Thousands of patients using blockbuster weight-loss jabs may be suffering side effects that have previously flown under the radar in official research, a new study suggests.
An analysis of more than 400,000 online forum posts has flagged potential links between GLP-1 drugs such a Wegovy and Mounjaro and symptoms such as irregular periods and fever-like chills.
Researchers say these issues are being reported by patients themselves, pointing to possible gaps between clinical data and real-world experiences.
Scientists from Penn Engineering trawled posts on social media forum Reddit to identify lesser-known problems linked to the widely used injections, which are prescribed for both weight loss and diabetes.
At least 1.6 million Britons are thought to be using the appetite-suppressing drugs, with a further 3.3 million expected to seek them this year.
While the jabs have already been linked to a range of troubling side effects – including vision problems, muscle loss and mental health issues – clinical trials tend to pick up the most common reactions.
This is because they track a limited number of patients over a set period of time.
The researchers wanted to see whether now millions had been using the medications for years, whether patients were raising other concerns online.
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
Their analysis suggested that changes to menstrual cycles and sudden shifts in body temperature – including chills, hot flushes and fever-like symptoms – may warrant closer investigation.Â
Sharath Chandra Guntuku, the study’s senior author and a professor in Computer and Information Science, 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.
‘The underreported symptoms are leads that came from patients themselves, unprompted, and clinicians could potentially pay attention to them.’
Around four per cent of Reddit users in the study reported menstrual irregularities – usually linked to factors such as stress, polycystic ovary syndrome or thyroid disorders.
These can include missed periods, unusually heavy bleeding, or cycles that vary sharply in length.
Neil Sehgal, the study’s first author and a doctoral student in Computer and Information Science, said the proportion could be ‘even higher’ in female-only research, adding: ‘We think that’s a signal worth investigating.’
The researchers noted that Reddit users are more likely to be male and are predominantly based in the United States.
Overall, about 44 per cent of users analysed reported at least one side effect, most commonly stomach problems such as pain and nausea. Fatigue was the second most frequently reported complaint.
Others described chills, feeling unusually cold, hot flushes and fever-like symptoms.
Jena Shaw Tronieri, a senior research investigator at Penn and co-author of the study, 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 Computer and Information Science and a co-author on the study, said clinical trials ‘generally identify the most dangerous side effects of drugs’ but can 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 said: ‘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 said the researchers could not prove the GLP-1 drugs were ‘actually causing these symptoms’, but argued the findings deserved further scrutiny.
Semaglutide first became available as a weight-loss treatment in the UK in 2018, and around 10.2 million prescriptions have been issued over the past five years.
Most users are thought to be accessing the drugs privately rather than via the NHS, with use doubling between 2024 and 2025.
