Scientists used AI to read 150,000 Reddit posts about a gray-market weight-loss drug, and the top complaint was the opposite of what it should do
Retatrutide is the next generation of weight-loss drug after Ozempic and Mounjaro, and it is still in clinical trials, not approved anywhere. That has not stopped people from buying unapproved versions online and discussing their experiences on Reddit. Researchers at the University of Pennsylvania used AI to analyze nearly 150,000 of those Reddit posts and comments from about 40,000 users. They compared the side effects people described with those reported in the drug’s clinical trials.
The most common complaint among people who said they were taking it was increased appetite. That is the opposite of what the drug is designed to do. In clinical trials, retatrutide suppresses hunger, and the most frequent side effects were digestive problems such as nausea.
Compared with Reddit posts about semaglutide and tirzepatide, the drugs behind Ozempic, Wegovy, Mounjaro and Zepbound, discussions of gray-market retatrutide also included proportionally more mentions of sleep problems, a racing heart and muscle-related symptoms, and fewer mentions of nausea and other digestive complaints.
“We can’t say that retatrutide is causing these symptoms, or even that Reddit users are actually taking genuine retatrutide, since the drug is not legally available, and gray market formulations vary,” said co-author Lyle Ungar, a professor of computer and information science at Penn.
The legitimate version of the drug has impressive results. In its phase 2 trial, published in the New England Journal of Medicine in 2023, people taking the highest dose lost about 24% of their body weight after 48 weeks. Retatrutide acts on three hormone receptors involved in appetite and blood sugar, while semaglutide acts on one and tirzepatide on two. It is now in phase 3 trials, the last step before regulators can consider approval.
To study the gray market, the team used large language models, the technology behind modern chatbots, to sort through the posts. They identified about 14,000 users who said they had personally taken unapproved retatrutide, then had the AI pull out each symptom they described and translate it into standard medical terms. After setting aside the intended effects, such as weight loss and lower appetite, 7,823 users reported at least one symptom that could be classified.
“This is the first glimpse into the experiences of a population that is otherwise outside conventional drug safety monitoring,” said first author Neil Sehgal, a doctoral student at Penn.
The second finding is about the method itself. The same group previously used AI to analyze more than 400,000 Reddit posts about semaglutide and tirzepatide and spotted reports of menstrual changes. Another team then found a similar signal in the FDA’s database of adverse event reports. The researchers hope this study triggers the same kind of independent check.
For anyone tempted by gray-market weight-loss drugs, the findings are a warning about uncertainty more than about retatrutide itself. Products sold as research chemicals are not tested for what they contain, how pure they are or how much drug is in each vial, and the people using them fall outside every safety system. Diet and exercise remain powerful, as a recent trial in young adults with type 2 diabetes showed, and what we eat shapes how the body handles hunger and insulin, as a study on ultra-processed meals found.
There are limits to what this shows. Reddit users are not a representative sample, their reports cannot be verified, and many may be taking other experimental peptides at the same time. The study shows what people say they experience, not what the drug does. One co-author, Jena Shaw Tronieri, has received a research grant from Novo Nordisk, which makes semaglutide, and consulting fees from Currax Pharmaceuticals.
Source
Sehgal NKR, Tronieri JS, Rader B, Ungar L, Guntuku SC. “Self-Reported Adverse Effects Among Social Media Users Taking Nonapproved Retatrutide.” JAMA Network Open, 2026.
DOI: 10.1001/jamanetworkopen.2026.38929
Also cited
Jastreboff AM, Kaplan LM, Frías JP, et al. “Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial.” New England Journal of Medicine, 2023.
DOI: 10.1056/NEJMoa2301972
