Penn study uses AI to scan 400,000 Reddit posts for GLP-1 side effects

Researchers at the University of Pennsylvania used artificial intelligence to analyze more than 400,000 Reddit posts about GLP-1 medications and found that patients frequently discuss symptoms that get less attention in clinical trials, including menstrual irregularities, chills, hot flashes and fatigue. The study was published in Nature Health ^[1].

The analysis covered posts written by nearly 70,000 users over a span of more than five years ^[1]. About 44% of users in the study mentioned at least one side effect, most commonly gastrointestinal problems ^[1]. Nausea, a well-known GLP-1 side effect, showed up often — which the researchers said confirms the method is picking up a real signal rather than noise ^[1].

Fatigue stood out. It ranked as the second most common symptom reported by Reddit users, even though it appears less prominently in many clinical trials ^[1].

Reproductive symptoms also emerged. Nearly 4% of users who reported side effects described reproductive issues, including irregular menstrual cycles, intermenstrual bleeding and heavy bleeding ^[1]. First author Neil Sehgal, a doctoral student in computer and information science, said nearly 4% of the Reddit users in the sample reported menstrual irregularities, a share he said would be higher in a female-only sample ^[1].

Other users described temperature-related symptoms such as chills, feeling cold, hot flashes and fever-like sensations ^[1].

"We can't say that GLP-1s are actually causing these symptoms," Sehgal said. "But nearly 4% 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" ^[1].

Jena Shaw Tronieri, a senior research investigator at Penn's Center for Weight and Eating Disorders and a co-author, noted that these drugs are thought to work by engaging the hypothalamus, a brain region that 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," she said ^[1].

The researchers used large language models such as GPT and Gemini to process the posts and map the language onto the Medical Dictionary for Regulatory Activities, the standardized terminology clinicians use to classify symptoms ^[1]. Senior author Sharath Chandra Guntuku said clinical trials remain the gold standard but are slow by design. "This is not a replacement for trials, but it can move much faster, and that speed matters when a drug goes from niche to mainstream almost overnight," he said ^[1].

The study has limits the authors state plainly. It does not prove that GLP-1 medications caused any of the symptoms discussed online ^[1]. Reddit users also do not perfectly represent the general population: they tend to be younger, more likely to be male and disproportionately based in the United States ^[1]. Co-author Lyle Ungar said it is not yet known whether the patterns seen on Reddit reflect the experience of GLP-1 users globally or are particular to people who post on Reddit in the US ^[1].

The authors report no outside funding. Tronieri disclosed receiving an investigator-initiated grant, on behalf of the University of Pennsylvania, from Novo Nordisk, and consulting fees from Currax Pharmaceuticals ^[1]. Novo Nordisk makes semaglutide products including Ozempic, Wegovy and Rybelsus.

Why it matters for patients

Clinical trials are designed to catch the most dangerous side effects, but they can miss the symptoms patients care about most, Ungar said ^[1]. The study's list — menstrual changes, chills, hot flashes, fatigue — points to experiences that may not be fully reflected in current drug labeling or standard adverse event reporting systems ^[1].

That cuts two ways. A symptom appearing in this analysis is not evidence that semaglutide or tirzepatide caused it, and the researchers say so directly ^[1]. Nor does the paper say anything about how common these symptoms are across all GLP-1 users, how long they last, or whether they resolve. Those questions have not been answered by this research. For anyone weighing a symptom against a medication decision, the study offers a prompt to raise it with a clinician rather than a conclusion to act on.

What happens next

The team plans to expand the analysis beyond Reddit and beyond English-speaking communities to see whether similar patterns appear on other platforms and in other countries ^[1]. No timeline for that work has been disclosed, and no regulatory review, label change or follow-up clinical study tied to these findings has been reported ^[1].

Guntuku framed the approach as a fast first filter rather than a final word: "The whole point of this kind of approach is that it can move quickly, and that's exactly when it's most valuable," he said ^[1].