Researchers at the University of Pennsylvania used large language models to read more than 400,000 Reddit posts from nearly 70,000 people discussing Ozempic, Wegovy, Mounjaro and Zepbound, then counted which symptoms kept coming up among users of these GLP-1 drugs. Beyond the expected nausea, the software flagged two complaints that rarely appear in drug labels: menstrual changes and chills.
The study, led by doctoral student Neil Sehgal and senior author Sharath Chandra Guntuku, was published in the journal Nature Health, and Penn Engineering announced it on April 10, 2026. Republished summaries kept appearing through September, so this is an explainer of a months-old paper and its caveats, not a new release.
How the software read 400,000 posts
The team collected posts from Reddit communities where people discuss semaglutide, the drug in Ozempic, Wegovy and Rybelsus, and tirzepatide, the drug in Mounjaro and Zepbound. The dataset, described in a ScienceDaily summary of the paper, covered more than 400,000 posts from about 70,000 users over more than five years. The approach is called computational social listening: the researchers used models including GPT and Gemini to translate informal patient language, such as a complaint about feeling freezing all the time, into standardized medical terms from the MedDRA vocabulary used in drug safety reporting.
Standardizing the language matters because patients rarely write in clinical terms. One person says chills, another says feeling cold, a third describes fever-like shivering, and a conventional keyword search would count them separately or miss them. Once mapped to a common vocabulary, the posts could be tallied like adverse event reports.
What turned up most often, and what turned up unexpectedly
About 44 percent of users described at least one side effect, and gastrointestinal problems led the list. Guntuku treated that as validation: well-known effects such as nausea appearing in the data showed, he said in the Penn Engineering announcement, that the method “is picking up a real signal.” Fatigue ranked second among complaints, even though it is thinly represented in clinical trial reporting.
Two categories stood out as underreported. Nearly 4 percent of users reported menstrual irregularities, including bleeding between periods, heavy bleeding and irregular cycles. Others described chills, feeling unusually cold, hot flashes and symptoms resembling a fever. Sehgal was careful about what that number means, saying the researchers cannot claim that GLP-1 drugs are causing these symptoms even though nearly 4 percent of the Reddit users in the sample reported menstrual irregularities, per the ScienceDaily account.
Lyle Ungar, a Penn computer scientist and co-author, explained why such gaps exist. “Clinical trials generally identify the most dangerous side effects of drugs, but they can fail to find what symptoms patients are most concerned about,” he said, according to the EurekAlert release. Trials are built to catch serious harms in a defined window, and they seldom probe questions about periods or temperature. Reddit threads, by contrast, run for years and range across whatever patients happen to be worried about, which is the kind of unstructured signal a language model can now count.
A plausible mechanism and a long list of limits
Jena Shaw Tronieri, a senior research investigator at Penn’s Center for Weight and Eating Disorders, offered a hypothesis for the hormonal signals. “These drugs are thought to work by engaging part of the brain called the hypothalamus, which helps regulate a wide variety of hormones,” she said. The hypothalamus also helps control body temperature, which would fit the chills, though the study did not test that link. Tronieri disclosed an investigator-initiated grant from Novo Nordisk and consulting fees from Currax Pharmaceuticals, while the other authors reported no conflicts and the work received no outside funding.
The limits are substantial. The findings are associations only, and the paper describes Reddit users as “younger, are more likely to be male, and are disproportionately located in the United States” than people taking these drugs overall. The analysis was also restricted to English-language communities, and the 44 percent figure covers users who mentioned a side effect in their posts, not the share of all patients who experience one. A post reporting a symptom does not show the drug caused it, and people who take the medications may have other reasons for irregular periods, including weight loss itself, illness or age.
The earlier ScienceDaily write-up of the paper said the team plans to extend the analysis beyond Reddit to other platforms and to communities that do not write in English. The paper itself, Self-reported side effects of semaglutide and tirzepatide in online communities, is in Nature Health. Whether the roughly 4 percent menstrual signal survives in a clinical dataset with confirmed diagnoses is the question the authors leave for follow-up work.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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