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 Menstrual Bleeding — What You Need To Know
Mounjaro (tirzepatide) may affect your menstrual cycle in some women. Weight loss itself can influence period regularity, and changes to stomach function may affect how oral medications work — including hormonal contraceptives. This NHS UK guide explains what menstrual changes may occur, when to contact your GP, and how to manage your contraception safely if you're taking Mounjaro as a prescription weight management injection.
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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 May Affect Your Menstrual Cycle
Mounjaro (tirzepatide), a dual GIP and GLP-1 receptor agonist, may indirectly affect your period through several mechanisms. Weight loss itself can alter hormone levels and menstrual regularity — losing 5–10% of body weight may cause temporary changes to your cycle. Additionally, Mounjaro slows gastric emptying (how quickly food leaves the stomach), which may reduce the absorption of oral contraceptive pills.
- Some women report lighter or irregular periods after starting Mounjaro
- Others experience temporary amenorrhoea (missed periods) during gradual weight management
- Contraceptive absorption may be reduced due to slower stomach emptying
- These effects are usually temporary and resolve as your body adapts
If you experience significant menstrual changes, contact your GP. They can assess whether changes are related to weight loss, Mounjaro itself, or another cause. Do not assume your contraception is less effective — seek advice before relying solely on your pill.
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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.
Oral Contraceptives And Mounjaro — UK Safety Guidance
The UK prescribing information for Mounjaro specifically advises women using oral contraceptives to 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 is because reduced stomach emptying may lower pill absorption, potentially reducing contraceptive efficacy.
- Alternative contraceptive methods to consider: IUD, implant, injection, patch, or ring
- Inform your prescriber and GP about all medications, including contraception
- Your GP can advise on the safest contraceptive option for you while taking tirzepatide
- Track your menstrual cycle using our free progress tracker to monitor any changes
- 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 Mounjaro stop your period?
gradual weight management from any medication, including Mounjaro, can cause temporary amenorrhoea (missed periods). This is usually temporary and related to weight loss rather than the medication itself. However, contact your GP if periods stop or become severely irregular — they can rule out other causes and check your health.
Will my period return to normal after stopping Mounjaro?
Yes, for most women menstrual regularity returns once weight stabilises and you stop taking Mounjaro. However, everyone's body responds differently. Your GP can advise on what to expect based on your individual situation and health history.
Is my contraceptive pill still effective while taking Mounjaro?
Mounjaro may reduce oral contraceptive absorption due to slower stomach emptying. NHS guidance recommends using a barrier method (condoms) alongside your pill, or switching to a non-oral method (IUD, implant, injection) for safer protection. Always discuss this with your GP or prescriber.
When should I contact my GP about menstrual changes on Mounjaro?
Contact your GP if you experience: periods stopping completely (beyond a temporary missed cycle), severe pain, very heavy bleeding, or if you're concerned about contraceptive effectiveness. These changes warrant professional assessment to rule out other causes.
Can I use the NHS weight loss calculator if I'm on Mounjaro?
Yes. Our free GLP-1 weight loss calculator can help you estimate potential weight loss with tirzepatide at different doses. This can be useful to discuss with your GP and to track realistic expectations alongside monitoring menstrual changes during weight loss.
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.