Infants, endurance athletes, people with psychiatric disorders, older adults, and military trainees face the greatest risk of developing water toxicity. The condition occurs when someone takes in more water than their kidneys can excrete, diluting sodium in the blood to dangerous levels. Healthy kidneys can process roughly 600 to 900 mL of water per hour, so exceeding that rate consistently is where trouble begins.
Infants Under Six Months
Babies are the single most vulnerable group. Their kidneys are immature and far less efficient at filtering excess water, and their stomachs are tiny: a one-month-old’s stomach is about the size of an egg. When a baby drinks water, it rapidly dilutes the sodium in their bloodstream, a condition that can trigger seizures, coma, and permanent brain damage. The risk is highest when caregivers dilute formula with extra water to stretch a supply, or offer plain water between feedings. Breast milk and formula already contain all the water an infant needs, so pediatric guidelines consistently advise against giving babies water before six months of age.
Endurance Athletes
Between 1% and 22% of marathon runners develop some degree of exercise-associated hyponatremia, with an average prevalence around 8%. The risk climbs with distance. One study comparing short-course and Ironman triathletes found that water toxicity only appeared in the Ironman group. The core problem is overdrinking: most symptomatic cases involve athletes who gain weight during a race rather than losing it. Runners who gained 4% of their body weight during exercise had an 85% chance of developing the condition.
Women appear to be at higher risk during endurance events, possibly because of smaller body size and differences in hormone signaling. Hot weather also increases risk, partly because athletes drink more aggressively when they feel overheated. Sodium supplements during ultra-distance events have been studied, but they don’t reliably prevent the problem if fluid intake still outpaces losses through sweat, breathing, and urination. The most effective protection is drinking to thirst rather than forcing fluids on a schedule.
People With Psychiatric Disorders
Compulsive water drinking, called psychogenic polydipsia, affects an estimated 1% to 20% of people with chronic schizophrenia. It involves consuming more than three liters of fluid daily, sometimes far more. The behavior can accompany delusional states, obsessive-compulsive patterns, or simply the dry mouth caused by many psychiatric medications. Hospitalized patients with eating disorders or alcohol use disorder sometimes drink large volumes of water to create a false sense of fullness, producing the same dilutional effect on blood sodium.
The danger is compounded by medication. Antidepressants, particularly SSRIs, are the most common drug class linked to a hormonal disruption that causes the body to retain water instead of excreting it. Antipsychotics and anticonvulsants can trigger the same response. When a person is already drinking excessive water and their medication simultaneously signals the kidneys to hold onto it, sodium levels can plummet quickly. In half of severe cases, the resulting brain swelling can be fatal.
Older Adults
People over 60, and especially those over 80, are disproportionately affected. In one study of hospitalized patients with dangerously low sodium, over 81% were older than 60. Several factors converge in this age group: kidney function naturally declines, making it harder to excrete excess water. Many older adults take diuretics or multiple medications that interfere with sodium balance. Dementia can impair the ability to regulate fluid intake or recognize early symptoms like confusion, which may be mistakenly attributed to the dementia itself.
Low body weight, which is more common in older adults, also increases vulnerability because there’s less total body water to buffer changes in sodium concentration. Even mild, chronic low sodium raises the risk of bone fractures and falls in this group, making it a concern well before it becomes a medical emergency.
Military Recruits and Outdoor Laborers
Military training programs have historically emphasized aggressive hydration to prevent heat illness, and that well-intentioned guidance has led to documented fatalities. At least one Army basic trainee died after developing catastrophic brain and lung swelling from acute water overload. His early symptoms of water toxicity, including confusion and nausea, were mistaken for dehydration, prompting medics to push even more fluids. This tragic feedback loop, where the symptoms of overhydration mimic those of dehydration, is a recurring pattern in heat-exposed populations.
The same risk applies to construction workers, agricultural laborers, and anyone performing intense physical work in hot or humid conditions. When people drink on a rigid schedule rather than in response to thirst, or when they hydrate exclusively with plain water without replacing lost sodium, they can overwhelm the kidneys’ ability to keep up.
Women and People With Low Body Weight
Female sex is an independent risk factor across multiple settings, from endurance sports to medication side effects. Smaller body size means less total blood volume, so the same amount of excess water produces a larger drop in sodium concentration. Hormonal differences in how the body manages water retention also play a role, particularly during menstruation when estrogen and progesterone can affect kidney signaling.
Low body weight amplifies risk regardless of sex. A person weighing 50 kg who drinks the same volume of water as someone weighing 90 kg will experience a proportionally greater dilution of their blood sodium. This is one reason children, small-framed women, and underweight older adults appear repeatedly among severe cases.
How Symptoms Progress
Early signs of water toxicity are deceptively mild: nausea, a sense of heaviness, and impaired taste. As sodium drops further, muscle cramps, weakness, and confusion set in. When levels fall sharply below 120 mmol/L within a few hours, the brain begins to swell. At that point, headache and vomiting can rapidly escalate to seizures, respiratory arrest, and death. The speed of onset matters enormously. A gradual decline over days gives the brain time to adapt, while an acute drop over hours is far more dangerous.
The practical takeaway is that water toxicity is not just about volume. It’s about how fast you drink, how well your kidneys can respond, how much sodium you’re replacing, and whether your body size and medications shift the threshold. For most healthy adults, drinking to thirst is sufficient and safe. The people who get into trouble are those whose physiology, behavior, or circumstances push them past the kidneys’ ability to keep pace.