Thousands post unrecognised side effects of weight-loss drugs online

Thousands of people taking weight-loss jabs have posted on Reddit about side effects of the drugs that are ‘unrecognised’ by clinical trials, a new study has found.

A large-scale analysis of more than 400,000 Reddit posts by researchers from the University of Pennsylvania discovered numerous reports of potential GLP-1 side effects that may not be captured in clinical trials, including menstrual changes, fatigue, and temperature sensitivities.

Researchers used artificial intelligence (AI) to scan the posts and identified 67,008 people who said they were using weight loss medications.

Of these 43.5% described at least one side effect, with about 4% reporting menstrual changes like irregular cycles (0.7%), heavy bleeding (0.9%), or spotting between periods (0.9%), which have not previously been reported with semaglutide or tirzepatide.

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The research, published in Nature on 10 April, said: ‘While weight loss may alleviate some reproductive issues, both weight gain and loss are linked to menstrual irregularities in women with obesity. Further, GLP-1 RAs are thought to impact food intake, in part, by engaging receptors in the hypothalamus, a brain region that also plays a central role in menstrual cycle regulation.’

Some 1-4% of people also described temperature-related symptoms such as chills and hot flushes, which the researchers said needed further investigation due to glucagon’s association with increased heat production.

The study used AI to scan 410,198 Reddit posts published across nine large subreddits that discuss GLP-1s between May 2019 and June 2025.

Of the 29,172 users who self-disclosed at least one side effect, 17,937 (61.5%) exclusively mentioned semaglutide formulations and 7,125 (24.4%) exclusively mentioned tirzepatide formulations.

Consistent with clinical trial findings, gastrointestinal issues were the most reported side effects including nausea (39.4%), vomiting (18%), diarrhoea (12.5%) and constipation (12.9%).

Additional symptoms – reported by about 5% of users – including decreased appetite, burping, abdominal pain, headache and dizziness have also been noted in clinical trials as more common among those taking weight loss drugs.

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Fatigue was the second most common symptom reported by people on Reddit (16.7%), yet it has only been significantly associated with semaglutide and tirzepatide in a small number of clinical trials.

The findings of this study do not prove that the medications caused these symptoms, but the authors said that the patterns may warrant further study.

They added that the results should be viewed as ‘hypothesis-generating signals’ rather than fact that require further confirmation through ‘prospective studies in representative populations’.

The study noted that Reddit users tend to be younger, more likely to be male, and disproportionately located in the US. People were also not prompted to disclose all their side effects, so it is not possible to estimate their true prevalence.

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‘The symptoms discussed here may be attributable to weight loss itself, changes in comorbidities or other unrelated factors. Additionally, dosage, route of administration and treatment duration were inconsistently reported in posts and were not analysed,’ the researchers added.

Another recent study found that variations in two genes involved in gut hormone pathways, which regulate appetite and digestion, may help to explain different weight-loss outcomes or side effects when taking GLP-1 medicines.