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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.<\/p>\n
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.<\/p>\n
Researchers say these issues are being reported by patients themselves, pointing to possible gaps between clinical data and real-world experiences.<\/p>\n
Scientists from Penn Engineering trawled posts on social media forum\u00a0Reddit<\/a>\u00a0to identify lesser-known problems linked to the widely used injections, which are prescribed for both weight loss<\/a> and diabetes<\/a>.<\/p>\n 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.<\/p>\n While the jabs have already been linked to a range of troubling side effects \u2013 including vision problems, muscle loss and mental health issues \u2013 clinical trials tend to pick up the most common reactions.<\/p>\n This is because they track a limited number of patients over a set period of time.<\/p>\n The researchers wanted to see whether now millions had been using the medications for years, whether patients were raising other concerns online.<\/p>\n 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<\/p>\n<\/div>\n Their analysis suggested that changes to menstrual cycles and sudden shifts in body temperature \u2013 including chills, hot flushes and fever-like symptoms \u2013 may warrant closer investigation.\u00a0<\/p>\n 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.<\/p>\n ‘The underreported symptoms are leads that came from patients themselves, unprompted, and clinicians could potentially pay attention to them.’<\/p>\n Around four per cent of Reddit users in the study reported menstrual irregularities \u2013 usually linked to factors such as stress, polycystic ovary syndrome or thyroid disorders.<\/p>\n These can include missed periods, unusually heavy bleeding, or cycles that vary sharply in length.<\/p>\n 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.’<\/p>\n The researchers noted that Reddit users are more likely to be male and are predominantly based in the United States.<\/p>\n 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.<\/p>\n Others described chills, feeling unusually cold, hot flushes and fever-like symptoms.<\/p>\n 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.<\/p>\n ‘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.’<\/p>\n 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’.<\/p>\n He added: ‘Even though social media is not necessarily representative, a large collection of posts may reflect additional concerns.’<\/p>\n Prof Ungar, who has long studied the use of social media to identify adverse drug effects, compared online patient communities to ‘a neighbourhood grapevine’.<\/p>\n 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.’<\/p>\n Sehgal said the researchers could not prove the GLP-1 drugs were ‘actually causing these symptoms’, but argued the findings deserved further scrutiny.<\/p>\n 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.<\/p>\n Most users are thought to be accessing the drugs privately rather than via the NHS, with use doubling between 2024 and 2025.<\/p>\n<\/div>\n