A new analysis of hundreds of thousands of online forum posts has uncovered potential side effects of popular weight-loss drugs like Wegovy and Mounjaro that weren't captured in official clinical trials, including irregular menstrual cycles and unexplained fever-like symptoms. Researchers from Penn Engineering reviewed more than 400,000 Reddit posts to identify lesser-known problems linked to GLP-1 drugs, which are now used by approximately 1.6 million people in the UK alone .

What Side Effects Are Patients Reporting That Doctors Might Miss?

The study flagged two particularly concerning patterns: changes to menstrual cycles and sudden shifts in body temperature, including chills, hot flushes, and fever-like symptoms. Around 4% of Reddit users in the study reported menstrual irregularities, though researchers believe this proportion could be significantly higher in female-only research populations . These menstrual changes typically include missed periods, unusually heavy bleeding, or cycles that vary sharply in length.

Overall, about 44% of Reddit users analyzed reported at least one side effect, with the most common complaints being gastrointestinal issues such as stomach pain and nausea, followed by fatigue . While nausea and digestive problems are well-documented in clinical trials, the menstrual and temperature-related symptoms appear to be underreported in official safety data.

"Some of the side effects we found, like nausea, are well known, and that shows that the method is picking up a real signal. The underreported symptoms are leads that came from patients themselves, unprompted, and clinicians could potentially pay attention to them," said Sharath Chandra Guntuku, senior author of the study and a professor in Computer and Information Science at Penn Engineering.

Sharath Chandra Guntuku, Professor in Computer and Information Science, Penn Engineering

Why Do Clinical Trials Miss These Symptoms?

Clinical trials are designed to identify the most dangerous side effects of drugs, but they have inherent limitations. Trials track a limited number of patients over a set period of time, which means they may not capture symptoms that develop slowly or affect smaller subsets of users. Now that millions of people have been using GLP-1 drugs for years, patients are sharing experiences online that rarely make it into a doctor's office visit or an official safety report .

The hypothalamus, a part of the brain that helps regulate a wide variety of hormones, is thought to be the mechanism through which these drugs work. This connection to hormone regulation may explain why patients are reporting menstrual changes and body temperature fluctuations . However, researchers emphasize they cannot yet prove the GLP-1 drugs are actually causing these symptoms, only that the findings deserve further investigation.

"These drugs are thought to work by engaging part of the brain called the hypothalamus, which 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," explained Jena Shaw Tronieri, a senior research investigator at Penn.

Jena Shaw Tronieri, Senior Research Investigator, Penn

How Can Social Media Data Complement Traditional Research?

Researchers compared online patient communities to "a neighbourhood grapevine," where people living with medications swap notes with each other in real time, sharing experiences that rarely make it into formal medical channels . While social media data is not necessarily representative of the entire population using these drugs, a large collection of posts can reflect additional concerns that warrant investigation.

The Reddit analysis method moves much faster than traditional clinical trials, which is particularly important when a drug goes from niche to mainstream almost overnight. Semaglutide, one of the most commonly used GLP-1 drugs, first became available as a weight-loss treatment in the UK in 2018, and around 10.2 million prescriptions have been issued over the past five years . Use of these drugs doubled between 2024 and 2025, with most users accessing them privately rather than through the NHS.

Steps to Monitor Your Health While Using GLP-1 Weight-Loss Drugs

• Track menstrual changes: Keep a record of your period dates, flow intensity, and any irregularities to discuss with your healthcare provider, as these may warrant closer medical attention.
• Monitor body temperature symptoms: Note any unexplained chills, hot flushes, or fever-like symptoms and report them to your doctor, even if they seem minor or unrelated to the medication.
• Report all side effects: Share both common and unusual symptoms with your healthcare provider, as patient reports help identify patterns that clinical trials may have missed.
• Maintain regular check-ups: Schedule consistent appointments with your doctor to discuss your experience on the medication and any changes in your health or symptoms.

Neil Sehgal, the study's first author and a doctoral student in Computer and Information Science, noted that the proportion of women experiencing menstrual irregularities could be "even higher" in female-only research, adding that "we think that's a signal worth investigating" . The findings suggest that women taking GLP-1 drugs should be particularly attentive to changes in their menstrual cycles and report them promptly to their healthcare providers.

"Clinical trials are the gold standard, but by design, they are slow. 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," said Lyle Ungar, a professor in Computer and Information Science and co-author on the study.

Lyle Ungar, Professor in Computer and Information Science, Penn

The research underscores an important gap in how we understand medication safety in the real world. As GLP-1 drugs continue to be prescribed to millions of people globally, the experiences shared by patients online may provide crucial early warning signals about side effects that deserve formal investigation. Researchers emphasize that this approach complements, rather than replaces, traditional clinical trials, but it offers a faster way to identify concerns that warrant further study .