Does drinking eight glasses of water have any evidence behind it?

What the evidence says

The idea that everyone needs exactly eight 8-ounce glasses of water a day is a myth without a clear scientific source. A well-known scientific review looked for the origin of this advice and could not find any study that actually tested or proved it.

Health authorities, including Health Canada, do not recommend a fixed number of glasses. Instead, they set a 'total water' target that includes water from food and all beverages, not just plain water. Health Canada's general guidance is that adults should get roughly two litres of fluids a day from all sources combined, and most people meet this without keeping count, mainly by drinking when they feel thirsty.

Researchers have actually tested the '8 cups a day' advice in a randomized trial with teenagers trying to lose weight. Giving them this specific advice did not lead to any extra weight loss compared with a normal diet plan, which suggests the number itself has no special power.

Thirst is a very reliable internal signal for healthy people in normal conditions. Needs go up with exercise, heat, pregnancy, breastfeeding, or certain illnesses, and in those situations more fluid is genuinely needed. But for an average healthy adult sitting at a desk in a mild climate, there's no proof that forcing down eight specific glasses adds any measurable health benefit over just drinking normally.

Some observational studies have looked at higher fluid intake and kidney health, with mixed results, but these are not controlled trials and can't prove cause and effect.