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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How Mounjaro May Affect Your Menstrual Cycle
Mounjaro (tirzepatide) is a prescription-only GLP-1 and GIP receptor agonist used for weight management in the UK. Some women have reported changes to their menstrual cycle when taking Mounjaro, though this is not listed as a common side effect in the official product information. Understanding how this medication might affect your period, and what changes warrant a conversation with your GP, is important for informed healthcare decisions.
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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.
Why Mounjaro May Influence Menstrual Changes
Menstrual cycle changes are not formally listed as a side effect of Mounjaro in clinical trials, but weight loss itself—which Mounjaro facilitates—can significantly affect hormone levels and menstruation. gradual weight management can temporarily disrupt the hypothalamic-pituitary-ovarian axis, the hormonal pathway that regulates your cycle. Additionally, Mounjaro slows gastric emptying and reduces appetite, which may affect nutrient absorption. If you are not consuming enough calories, iron, or other micronutrients essential for menstrual regulation, changes may occur. The dose titration process—starting at 2.5 mg and escalating to your maintenance dose over weeks—gives your body time to adapt, but individual responses vary significantly. Any noticeable changes to your cycle, such as delayed periods, heavier or lighter bleeding, or increased cramping, should be discussed with your GP, particularly if they persist beyond the initial adjustment phase.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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What to Do If You Notice Menstrual Changes
If your period changes after starting Mounjaro, contact your GP to rule out other causes and ensure you're meeting your nutritional needs. Keep a record of any changes—timing, flow, duration, and associated symptoms—to share with your healthcare provider. Your GP may recommend checking iron levels and other nutritional markers, especially if you're experiencing heavier bleeding or longer cycles. If you're taking hormonal contraception, be aware that gradual weight management or changes in absorption could theoretically affect its efficacy, though this is not specifically documented with Mounjaro. Consider whether you're consuming sufficient calories for your activity level; many women find that adding 100–200 kcal of nutrient-dense food supports cycle regularity whilst still losing weight. Discuss any concerns openly with your prescriber—they can adjust your approach or explore whether continuing Mounjaro is 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 Mounjaro directly cause period changes?
Mounjaro is not listed as causing menstrual changes in clinical trials. However, the weight loss it facilitates can influence hormone levels. Rapid weight reduction, calorie deficiency, or reduced nutrient absorption may affect your cycle. If you notice changes, consult your GP to rule out other factors and ensure adequate nutrition.
Is it safe to take Mounjaro if I'm on contraception?
Mounjaro may reduce absorption of oral medications, including some hormonal contraceptives, during the first four weeks after starting and after each dose increase. The NHS recommends adding a barrier method or switching to a non-oral contraceptive (implant, coil) during these periods. Discuss your specific contraception with your prescriber.
When should I contact my GP about menstrual changes?
Contact your GP if your period is significantly delayed, flow changes dramatically, you experience severe cramping, or bleeding is unusually heavy or prolonged. Changes that persist beyond two to three cycles warrant medical review to rule out other health conditions and assess your nutritional intake.
Could nutrient deficiency from Mounjaro affect my cycle?
Mounjaro doesn't directly cause malabsorption, but reduced food intake and appetite suppression mean some women may eat fewer nutrients, particularly iron and B vitamins essential for menstruation. Eating a balanced, nutrient-dense diet and having your GP check iron levels can help maintain cycle regularity.
Should I stop Mounjaro if my period becomes irregular?
Do not stop Mounjaro without discussing it with your GP first. Menstrual changes may be temporary and linked to weight loss adjustment, not the medication itself. Your prescriber can help determine whether continuing is appropriate and whether nutritional or lifestyle adjustments might help.
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