weight loss injection.
Ready to take control of your weight loss journey?
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