Drinking overnight tea once in a while will not cause serious harm to the body. The main harm of overnight tea is that it is easy to breed bacteria because it has been placed for too long, which may cause diarrhea, abdominal pain and other uncomfortable symptoms after drinking, but generally no serious harm. The reason why it is not recommended to drink overnight tea is because the tea has been placed for too long, the decomposition of the substances in the tea leads to the breeding of bacteria, and prolonged exposure to the air will also lead to a large number of bacterial reproduction. After drinking overnight tea, the bacteria enter into the stomach and intestines, which will cause diarrhea, abdominal pain and other symptoms. However, drinking overnight tea once in a while will not cause serious harm to the body, so patients do not need to worry too much. And because the tea is left for too long, the active ingredients inside are lost and have no nutritional value to the human body. So it is not recommended to drink overnight tea.