Diarrhea is always a sign that the body isn’t happy about something, although there are far more possible causes for diarrhea than could be listed in any one article. As a general medical symptom, it always indicates that there’s something that needs to be examined further, whether that’s an internal parasite or something like serotonin syndrome instead.
Diarrhea is a potentially dangerous medical symptom that can be associated with further symptoms like fast dehydration and organ failure. Sometimes it might be associated with temporary conditions like food poisoning, but many other times it can be more dangerous conditions behind them that warrant immediate medical attention.
If you develop sudden and uncontrollable diarrhea that conventional over-the-counter treatments can put a stop to, visit the emergency room as soon as possible.