Researchers have trawled internet forums to identify lesser known weight loss jab side effects.
And according to the new study, it seems that irregular periods and fever-like symptoms are among the most concerning side effects of weight-loss drugs that are not fully captured in clinical trials.
Researchers from Penn Engineering analysed more than 400,000 Reddit posts to identify lesser-known issues linked to widely used GLP-1 medications semaglutide – the active ingredient in Ozempic and Wegovy – and tirzepatide, found in Mounjaro and Zepound.
At least 1.6million Britons are believed to be using the appetite suppressing jabs – which are also prescribed for diabetes – with an estimated 3.3million more expected to seek them this year.
While the drugs have been associated with a range of worrying side effects – such as vision issues, cancer risk, muscle loss and mental health problems – clinical trials tend to focus on the most common or severe reactions.
However, the researchers set out to uncover symptoms that may be underreported in formal studies.
Their analysis highlighted that changes to menstrual cycles for women and drastic temperature changes such as chills and hot flashes may require further investigation.
Sharath Chandra Guntuku, the study’s senior author and a Professor in Computer and Information Science (CIS), said: ‘Some of the side effects we found, like nausea, are well known, and that shows that the method is picking up a real signal.
Irregular periods and fever-like symptoms may be the most concerning side effects of weight-loss drugs that are not fully captured in clinical trials, a new study suggests
‘The underreported symptoms are leads that came from patients themselves, unprompted, and clinicians could potentially pay attention to them.’
Around 4 per cent of Reddit users in the study reported menstrual irregularities – usually linked to factors such as stress, polycystic ovary syndrome (PCOS), or thyroid disorders.
These irregularities can include missed periods, heavy bleeding, or cycles that vary significantly in length.
Neil Sehgal, the study’s first author and a doctoral student in CIS, said the proportion could be ‘even higher’ in female-only research, adding: ‘We think that’s a signal worth investigating.’
The research noted that Reddit users tend to mainly be male and are predominantly based in the United States.
Overall, around 44 per cent of users analysed reported at least one side effect, most commonly stomach issues such as pain and feelings of sickness. Fatigue was the second most frequently reported complaint.
Others described chills, feeling unusually cold, hot flashes and fever-like symptoms.
Jena Shaw Tronieri, senior research investigator at Penn and the study’s co-author, 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.’
Lyle Ungar, a professor in CIS and a co-author on the study, said clinical trials ‘generally identify the most dangerous side effects of drugs’ but believes they sometimes ‘fail to find what symptoms patients are most concerned about’.
He added: ‘Even though social media is not necessarily representative, a large collection of posts may reflect additional concerns.’
Prof Ungar, who has long studied the use of social media to identify adverse drug effects, compared online patient communities to ‘a neighbourhood grapevine’.
He continued: ‘People who are living with these medications are swapping notes with each other in real time, sharing experiences that rarely make it into a doctor’s office visit or an official report.’
Sehgal admitted that they can’t prove that the GLP-1 drugs are ‘actually causing these symptoms’ but the researchers believe their findings warrant further scrutiny.
Prof Guntuku added: ‘Clinical trials are the gold standard, but by design, they are slow.
‘This is not a replacement for trials, but it can move much faster, and that speed matters when a drug goes from niche to mainstream almost overnight.’
Semaglutide first became available to use as a weight loss aid in the UK in 2018, and in the past five years there has been an approximate 10.2millions prescriptions of the drug handed out.
The vast majority of these users are accessing the medication privately rather than through the NHS, with usage doubling between 2024 and 2025.
