Chills, Cycle Changes, and Fatigue: Side Effects Penn's AI Found in 400,000 Reddit Posts From GLP-1 Users

On September 12-13, 2026, a University of Pennsylvania School of Engineering team led by Neil K.R. Sehgal, Sharath Chandra Guntuku, Lyle Ungar, and Jena Shaw Tronieri published in Nature Health (Vol. 1, Issue 8, page 806; DOI 10.1038/s44360-026-00108-y) an AI-driven analysis of 410,198 Reddit posts from 67,008 self-reported users of semaglutide (Ozempic, Wegovy, Rybelsus) or tirzepatide (Mounjaro, Zepbound). Posts covered a span of more than five years, from May 2019 through June 2025.

The well-known gastrointestinal side effects showed up as expected: nausea (36.9% of side-effect mentions), fatigue (16.7%), vomiting (16.3%), constipation (15.3%), and diarrhea (12.6%). Of the users the model tagged as GLP-1 users, 43.5% mentioned at least one side effect.

Two groups of symptoms came up more often than the current drug labels would suggest. The first was reproductive symptoms in women, including changes in menstrual cycle length, heavier bleeding, and irregular periods, mentioned by roughly 4% of women in the sample. The second was body-temperature complaints — chills, unusual coldness, and hot flashes — mentioned across both sexes. Fatigue was the second-most-common symptom overall, above what clinical trial write-ups usually give it.

The study does not prove that GLP-1 drugs cause any of these effects. What it flags are signals worth investigating with the kind of prospective evidence a randomized trial provides. This piece explains what a Reddit dataset can and cannot tell you, why the hypothalamus makes some of these signals biologically plausible, and how to think about your own symptoms if you are on one of these drugs.

Reddit is a public forum where people post about their own experiences, often with more candor than they show a clinician. The Penn team pulled every post from May 2019 through June 2025 that mentioned semaglutide, Ozempic, Wegovy, Rybelsus, tirzepatide, Mounjaro, or Zepbound. They then trained a language model to identify posts where the writer self-reported using one of these drugs, and to classify each side-effect mention into a symptom category.

The final dataset covered 410,198 posts written by 67,008 unique users. Of those users, 43.5% mentioned at least one side effect somewhere in their posting history.

What the researchers did next matters. They compared the frequency of each symptom category in the Reddit data to the frequency reported in the drug labels (which come from clinical trials). Where the Reddit rate was much higher than the label rate, the researchers flagged the symptom as a potential underreported signal.

Several important caveats: Reddit users skew younger, more male, and more U.S.-based than the general GLP-1 patient population, which now includes many older adults, more women, and patients in other countries. Anyone posting to Reddit already has enough motivation to write about their treatment. And the AI classifier can misread ambiguous phrasing. The authors are direct about these limits and describe the work as hypothesis-generating, not confirmatory.

Still, when a symptom shows up in tens of thousands of posts and appears rarely in trial data, that gap is worth taking seriously.

Nausea led the list at 36.9% of side-effect mentions among users. That fits the drug labels, where nausea shows up as the most common adverse event in every GLP-1 trial in adults. Vomiting (16.3%), constipation (15.3%), and diarrhea (12.6%) all track with trial data too. Together these gastrointestinal effects account for the bulk of what people talk about, and the reasons are well-known: GLP-1 drugs slow how quickly the stomach empties and change how gut nerves signal fullness. Most people either get past the worst of it in the first few weeks or find a dose they tolerate.

The injection-site reactions, headaches, and reflux that also show up in the Reddit data track with what you would expect from a large, real-world user base.

One finding in this group is worth pausing on: fatigue was the second-most-common symptom mentioned, at 16.7% of side-effect mentions. In the pooled label data for semaglutide and tirzepatide, fatigue is listed but at a lower reported rate. Fatigue often gets folded into non-specific categories in trials or attributed to the calorie deficit itself. The Reddit signal suggests that at least a large share of users experience fatigue as its own distinct problem beyond the calorie-deficit explanation.

Two categories in the Reddit data came up more often than clinical trials would predict.

