FAERS is the FDA's Adverse Event Reporting System — a public database of side-effect reports submitted after a drug reaches the market. Anyone can file one: patients, caregivers, pharmacists, doctors, and (by legal requirement) manufacturers. The FDA mines it for "signals," patterns that might indicate a problem the clinical trials were too small or too short to catch.
Many of the headlines about GLP-1 side effects — hair loss, mood changes, suicidal thoughts, gastroparesis — started as FAERS analyses. Those signals are worth taking seriously, but the database has hard limits: reports are unverified, one event can be filed several times, and there is no denominator, so FAERS cannot say how often a side effect happens per person taking the drug. Media attention also drives reporting up on its own. A FAERS signal is a reason to study something, not proof of cause. See mood changes and anhedonia on a GLP-1 for a worked example of how such a signal gets investigated.