When your body’s sodium levels drop too low, water shifts into your cells and causes them to swell. This is especially dangerous in the brain, where even a small increase in volume can cause serious problems. The medical term for low blood sodium is hyponatremia, and it’s defined as a sodium level below 135 mEq/L. Mild cases might leave you feeling a bit off, while severe drops can trigger seizures, coma, or death.
Why Low Sodium Makes Your Cells Swell
Sodium plays a critical role in controlling how water moves between your cells and the fluid surrounding them. When sodium levels in your blood drop, the fluid outside your cells becomes more dilute than the fluid inside them. Water naturally flows toward the more concentrated side to even things out, a process called osmosis. The result: your cells absorb extra water and swell.
This happens throughout your body, but the brain is uniquely vulnerable. Your skull is rigid, so there’s no room for the brain to expand. An increase in brain volume of just 8% to 10% can be fatal. That’s why the most noticeable symptoms of low sodium are neurological: headaches, confusion, difficulty thinking clearly, and in serious cases, seizures and loss of consciousness.
Early Symptoms vs. Severe Symptoms
The first signs of low sodium are easy to dismiss. You might feel nauseous, lose your appetite, get a headache, or just feel unusually tired and sluggish. Muscle cramps and general weakness are common too. Many people chalk these up to dehydration, stress, or a bad night’s sleep.
As sodium drops further, symptoms become harder to ignore. Confusion, restlessness, irritability, and vomiting set in. If levels fall below roughly 125 mEq/L (classified as severe hyponatremia), the situation can deteriorate fast. In acute cases where sodium plummets quickly, symptoms can progress from a headache to full seizures and respiratory arrest within as little as 20 minutes. Focal neurological problems like slurred speech, weakness on one side of the body, tremor, and problems with coordination have also been reported, even without any underlying brain injury.
The speed of the drop matters enormously. Chronic hyponatremia, where levels decline gradually over 48 hours or more, tends to produce milder symptoms because the brain has time to adapt by pushing some of its internal solutes out, reducing the osmotic pull. Acute hyponatremia, where levels crash rapidly, gives the brain no time to compensate and is far more dangerous.
What Causes Sodium to Drop
Low sodium rarely happens because you aren’t eating enough salt. The more common culprits involve too much water relative to sodium, or conditions that prevent your kidneys from maintaining the right balance.
- Drinking too much water. Overhydration dilutes the sodium in your blood. This is particularly common in endurance athletes (more on that below) and in people who drink large volumes of water without replacing electrolytes.
- Certain medications. Diuretics (water pills), some antidepressants, and pain medications can interfere with how your kidneys handle sodium and water.
- Hormonal imbalances. A condition called SIADH causes your body to retain too much water. Thyroid and adrenal gland problems can also disrupt sodium regulation.
- Kidney, liver, or heart problems. These conditions can cause fluid to build up in your body, diluting sodium concentrations.
- Severe vomiting or diarrhea. Losing large amounts of fluid and electrolytes at once can throw your sodium levels off.
Why Endurance Athletes Are at High Risk
Marathon runners, triathletes, and ultramarathon participants face a specific and well-documented risk. For decades, athletes were encouraged to drink as much fluid as possible during long events. That advice, meant to prevent dehydration, inadvertently led to a rise in exercise-associated hyponatremia, particularly in the United States.
The problem is straightforward: during hours of sustained exercise, athletes sweat out sodium while replacing it with plain water. Over the course of a four- or five-hour event, the sodium in their blood gets progressively diluted. Smaller athletes and slower finishers tend to be at greater risk because they have more time to overdrink relative to their sweat losses. The symptoms can appear during or shortly after the event, and severe cases have been fatal. Current guidance for endurance athletes emphasizes drinking to thirst rather than forcing fluid intake on a schedule, and using electrolyte-containing drinks during prolonged efforts.
Premenopausal Women Face Greater Danger
One group is especially vulnerable to the worst outcomes of acute low sodium: premenopausal women. Estrogen and progesterone appear to inhibit the brain’s ability to push excess solutes out of its cells, a key adaptation that normally limits swelling. This means the osmotic gradient between the brain and the surrounding blood remains steeper, pulling more water into brain tissue. The result is more severe cerebral edema for the same degree of sodium loss compared to men or postmenopausal women.
How Low Sodium Is Treated
Treatment depends on how severe the drop is and how fast it happened. For mild, chronic cases with no serious symptoms, the first step is usually restricting fluid intake. Drinking less allows the kidneys to gradually rebalance sodium concentrations on their own. Addressing the underlying cause, whether that’s adjusting a medication or treating a hormonal issue, is equally important.
Severe or rapidly developing hyponatremia with neurological symptoms like seizures or significant confusion is a medical emergency. In these situations, doctors carefully raise sodium levels using concentrated salt solutions given intravenously. The key word is “carefully.” Correcting sodium too quickly creates its own serious problem.
The Danger of Correcting Too Fast
When sodium has been low for more than a day or two, brain cells adapt by shedding internal solutes to reduce swelling. If sodium levels in the blood are then raised too rapidly, water rushes back out of those adapted brain cells, causing them to shrink. This can destroy the protective myelin coating on nerve cells, particularly in a region of the brainstem called the pons. The result is osmotic demyelination syndrome, a condition that can cause permanent neurological damage including difficulty speaking, swallowing, and moving.
This is why hospitalized patients with low sodium are monitored closely, with sodium levels checked frequently during correction. The goal is a slow, controlled rise, typically no more than a few points per day, to give brain cells time to readjust without being damaged in the process. It’s a narrow therapeutic window: too little correction leaves the brain swollen, too much correction strips its nerve insulation.
Mild Low Sodium and Everyday Health
Not all low sodium is a medical emergency. Mildly low levels (130 to 134 mEq/L) often cause subtle symptoms like fatigue, mild nausea, or difficulty concentrating. Some people with chronic mild hyponatremia don’t notice obvious symptoms at all, but research suggests even modest reductions in sodium can affect balance and attention, increasing fall risk in older adults.
For most healthy people, the body does an excellent job regulating sodium through the kidneys. The WHO recommends adults consume less than 2,000 mg of sodium per day (just under a teaspoon of salt) for heart health, and deficiency from diet alone is rare in countries with typical Western eating patterns. Low sodium becomes a concern primarily when another factor, whether a medication, medical condition, excessive water intake, or prolonged intense exercise, disrupts the balance your kidneys normally maintain.