For decades, endurance athletes were told to drink frequently and in large quantities, even before feeling thirsty, to avoid dehydration. However, recent guidance from sports medicine has shifted dramatically. The key insight is that overhydration, not dehydration, is the more common and dangerous issue among runners, especially during long-distance events. When athletes consume more water than their bodies can process, it can lead to a condition called exercise-induced hyponatremia, where the blood's sodium levels drop dangerously low. This happens because the kidneys can only eliminate about 0.8 to 1 liter of water per hour, and this capacity decreases during physical exertion. Excess water dilutes the blood, leading to brain swelling, confusion, nausea, and other serious symptoms. Previously, the common belief was that thirst was a late signal of dehydration, and that athletes should drink regularly to avoid it. This advice was based on the idea that losing water through sweat was more dangerous than drinking too much. However, research has since shown that the body’s response to physical stress, such as running, can trigger the release of antidiuretic hormone (ADH), which slows the kidneys’ ability to eliminate excess water. This means that during intense activity, drinking too much can lead to a buildup of water that the body can’t process, further lowering sodium levels and increasing the risk of hyponatremia. The symptoms of hyponatremia can closely resemble those of dehydration, such as confusion and nausea. As a result, a runner experiencing these symptoms may be mistakenly given more water, which can worsen the condition. Studies on endurance athletes have shown that those who consume more water than their bodies need are at a significantly higher risk of developing hyponatremia. In contrast, dehydrated runners typically lose weight during the event, while overhydrated athletes may even gain weight, a key indicator of excessive water intake. This distinction has led to the use of pre- and post-race weighing as a tool to detect potential cases of hyponatremia. In response to these findings, new guidelines now advise athletes to drink based on their thirst, not on a fixed schedule. This approach has been endorsed by international medical organizations, including the Wilderness Medical Society, which recommends that athletes should not be given specific water volume targets. Instead, they should drink only when thirsty. This shift is also reflected in the organization of races, with hydration stations now spaced farther apart. As the season of long-distance races begins, this updated approach is being incorporated into race instructions, emphasizing that drinking too much water can be as harmful as not drinking enough.