SLIMMERS were today warned that weight loss jabs could trigger two new side effects not listed by manufacturers.
Once aimed at diabetes patients, drugs like Ozempic and Mounjaro are now famous for bringing about rapid weight loss by reducing appetite.
But the injections often come with side effects, from nausea and constipation and, in most severe cases, life-threatening organ damage.
Now, scientists have warned the medication may also help trigger reproductive symptoms such as irregular menstrual cycles and temperature-related issues including chills and hot flashes.
Experts believe the jabs, known medically as GLP-1 medications, do not directly target reproductive hormones or temperature regulation.
Instead, the physiological effects of rapid weight loss, reduced calorie intake and changes in metabolism could indirectly influence period regularity and how the body adapts to a lower energy state.
The findings from US researchers were uncovered in a social media focused study that assessed more than half a decades worth of posts between May 2019 and June 2025 from nearly 70,000 Reddit users.
Published in the journal Nature Health, they found nausea (36.9 per cent), fatigue (16.7 per cent), vomiting (16.3 per cent), constipation (15.3 per cent) and diarrhoea (12.6 per cent) were the four most commonly reported symptoms.
But, “notably, reproductive symptoms, for example, menstrual irregularities and temperature-related complaints, for example, chills and hot flushes, emerged as unrecognized potential effects,” they wrote.
“These findings highlight patient concerns not well captured in current labelling or trials.”
In addition, fatigue ranked as the second most common complaint among Reddit users, despite reaching reporting thresholds in relatively few clinical trials.
Neil Sehgal, study lead author and doctoral student in computer and information science at the University of Pennsylvannia, said: “We can’t say that GLP-1s are actually causing these symptoms.
“But nearly four per cent of the Reddit users in our sample reported menstrual irregularities, which would be even higher in a female-only sample.
“We think that’s a signal worth investigating.”
Professor Lyle Ungar, a professor of computer and information science at the University of Pennsylvannia, added: “Clinical trials generally identify the most dangerous side effects of drugs.
“But they can fail to find what symptoms patients are most concerned about – even though social media is not necessarily representative, a large collection of posts may reflect additional concerns.”
“We don’t really know yet whether what we’re seeing on Reddit reflects the experience of GLP-1 users globally, or whether it’s particular to the kind of person who posts on Reddit in the United States.”
Research has previously suggested that rapid weight loss can trigger changes in menstrual symptoms and body temperature.
This is because when body fat decreases rapidly, oestrogen levels may fluctuate, potentially leading to irregular periods, lighter bleeding, or temporary amenorrhoea – absence of menstruation.
Conversely, for women with polycystic ovary syndrome (PCOS) or obesity-related menstrual irregularities, weight loss can actually restore ovulatory cycles and improve period regularity.
Dr Jena Shaw Tronieri, a senior research investigator at the Centre for Weight and Eating Disorders at the University of Pennsylvannia, also said: “These drugs are thought to work by engaging part of the brain called the hypothalamus, which helps regulate a wide variety of hormones.
“That doesn’t mean the medications are necessarily causing these symptoms, but it could suggest that reports of menstrual changes and body temperature fluctuations are worth studying more systematically.”
The researchers believe, that while these findings can’t yet be proven, AI-assisted social media analysis could become a useful way to spot early warning signs around emerging drugs trends.
For substances that are also increasingly sold in unregulated markets, like injectable jabs, patient discussions on platforms like Reddit and TikTok may offer one of the earliest clues to what users are actually experiencing.
More than 1.6 million adults in the UK are now believed to be using weight loss jabs like Wegovy and Mounjaro.
Like any medication, the jabs can cause side effects that vary in both frequency and severity.
Earlier this year, the UK medicines regulator the MHRA, slapped warnings on semaglutide jabs – Wegovy and Ozempic – warning that the drug could raise the risk of two separate conditions.
These include nonarteritic ischemic anterior optic neuropathy (NAION), also known as an “eye stroke”, and acute pancreatitis, which is often linked to gallstones.
The condition occurs when the pancreas – a small organ, located behind the stomach that aids digestion – becomes inflamed over a short period of time.
However, overall risk of developing the condition while on the jabs remained “very rare”, UK medicines watchdog, the Medicines and Healthcare products Regulatory Agency (MHRA), said.
The 7 fat jab mistakes stopping you losing weight
WHILE weight loss jabs have been hailed as a breakthrough in helping tackle Britain’s obesity crisis, some users say they’re missing out on their waist-shrinking powers – and it could be down to some simple mistakes…
POOR PENMANSHIP
Many people don’t correctly use the injection pen, according to Ana Carolina Goncalves, a pharmacist at Pharmica in Holborn, London.
Make sure to prime your weight loss pen correctly, as per the instructions. If nothing comes out, try again, and if it still doesn’t work, switch the needle or ask a pharmacist for help.
It’s also recommended to rotate injection sites between the abdomen, thigh and upper arm to avoid small lumps of fat under the skin.
TIME IS OF THE ESSENCE
Make sure you’re using the jabs on the most effective day of your schedule.
For example, taking the jab right before a takeaway or party won’t stop you from indulging, says Jason Murphy, head of pharmacy and weight loss expert at Chemist4U.
Weight loss injections need time to build up in your system, so if you’re planning for a heavier weekend, inject your dose mid-week.
MAKING A MEAL OF IT
You may not feel the urge to overeat at mealtimes due to the jabs. But skipping meals altogether can backfire, says Dr David Huang, director of clinical innovation at weight loss service Voy.
If a person is extremely malnourished, their body goes into emergency conservation mode, where their metabolism slows down.
FOOD FOR THOUGHT
A key mistake using weight loss jabs is not eating the right foods.
As well as cutting out sugary drinks and alcohol, Dr Vishal Aggarwal, Healthium Clinics recommends focusing on your protein intake.
DE-HYDRATION STATIONS
Dehydration is a common side effect of weight loss injections. But it’s important to stay hydrated in order for your body to function properly.
Dr Crystal Wyllie, GP at Asda Online Doctor, says hydration supports metabolism, digestion, and can reduce side effects like headaches, nausea and constipation.
MOVE IT, MOVE IT
It can be easy to see the jabs as a quick fix, but stopping exercising altogether is a mistake, says Mital Thakrar, a pharmacist from Well Pharmacy.
Exercise helps maintain muscle mass and help shape the body as you lose weight, which may be crucial if you’re experiencing excess skin.
QUIT IT
While there’s the tendency to ditch the jabs as soon as you reach your desired weight, stopping them too soon can cause rapid regain.
Mr Thakrar recommends building habits like healthier eating during treatment for sustaining results.
