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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Does Mounjaro Affect Estrogen Levels?
There is no direct evidence that Mounjaro (tirzepatide) affects estrogen levels themselves. However, Mounjaro may affect how your body absorbs oral contraceptives containing estrogen, which is an important consideration for women of reproductive age. This happens because Mounjaro slows gastric emptying—how quickly food and medicines move through your stomach. If you take the combined oral contraceptive pill alongside Mounjaro, your prescriber will recommend adding a barrier method or switching contraception for specific periods. Understanding this distinction between direct hormonal effects and medication interactions is essential before starting 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.
Mounjaro, Oral Contraceptives, and Absorption
Mounjaro does not directly alter estrogen production or hormonal pathways. As a dual GIP and GLP-1 receptor agonist, it works on appetite regulation and glucose control—not on reproductive hormones. However, Mounjaro does slow gastric emptying, which affects how quickly medications dissolve and absorb in your digestive system. This means oral contraceptives taken at the same time as Mounjaro may not be absorbed as effectively, potentially reducing contraceptive protection.
Specific guidance from UK prescribers:
- Women taking the combined oral contraceptive pill should use a barrier method (condoms) for 4 weeks after starting Mounjaro
- Add a barrier method again for 4 weeks after each dose increase (every 4 weeks during titration)
- Alternatively, discuss switching to a non-oral contraceptive (IUD, implant, injection, or patch)
- Progestin-only pills ("mini pill") may have similar absorption issues—discuss with your GP or prescriber
This precaution is specific to tirzepatide (Mounjaro) due to its effect on stomach function, though similar guidance applies to other GLP-1 medications like Wegovy.
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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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Mounjaro vs Wegovy—Hormonal Effects and Contraception
Both Mounjaro and Wegovy (semaglutide) slow gastric emptying and therefore carry the same contraception absorption considerations. Mounjaro uses a dual GIP and GLP-1 mechanism, while Wegovy activates GLP-1 only—but neither directly affects estrogen production. The key difference lies in titration speed: Mounjaro escalates from 2.5 mg to a maximum of 15 mg over roughly 20 weeks, whilst Wegovy titrates from 0.25 mg to 2.4 mg over approximately 16 weeks. Regardless of which medication you use, contraceptive planning should be discussed with your prescriber during your clinical assessment. Try our free GLP-1 weight loss calculator to understand potential outcomes, and use the BMI calculator to establish your baseline before treatment begins.
- 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 change hormone levels?
No. Mounjaro (tirzepatide) does not directly alter estrogen, progesterone, or other reproductive hormones. It works on appetite and blood sugar regulation. However, it slows stomach emptying, which may reduce how well oral contraceptives are absorbed. This is a medication interaction, not a hormonal effect. Always inform your prescriber if you use hormonal contraception.
Can I take the pill with Mounjaro?
Yes, but with precautions. If you take the combined oral contraceptive pill, use a barrier method (condoms) for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. Alternatively, ask your GP about switching to a non-oral contraceptive like an IUD, implant, or injection to avoid absorption issues.
Do I need to stop contraception while taking Mounjaro?
No—you should not stop contraception. Instead, add a barrier method or switch to a non-hormonal or long-acting reversible contraceptive (LARC) option. Stopping contraception entirely leaves you at risk of unplanned pregnancy. Discuss your options with your prescriber before starting treatment.
Does Wegovy affect estrogen like Mounjaro does?
Wegovy (semaglutide) has the same contraceptive absorption considerations as Mounjaro because both slow gastric emptying. If you take oral contraceptives with Wegovy, use a barrier method for the same timeframes (4 weeks after starting, 4 weeks after each dose increase). Neither medication directly changes estrogen levels.
How long do I need barrier contraception with Mounjaro?
Use a barrier method for 4 weeks after your first Mounjaro injection, then for 4 weeks after each dose increase. Once you reach your maintenance dose (typically 10–15 mg) and stop increasing, barrier contraception is no longer required—though you may choose to continue for other reasons. Clarify timing with your prescriber.
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