What Causes Overhydration and Water Intoxication?

January 6, 2026 •

3 min read

While severe cases of overhydration are rare, with a healthy person's kidneys able to excrete up to one liter of water per hour, it can be a life-threatening condition when it occurs. Understanding what causes overhydration is crucial for prevention, especially for endurance athletes and individuals with certain medical issues.

Quick Summary

Overhydration is caused by either excessive water intake or the body's inability to excrete water effectively. It can lead to a dangerous electrolyte imbalance, most notably low sodium levels (hyponatremia). Athletes, those with medical conditions like heart or kidney disease, and people with certain psychiatric disorders are at higher risk.

Key Points

Hyponatremia is Key: Overhydration leads to dangerously low blood sodium levels, a condition known as hyponatremia, which causes fluid shifts and cellular swelling.
Two Primary Routes: Overhydration is caused either by consuming more water than the kidneys can process, or by a medical condition that prevents the body from excreting fluid properly.
Athletes are at Risk: Endurance athletes who drink large amounts of plain water to avoid dehydration are highly susceptible to dilutional hyponatremia due to high sweat loss of sodium.
Medical Conditions are Major Drivers: Chronic issues like congestive heart failure, liver disease, and kidney problems are frequent causes of overhydration due to impaired organ function.
Hormonal Influence: The syndrome of inappropriate antidiuretic hormone (SIADH) is a specific hormonal condition that causes excessive water retention.
Infants are Vulnerable: Due to their low body mass and immature kidney function, infants should not be given large amounts of plain water, as they are highly susceptible to water intoxication.
Medications Can Play a Role: Certain drugs, including diuretics, antidepressants, and antipsychotics, can increase the risk of overhydration by affecting fluid balance.

What is Overhydration (Water Intoxication)?

Overhydration, also known as water intoxication, is a condition that occurs when the body contains too much water relative to its sodium content. This can result from consuming an excessive amount of water or from an underlying medical condition that causes the body to retain fluid. The core problem in overhydration is a dilution of blood sodium, a condition called hyponatremia. As sodium levels drop, the balance of fluids shifts, causing cells—especially brain cells—to swell. This cellular swelling can lead to severe and life-threatening complications.

Causes of Increased Water Intake

For a healthy individual, accidentally consuming enough water to cause overhydration is rare, as the kidneys are highly efficient at filtering and removing excess fluid. However, certain scenarios and behaviors can lead to drinking excessive amounts of water, overwhelming the body's systems.

Endurance Athletes: Marathon runners, triathletes, and other endurance athletes can over-hydrate by drinking large volumes of water without replacing the sodium lost through sweat. This is particularly risky when they mistake symptoms of mild hyponatremia for dehydration and drink more plain water.
Psychogenic Polydipsia: This is a psychiatric disorder that causes a compulsive desire to drink excessive amounts of water. It is sometimes associated with other mental illnesses, such as schizophrenia.
MDMA Use: The drug MDMA (ecstasy) can cause extreme thirst and overexertion. Combined with the drug's effect of increasing antidiuretic hormone (ADH) and causing urine retention, this can lead to severe water intoxication.
Water Drinking Contests or Forced Consumption: While uncommon, documented cases of fatal water intoxication have occurred due to water drinking contests or as a form of abuse, forcing the rapid consumption of massive volumes of water.

Medical Conditions That Cause Water Retention

In many cases, overhydration is not the result of drinking too much water but rather the body's inability to excrete it properly due to an underlying health issue. These conditions impair the kidneys' function or interfere with hormone regulation.

Congestive Heart Failure (CHF): This condition impairs the heart's ability to pump blood effectively, causing a buildup of fluid in the body that the kidneys cannot process.
Kidney Disease: Various kidney problems, including chronic kidney disease, can prevent the kidneys from filtering excess water from the blood, leading to fluid retention.
Liver Disease: Conditions like cirrhosis of the liver can cause fluid to accumulate in the body, which can dilute blood sodium levels.
Syndrome of Inappropriate Antidiuretic Hormone (SIADH): In SIADH, the pituitary gland secretes too much vasopressin (ADH), which signals the kidneys to conserve water unnecessarily, leading to fluid overload.

Medications and Other Factors

Certain medications and temporary conditions can also increase the risk of overhydration by affecting fluid balance.

Medications: Some diuretics, antidepressants, and antipsychotic medications can cause or exacerbate fluid retention issues or increase thirst.
Hormonal Imbalances: Conditions like hypothyroidism and Addison's disease can disrupt the body's electrolyte balance and fluid regulation.
Infants: Due to their low body mass and immature kidneys, infants can become overhydrated from drinking too much plain water, as their hydration needs are typically met through breast milk or formula.

Comparison of Overhydration Causes


Cause	Mechanism	At-Risk Population	Severity Potential
Excessive Intake	Overwhelming kidney's excretion capacity	Endurance athletes, psychogenic polydipsia patients, MDMA users	Moderate to severe (potentially fatal)
Heart/Kidney Disease	Impaired organ function prevents water excretion	Individuals with pre-existing heart, kidney, or liver conditions	Gradual onset, chronic, potentially life-threatening
SIADH	Hormonal overproduction causes water retention	People with tumors, certain neurological conditions	Can cause significant hyponatremia and neurological symptoms
Infants	Immature kidneys and low body mass	Children under one year of age	High risk of rapid water intoxication

Conclusion

Overhydration is a complex condition that can stem from behavioral factors or serious underlying medical issues. While healthy kidneys are remarkably adept at handling excess fluid, the balance can be easily disrupted by excessive intake during intense exercise or a pre-existing health problem. Recognizing the risk factors, such as specific medical conditions or high-intensity athletic activities, is the first step toward prevention. Paying attention to your body’s thirst signals and consulting a doctor if you have persistent symptoms or medical concerns are crucial steps for maintaining proper hydration. The ultimate goal is to achieve a healthy balance, not an excessive one.

For more detailed guidance on managing your hydration needs, especially if you have an underlying health condition, consult a medical professional. For general information on maintaining a healthy fluid balance, organizations like the Mayo Clinic provide helpful resources.

Frequently Asked Questions

For a healthy person with normal kidney function, it is very difficult to accidentally drink enough water to cause overhydration. The kidneys are highly efficient, able to excrete about one liter of fluid per hour, and your thirst mechanism usually stops you from drinking too much.

The main danger of overhydration is hyponatremia, a condition where blood sodium levels become diluted. This causes cells, particularly in the brain, to swell. In severe cases, this can lead to headaches, confusion, seizures, coma, or even death.

Endurance athletes are at higher risk because they sweat profusely, losing both water and electrolytes like sodium. If they replace fluids with large amounts of plain water without also replenishing electrolytes, they can dilute their blood sodium, leading to hyponatremia.

A number of medical conditions can cause the body to retain fluid, leading to overhydration. These include congestive heart failure, kidney disease, liver disease, and the syndrome of inappropriate antidiuretic hormone (SIADH).

Early symptoms can be non-specific, including nausea, headaches, and bloating. A key indicator is persistently clear or colorless urine, which suggests that you are drinking more than your body needs. Severe symptoms include confusion, seizures, and difficulty breathing.

Treatment depends on the severity. Mild cases can be managed by restricting fluid intake and consuming salty snacks or electrolyte-containing drinks. More severe cases may require hospitalization for fluid and electrolyte monitoring and intravenous sodium replacement.

Yes, if left untreated, severe overhydration can lead to cerebral edema (brain swelling) due to hyponatremia. This can cause permanent brain damage, seizures, or death.