Reproductive symptoms in women. Roughly 4% of women in the sample mentioned changes in menstrual cycle length, unusually heavy bleeding, irregular cycles, or breakthrough bleeding while on semaglutide or tirzepatide. Some described the changes starting within a few weeks of a dose escalation; others reported cycles becoming more regular over several months on stable dosing. The Penn team also picked up posts about breast tenderness and libido changes. Clinical trial protocols for semaglutide and tirzepatide typically enroll women without menstrual endpoints and do not track menstrual cycles as a primary or secondary outcome, so this category has been genuinely under-measured.

Body temperature. Users posted about feeling colder than usual, chills unrelated to fever, and hot flashes that did not fit their usual pattern (including in premenopausal women who had not experienced them before). The Reddit signal here was strong enough that the Penn team called it out specifically in the paper.

The authors also flagged two additional patterns that need more work: psychiatric symptoms (low mood, anxiety, and a smaller group of posts about worsened depression) and skin or hair changes (hair thinning during rapid weight loss). Both categories overlap with things that happen during any large calorie deficit, which makes it hard to sort out drug-specific effects from generic weight-loss effects.

The hypothalamus is the part of the brain that regulates hunger, appetite, reproductive hormones, and body temperature. It sits at the base of the brain and coordinates several of the systems the Penn study flagged. This is why the reproductive and thermoregulation signals are worth taking seriously rather than dismissing as noise.

Semaglutide and tirzepatide work primarily by activating GLP-1 receptors (and, in tirzepatide's case, GIP receptors too) in brain regions including the arcuate nucleus of the hypothalamus. That is how they blunt appetite. The same neural circuits sit next to circuits that regulate the release of gonadotropin-releasing hormone (which drives the reproductive cycle) and the thermoregulatory set point (which decides whether you should feel warm or cold in a given environment). When one part of a densely connected neighborhood is being pushed by a drug, effects on nearby circuits are biologically plausible.

Rapid weight loss on its own also affects both reproductive and thermoregulation systems. A large calorie deficit can shift menstrual cycles and lower core body temperature. This means the Reddit signals could reflect direct drug effects on the hypothalamus, indirect effects mediated by weight loss, or some combination. Working out which is which will take dedicated studies with both a drug arm and a matched weight-loss arm.

Every Reddit-based study has three problems that limit what you can conclude. The Penn authors are careful about all three.

First, Reddit users are not the average GLP-1 patient. They skew younger (many in their 20s and 30s), more male, more U.S.-based, and more likely to be technologically comfortable. Older adults with type 2 diabetes on Ozempic are underrepresented. Any conclusion from Reddit should be treated as suggestive for the actual population using these drugs.

Second, people who post about side effects have chosen to talk about their experience. Someone who has no side effects is less likely to post about not having them. This is called selection bias, and it can inflate side-effect frequencies. The Penn team tried to correct for this by comparing rates across symptoms rather than treating any single rate as an absolute prevalence — but the correction is imperfect.

Third, the AI classifier can misread posts. If someone writes about a side effect their friend had, or asks whether a symptom might be from a GLP-1, the model may count that as a self-reported case. The authors ran spot checks on the classifier's accuracy but cannot rule out systematic errors in edge cases.

These limits do not sink the study. They frame it. The right use of the paper is as a set of hypotheses to be tested with prospective evidence, not as a replacement for the FDA label or your clinician's judgment.

The paper does not show that semaglutide or tirzepatide cause menstrual changes, temperature dysregulation, or new-onset fatigue. It shows that users report these symptoms more often than the drug labels would predict.

Why is that distinction important? Because a correlation in observational data can come from several sources beyond direct causation. Rapid weight loss itself changes hormone levels and metabolism. Behavioral changes that accompany a weight-loss journey (more time spent tracking symptoms, more attention to bodily changes) affect what people notice and report. Some symptoms may be caused by an underlying condition the drug is unmasking rather than by the drug itself. And Reddit demographics may over-represent a subgroup that responds differently than the average patient.

Confirming or ruling out any of these signals requires prospective studies designed specifically to measure them: randomized comparisons of GLP-1 users against matched weight-loss controls (calorie restriction, bariatric surgery, or placebo groups tracked with menstrual diaries and body-temperature logs). None of that data exists yet at the scale the Reddit dataset covered. Those studies will take years to run and will change the labels only if a signal reproduces in a controlled setting.

