Drinking water left in a glass overnight is generally safe, but experts advise caution after a certain period, particularly if the water has been exposed to contaminants or shared.

Kristen Smith, a dietitian and spokeswoman for the Academy of Nutrition and Dietetics, adheres to a 12-hour rule, discarding water left out longer. She notes that as long as the water remains uncovered and free from contaminants, drinking it after 12 hours is acceptable.

Microbiologist Jason Tetro, also known as "The Germ Guy," concurs with the 12-hour limit. Tetro suggests that the primary concern isn't airborne contaminants but potential bacteria from the tap. Studies have indicated that bacterial cell concentrations in drinking water can increase overnight. Infrequently used taps can also harbor high levels of bacteria.

Tetro explains that bacteria require time and a food source to multiply significantly. "For those first 12 hours, there’s not going to be enough food for the bacteria numbers to rise," he stated. "After those 12 hours, there will be food for the bacteria to multiply."

Drinking water that has sat for a full day, even if filtered, carries a risk of gastrointestinal issues, according to Tetro, who advises replacing it with fresh water. He describes day-old water as becoming "a growing environment for opportunistic pathogens."

The timeline for discarding water is accelerated when it is shared. "Once a person drinks directly from a bottle or glass, bacteria from their mouth can transfer to the remaining liquid and begin to multiply," Smith said. "So for that reason, after you put your mouth on a bottle, it’s best to finish it in one go and dispose of it rather than saving it for later, especially if you’re sharing a cup or bottle with someone else."

For extended storage, Tetro suggests bottled water, which is designed to inhibit bacterial growth for longer periods.

However, in situations of dehydration, consuming even day-old water is preferable to not drinking at all. "If the only option you have is that water bottle that’s been sitting around for a day, it’s definitely better to drink than to stay dehydrated," Smith added.