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
Mounjaro and Desogestrel — What You Need to Know
If you're taking desogestrel (a progestogen-only pill) and considering Mounjaro (tirzepatide), it's important to understand how these medications interact. Mounjaro may reduce the effectiveness of oral contraceptives because it slows how quickly food and medications move through your stomach. This guidance covers what the evidence shows, what your GP or prescriber needs to know, and how to stay protected. Always discuss any contraceptive concerns with your healthcare provider before starting Mounjaro.
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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 dual GIP and GLP-1 receptor agonist used for weight management in the UK. One of its effects is slowing gastric emptying — the rate at which food and medications leave your stomach. This delayed absorption can reduce how much of an oral contraceptive your body absorbs, potentially lowering its effectiveness.
Desogestrel (a progestogen-only pill or POP) carries a higher failure rate than combined oral contraceptives anyway — typically around 91–99% effective with typical use. If Mounjaro further reduces absorption, protection may decrease further.
The tirzepatide product information recommends that women taking oral contraceptives should add a barrier method (condoms) or switch to a non-oral contraceptive for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. This accounts for the slow absorption of the injection and the time it takes for gastric emptying to stabilise at higher doses.
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.
What You Should Do — Practical Steps
Before starting Mounjaro, tell your GP or prescriber that you take desogestrel. They can assess your individual risk and advise you on the best approach. Options include: continuing desogestrel with added barrier protection (condoms) during dose escalation phases; switching to a long-acting reversible contraceptive (LARC) such as an IUD, implant, or injection — these avoid the absorption issue; or choosing a combined oral contraceptive, though this also may be affected.
Use our free BMI calculator to check your baseline health metrics before discussing weight management options with your GP. Once you and your prescriber have a contraception plan in place, you can proceed confidently with Mounjaro if it's appropriate for you.
- 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 desogestrel?
You can take both, but Mounjaro may reduce desogestrel absorption and effectiveness. The product information recommends adding barrier contraception (condoms) for 4 weeks after starting Mounjaro and after each dose increase. Discuss this with your GP or prescriber before starting.
Will Mounjaro stop my contraceptive pill from working?
Mounjaro may reduce how much desogestrel your body absorbs because it slows gastric emptying. It doesn't stop it from working entirely, but protection may decrease. This is why healthcare providers recommend additional contraceptive methods during dose escalation phases.
What are better contraception options with Mounjaro?
Long-acting reversible contraceptives (IUD, implant, injection) avoid absorption issues entirely since they're not taken orally. If you prefer oral contraception, discuss barrier methods with your GP or consider switching contraceptive types once Mounjaro dosing is stable.
How long do I need extra contraception when taking Mounjaro?
The product information recommends additional barrier protection for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. This covers the time your body adapts to the injection and gastric emptying stabilises.
Should I tell my prescriber about my desogestrel use?
Yes, absolutely. Always inform your GP or online prescriber about all medications and contraceptives you use. They'll assess your individual situation and may suggest alternatives or additional precautions to ensure you stay protected while on Mounjaro.
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.