First things first, do not reheat the tea if it has been left at room temperature for anywhere between 4 to 8 hours. If you forgot to have your tea for a maximum of 10 to 15 minutes, it is safe to drink. The food poisoning bacteria grow in brewed teas that are exposed to heat between 41 to 140 degrees Fahrenheit. The case is even worse with milk teas, which can also gain an unpleasant taste and grainy texture when reheated. The presence of milk results in faster accumulation of bacteria, and only reheating the tea will not kill them. Problems might get dangerous if your tea develops moulds. They might not be visible to the eye and can be easily missed. Drinking such tea even when reheated can cause stomach upset, diarrhoea, cramps, nausea, inflammation and many other digestive issues.