Rachel* had only been on Mounjaro for a week when it started.
Like all patients starting out on the weight-loss injections, she was on the lowest dose – 2.5mg – and ‘felt otherwise great’.
‘But I noticed every time I went to the bathroom, blood was on the toilet paper,’ the 45-year-old mother of two told Daily Mail.
Rachel, from Sydney, at first assumed it must have been perimenopause.
‘I have been in the throes of perimenopause for a couple of years now, experiencing all the usual annoying symptoms – brain fog, memory loss and, of course, mood swings – so I assumed this was just another symptom I had to contend with,’ she said.
But to ease her mind, Rachel went to see her doctor, who sent her for blood tests.
When everything came back normal, she tried to forget about it – but a month later, she was still bleeding almost every time she wiped.
Worried, Rachel returned to her doctor. ‘My GP was great. She said I didn’t have to put up with this and the answer was to up my oestrogen,’ said Rachel.
Mia Schaumberg (pictured), an Associate Professor in physiology, explains how fat loss can affect menstruation, which may explain why women on weight-loss medication notice changes
Rachel* had an ‘eight-week non-stop period’ after she started on Mounjaro (stock image)
‘At the time I was already on hormone replacement therapy patches so she switched me to oestrogen cream and progesterone pills – it was a disaster.’
Hot flushes and mild depression joined a growing list of complaints for Rachel who was beginning to feel desperate. Her spotting had, by now, turned into full bleeding and her period was the heaviest it had been since her teenage years.
‘I knew to expect changes in energy levels on Mounjaro and hair loss – the doctor even had me get a scan of my gallbladder just to be safe, but nobody, not even my GP, knew about this.’
After two months, Rachel was down a dress size but her ‘eight-week non-stop period’ was not just uncomfortable – it was causing serious fatigue. ‘I have a demanding corporate job and I found myself having to come straight home and take a nap.’
Rachel decided to google ‘Mounjaro period’ and while there wasn’t a lot of information from medical websites, she did find dozens of Reddit threads where women were talking about their periods suddenly returning after years without them, or becoming heavier than they had been in decades.
‘There were even a few questions about heavier cramps, too,’ Rachel said.
But Rachel noticed some of these questions were met with scepticism. Several commenters suggested the period issues were simply due to obesity or sudden weight loss. However, Rachel knew this wasn’t true in her case: she wasn’t obese and by the second month she had only just graduated to the next dose, 5mg.
‘It seemed kind of dismissive to me.’
‘For women, especially those with underlying hormonal conditions like polycystic ovary syndrome (PCOS), even small shifts in insulin levels, body weight, or inflammation can have a ripple effect on oestrogen balance, ovulation patterns, and menstrual symptoms’ (stock image)
Paula* also experienced hormonal changes while taking Mounjaro.
‘I’d been on the contraceptive pill for years and would often skip the seven-day break, so I didn’t exactly have a regular cycle anyway, but a few months before I started on Mounjaro, I’d come off the pill and was starting to see a monthly bleed again,’ she said.
‘The first period I had after a month on 2.5mg of Mounjaro was extremely heavy and the PMS symptoms right beforehand were extreme. I was so full of rage but assumed it was because I’d just come off the pill and my hormones were getting back to normal.’
While this could indeed be the case, Paula felt sure it was a ‘double-whammy’ effect of adding Mounjaro into the mix. The following month she had full-on PMS symptoms but no bleed. The month after that, intermittent bleeding over a two-week period.
Like Rachel, she turned to Google and also found forums with hundreds of posts from women experiencing similar changes.
While there is no official link between Mounjaro and unpleasant changes to women’s menstrual cycles – and it is not listed on the pamphlet as a side effect – a growing number of complaints means some pharmacies that dispense the medication are now posting warnings on their websites, like this one, from Simple Online Pharmacy:
‘Mounjaro (tirzepatide) affects hormones that regulate both appetite and glucose metabolism (how the body uses glucose). So it is possible that changes in hormone levels and insulin sensitivity could also influence the menstrual cycle.’
Mia Schaumberg, an Associate Professor in physiology in the School of Health at the University of the Sunshine Coast, takes it one step further, explaining: ‘Oestrogen is primarily produced in the ovaries, but fat cells also produce oestrogen. This is why weight – and more specifically body fat – can affect menstruation.’
It stands to reason that sudden drops in body fat mean sudden drops in oestrogen, which can have a significant impact on regular menstrual cycles.
Indeed – in contrast to Rachel and Paula’s experience – some women have reported lighter periods and less cramping on Mounjaro, something medical experts theorise has to do with an overall improvement in metabolic health.
‘For women, especially those with underlying hormonal conditions like polycystic ovary syndrome (PCOS), even small shifts in insulin levels, body weight or inflammation can have a ripple effect on oestrogen balance, ovulation patterns, and menstrual symptoms’ agrees Dr Amit Kumar.
‘This means that although Mounjaro is not designed to act on reproductive hormones, its metabolic actions can influence menstrual health in meaningful ways.’
For Rachel, the bleeding seemed to stabilise after a few months, something she’s found is common among the other women who experience menstrual changes on the weight-loss medication. ‘The benefits for me – more energy, lower blood pressure and, of course, weight loss – mean I’m not stopping anytime soon,’ she said.
‘But these days, I take only the smallest dose and I’ve gone back to hormone therapy patches. Honestly, those two things are what’s saving me in perimenopause.’
Paula is considering going back on the pill to see if this improves her periods while on Mounjaro – but, like Rachel, her 11kg weight loss and improved energy levels won’t see her giving up the injections anytime soon.
*names have been changed