If you are on semaglutide or tirzepatide and notice any of the signals the Penn study flagged, a productive conversation with your prescriber covers a few specific points.

Bring a written log. Track what changed, when it started relative to a dose change, how often the symptom occurs, and what other changes you have made (diet, exercise, sleep, other medications). A menstrual-cycle app screenshot is a good record for period changes. A daily entry for temperature complaints or fatigue helps your clinician distinguish drug effects from other explanations.

Ask whether the symptom timing lines up with a dose escalation. Most side effects that come from the drug worsen within one to four weeks of a dose increase and stabilize or fade at a steady dose. A symptom that appeared at a stable dose after months of tolerance may point somewhere else.

Ask about rule-out testing. Menstrual changes have many causes — thyroid disease, polycystic ovary syndrome, perimenopause, pregnancy, IUD placement, and stress all matter. A brief workup can rule out unrelated causes before attributing the change to the drug. The same principle applies to temperature complaints (rule out thyroid dysfunction, anemia, infection) and fatigue (rule out anemia, thyroid, vitamin B12 deficiency, and depression).

Ask about dose adjustment. If the symptom lines up with a dose change and other causes have been ruled out, ask whether stepping the dose back to the previous tolerated level is an option. Some patients find that a smaller weight-loss trajectory at a lower dose fits their life better than a maximum dose with intolerable side effects. The dose that works is the one you can stay on.

Ask what to watch for that would justify stopping. Some symptoms — persistent severe abdominal pain, signs of gallbladder trouble, allergic reactions, or new suicidal thoughts — require the drug to stop right away. Others can be managed with dose changes, timing adjustments, or supportive treatment. Knowing the difference in advance helps you make a same-day call rather than waiting for an appointment.

Do not stop the drug on your own without talking to whoever prescribed it. Sudden stops can cause rebound appetite, weight regain, and blood-sugar swings for patients with type 2 diabetes.

For each category the Penn study flagged, the path from Reddit signal to updated drug label runs through the same kind of evidence.

For menstrual changes, a prospective study would enroll several thousand women starting a GLP-1 for the first time, ask them to log their cycles for six to twelve months before and after starting, and compare against a matched group losing similar weight without a GLP-1 (through calorie restriction plus a placebo, for example). If GLP-1 users show a higher rate of cycle changes than weight-matched controls, that would confirm a drug-specific effect. Existing GLP-1 trials have not been designed with menstrual endpoints; a dedicated follow-up study is what the field needs.

For temperature complaints, the design would be similar but simpler: continuous body-temperature monitoring (a smartwatch or wearable ring) in GLP-1 starters versus weight-matched non-users, plus a validated symptom diary. Temperature effects are quicker to measure than reproductive effects because they don't require tracking across menstrual cycles.

For fatigue, the harder problem is separating drug-related fatigue from calorie-deficit fatigue. Any large weight loss makes people tired. A well-designed study would need to control for total calorie intake and activity level and would look at whether GLP-1 users report more fatigue than would be predicted by their weight loss alone.

None of this will happen quickly. The people best positioned to fund and run these studies are the drug manufacturers (with an incentive to keep labels tight), the FDA (through post-market surveillance and required trials), academic centers (with limited funding), and independent groups working with real-world claims and prescription data. Expect the first prospective results within about two years and label changes, if any, another year or two after that.

Until then, the Penn paper does one useful thing: it gives patients and clinicians a specific vocabulary for symptoms that had no name in the drug labels. If a woman on tirzepatide asks her prescriber whether her three months of irregular periods might be from the drug, the answer is now "we don't know for sure, but it's a documented pattern in a large user sample; let's rule out other causes and think about your dose." That is better than what the label offered a year ago.

This piece is educational and reflects publicly available information as of September 13, 2026. It is not medical advice. If you notice new symptoms on a GLP-1 medication, talk to the clinician who prescribed it. Peptide profile background: semaglutide on Peptidelist.org, tirzepatide on Peptidelist.org.