While bottled water has an expiration date, the U.S. Food and Drug Administration (FDA) says it can last forever if left unopened and stored in a cool, dark place. The truth is that the water doesn't expire, but the bottle that holds it can suffer changes that ultimately affect water quality.

According to the journal Water Research, a 2008 study found that plastic bottles exposed to high temperatures can leak antimony. This chemical is a potential carcinogen and may also affect the lungs, skin, and digestive system, according to a 2010 review published in the International Journal of Environmental Research and Public Health.

Expired bottled water may also contain traces of polyethylene terephthalate (PET) and other plastics that can leak into the water over time, especially at high temperatures. Additionally, plastic materials can absorb odors from the environment and change the taste of water, so it's best to store the bottles away from other chemicals.

Bottled water won't spoil or start to taste rancid, but it can still make you sick if the bottle is stored for too long or in improper conditions. "A chemical called bisphenol-A, or BPA, along with other things used to manufacture plastic, can leach into your water if the bottle heats up or sits in the sun," said researcher Kellogg Schwab, Ph.D.

Expired bottled water can also become a breeding ground for bacteria, viruses, fungi, and other pathogens, incuding Escherichia coli, Helicobacter pylori, and Salmonella enterica. These microorganisms have been linked to digestive problems, lung disease, kidney failure, and respiratory illnesses, warns a 2019 review published in the journal Bottled and Packaged Water.