Weight-loss injections such as Ozempic and Mounjaro may cause side effects that pharmaceutical companies have not included on their official labels, according to new research.
An analysis published in the journal Nature Health has revealed menstrual irregularities and temperature-related complaints, including chills and hot flushes, have emerged as potential effects not currently recognised in clinical documentation.
American scientists examined posts from approximately 70,000 Reddit users spanning more than six years to identify symptoms being reported by those taking GLP-1 medications.
The drugs, originally developed for diabetes management, have gained widespread popularity for their ability to suppress appetite and deliver rapid weight reduction.
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The injections are already associated with various adverse reactions
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However, the injections are already associated with various adverse reactions, ranging from nausea and constipation to severe organ damage in extreme cases.
The study assessed social media posts spanning from May 2019 to June 2025, providing researchers with a substantial dataset of real-world patient experiences.
Nausea topped the list of reported symptoms at nearly 37 per cent, followed by fatigue at just under 17 per cent, vomiting at 16.3 per cent, and constipation at 15.3 per cent. Diarrhoea rounded out the five most frequent complaints at 12.6 per cent.
Crucially, the researchers noted reproductive symptoms and temperature-related issues "emerged as unrecognised potential effects" that are "not well captured in current labelling or trials".
Fatigue proved particularly noteworthy, ranking as the second most common concern among users despite rarely meeting reporting thresholds in formal clinical trials.
Scientists believe these medications do not directly affect reproductive hormones or temperature regulation, but rather the rapid physiological changes from swift weight loss may indirectly influence menstrual patterns and the body's adaptation to reduced energy intake.
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Neil Sehgal, the study's lead author and doctoral student at the University of Pennsylvania, cautioned: "We can't say that GLP-1s are actually causing these symptoms."
He added: "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, also from the University of Pennsylvania, observed while clinical trials typically identify the most dangerous adverse effects, they may miss symptoms that concern patients most.
Dr Jena Shaw Tronieri, a senior research investigator at the university's Centre for Weight and Eating Disorders, explained: "These drugs are thought to work by engaging part of the brain called the hypothalamus, which helps regulate a wide variety of hormones."
She suggested this connection makes reports of menstrual changes and temperature fluctuations worthy of more systematic investigation.
More than 1.6 million adults across the United Kingdom are now estimated to be using weight loss injections, including Wegovy and Mounjaro.
More than 1.6 million adults across the United Kingdom are now estimated to be using weight loss injections
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The MHRA issued fresh warnings earlier this year regarding semaglutide medications Wegovy and Ozempic, alerting patients to heightened risks of two additional conditions.
These include non-arteritic anterior ischaemic optic neuropathy, commonly termed an "eye stroke", alongside acute pancreatitis, an inflammation of the pancreas frequently associated with gallstones.
The medicines regulator stressed the overall likelihood of developing pancreatitis while taking these medications remains "very rare".
The researchers suggested AI-assisted analysis of social media platforms could prove valuable for detecting early warning signs around emerging drug trends, particularly for substances increasingly available through unregulated markets.
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