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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Can Mounjaro Stop or Affect Your Periods?
Mounjaro (tirzepatide) can potentially affect your menstrual cycle, though this is not a assured side effect. Some women report changes including irregular periods, lighter periods, or temporary cessation during treatment. This can happen for several reasons: gradual weight management itself affects hormone levels, the medication may influence gut hormones involved in the menstrual cycle, or underlying health conditions like PCOS may improve as weight reduces. If you're concerned about period changes while taking Mounjaro on the NHS or privately, it's essential to discuss this with your GP or prescriber. Individual experiences vary significantly, and most changes tend to stabilise over time.
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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
Weight loss and hormones: gradual weight management itself can disrupt oestrogen and progesterone balance, leading to irregular periods or temporary amenorrhoea (cessation of periods). This is separate from the Mounjaro KwikPen injection itself but occurs because of the weight loss it supports. Your body needs adequate fat stores to produce these hormones.
GLP-1 and gut hormone interactions: Mounjaro's dual GIP and GLP-1 mechanism may influence other hormone pathways. Since your gut produces hormones that affect fertility and menstrual regulation, changes to these pathways could theoretically contribute to cycle irregularities.
Underlying conditions improving: If you have polycystic ovary syndrome (PCOS), weight loss from Mounjaro may actually help regulate your periods by improving insulin sensitivity and hormone balance. For some women, this means longer, more regular cycles rather than shorter or absent ones.
Clinical data: The SURMOUNT trials did not list menstrual changes as a primary safety outcome, though some participants reported them informally. The UK MHRA's approved product information does not specifically warn against period changes as a known side effect, though they are reported by users.
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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.
When to Contact Your GP or Prescriber
Contact your GP immediately if you experience:
- Complete cessation of periods lasting more than three months
- Sudden, unexplained changes after months of stability on a consistent Mounjaro dose
- Severe menstrual bleeding or very prolonged periods
- Period changes accompanied by other symptoms like severe pain, fever, or hormonal signs (breast tenderness, mood changes beyond normal)
Your prescriber can assess whether the changes relate to Mounjaro, the weight loss itself, or an unrelated medical issue. If you're on oral contraceptives, remember that Mounjaro may reduce their absorption during dose changes — you may need additional contraception for 4 weeks after starting or increasing your dose. Always inform your GP of any period irregularities; they can perform relevant tests if needed.
- 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.
Is stopping my periods a known side effect of Mounjaro?
Period changes are not listed as a primary side effect in the official MHRA product information, but they are reported by some users during treatment. The most likely cause is gradual weight management rather than the medication itself. If your periods stop completely, contact your GP.
Will my periods come back when I stop Mounjaro?
In most cases, yes. Once weight loss stabilises and hormone levels re-equilibrate, periods typically return to their usual pattern. However, if you've reached a lower stable weight, your cycle may remain different if that weight naturally produces different hormone levels.
Can I use Mounjaro if I want to become pregnant?
Mounjaro is not recommended if you're planning to conceive — you should stop it at least 2 months before trying to get pregnant. Weight loss in preparation for pregnancy is beneficial, but Mounjaro's effects on the gut and metabolism are not studied in pregnancy. Always discuss family planning with your GP.
Does Mounjaro affect contraception?
Yes. Mounjaro may reduce absorption of oral contraceptive pills for 4 weeks after starting treatment or increasing your dose. During this time, use a barrier method (condoms) or switch to a non-oral contraceptive. Your GP can advise on alternatives.
Are period changes more common with Mounjaro than with Wegovy?
There's no direct comparative data on period changes between tirzepatide (Mounjaro) and semaglutide (Wegovy). Both cause weight loss, which independently affects menstrual cycles. Individual responses vary widely. Your GP can help decide which option suits your health profile.
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.