We Asked Experts Whether Drinking Days-Old Water Is Dangerous – And Their Answer Was Surprising

I regularly drink from stray glasses of water I see scattered around my house that were poured the day before. I think I’m being efficient and resourceful, but am I being safe? 

Experts say there is actually a tipping point at which it is better for your health to dump that glass of water and start afresh. 

Kristen Smith, a dietitian and spokeswoman for the Academy of Nutrition and Dietetics, said that she personally follows a 12-hour rule for a glass of water. After 12 hours, Smith will pour it out and drink a new glass. However, she said you can drink a day-old glass of water “as long as it hasn’t been exposed to contaminants or left uncovered”.

Microbiologist Jason Tetro, aka The Germ Guy, also said 12 hours was the limit for drinking leftover water in glasses.

To him, the potential bacterial problems wouldn’t come from air contaminants but from the tap. One study found that bacterial cell concentrations in drinking water increase overnight. Infrequently used drinking water taps can also harbour high levels of bacteria. 

“For those first 12 hours, there’s not going to be enough food for the bacteria numbers to rise,” Tetro said. “After those 12 hours, there will be food for the bacteria to multiply.”

In other words, if you drink water from the night before, you’re likely fine. But if the water has been sitting for a whole day, it’s better to get a new glass of water, even if you used a water filter, Tetro said, or you risk suffering from gastrointestinal issues. 

Day-old water “becomes a growing environment for opportunistic pathogens,” Tetro said. “And so what you want to do is just get a new glass of water.”

Sharing water with someone else speeds up the timeline of when you should tip it away.

“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.”

You can buy yourself more time by using bottled water, which is designed to avoid bacterial growth for longer than a day, Tetro said.

Of course, if you are parched, it’s better to stay hydrated than worry about how stale your water is.

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 said.