Low blood sodium, called hyponatremia, happens when sodium drops below 135 mEq/L. It’s one of the most common electrolyte problems, and the causes range from everyday medications to serious organ conditions. The underlying issue is almost always either too much water diluting sodium in the blood, too much sodium being lost from the body, or a hormonal signal gone wrong that disrupts the balance between the two.
How Low Sodium Is Classified
Doctors categorize low sodium by severity: mild (130 to 134 mEq/L), moderate (125 to 129 mEq/L), or severe (below 125 mEq/L). Mild cases often produce no noticeable symptoms and may be caught incidentally on a blood test. Moderate and severe cases can cause headache, nausea, confusion, and muscle cramps. At very low levels, particularly below 120 mEq/L, the brain begins to swell, which can trigger seizures, loss of consciousness, or coma.
Medications Are the Most Common Culprit
Certain prescription drugs are the single most frequent cause of low sodium, especially in older adults. Thiazide-type water pills (prescribed for high blood pressure) top the list as the leading cause of severe drug-induced hyponatremia. They work by making the kidneys excrete more water and salt, but in some people, the sodium loss outpaces the body’s ability to compensate.
Antidepressants are the second major drug category. SSRIs like fluoxetine and sertraline, along with the related SNRIs like venlafaxine and duloxetine, can cause the body to release too much antidiuretic hormone, which makes the kidneys hold onto water and dilute sodium. SNRIs often carry a higher risk than SSRIs.
Seizure medications, particularly carbamazepine and oxcarbazepine, are also well-known triggers, with oxcarbazepine showing a higher incidence. Beyond these, acid-reflux drugs (proton pump inhibitors like omeprazole), certain antibiotics, antipsychotics, opioids like tramadol, and even some chemotherapy drugs have been linked to dropping sodium levels. If you’re on any of these medications and experience new fatigue, confusion, or nausea, a simple blood test can check your sodium.
A Hormone Signal That Traps Water
Your brain produces antidiuretic hormone (ADH) to tell the kidneys when to conserve water. Normally, when sodium is low, the body dials ADH down so you urinate more and shed excess water. In a condition called SIADH (syndrome of inappropriate antidiuretic hormone), that signal stays locked in the “on” position even when sodium is already too low. The kidneys keep reabsorbing water, blood volume stays high, and sodium gets progressively diluted.
SIADH isn’t a disease on its own. It’s triggered by something else. The list of triggers is long: pneumonia and other lung infections, stroke, head trauma, brain infections, and surgery (likely from pain signals stimulating ADH release). Small-cell lung cancer is the most common tumor that produces ADH on its own, essentially hijacking the hormone system. Even recreational drugs like MDMA (ecstasy) directly stimulate ADH release, which is why dangerous hyponatremia occasionally occurs at parties or music festivals when people combine the drug with heavy water intake.
Heart Failure and Liver Disease
In heart failure, the heart’s weakened pumping creates a problem the body misreads. Because less blood reaches the arteries, the nervous system responds as if the body is dehydrated, even though fluid is actually backing up. This triggers a cascade: the kidneys activate hormones that retain salt and water, the brain releases more ADH, and thirst increases. The net result is excess water retention that dilutes sodium. In severe heart failure, ADH levels stay inappropriately high and don’t drop even when someone drinks extra fluid, making hyponatremia persistent and difficult to manage.
Liver cirrhosis creates a similar pattern. The damaged liver can’t regulate fluid properly, blood pools in the abdomen and other tissues, and the body again misinterprets this as low blood volume. The same water-retaining hormones kick in, diluting sodium. In both conditions, the total amount of sodium in the body may actually be normal or even elevated, but there’s so much extra water that the concentration falls.
Adrenal Gland Problems
The adrenal glands, which sit on top of the kidneys, produce two hormones critical for sodium balance: cortisol and aldosterone. In primary adrenal insufficiency (Addison’s disease), both hormones drop. Aldosterone is the hormone that tells the kidneys to hold onto sodium. Without enough of it, sodium pours out in the urine, blood volume drops, and people develop salt cravings, dizziness when standing, and low blood pressure. Hyponatremia is one of the most common lab findings in this condition, along with elevated potassium. Hypothyroidism and pituitary gland problems can also impair sodium regulation through similar hormonal disruptions.
Overhydration During Exercise
Marathon runners, ultramarathon participants, and other endurance athletes sometimes develop dangerously low sodium not from losing too much salt in sweat, but from drinking too much water. When fluid intake exceeds what the body loses through sweating, breathing, and urination, the excess water dilutes sodium in the blood. This is called exercise-associated hyponatremia.
The strongest predictor is weight gain during exercise. In almost all symptomatic cases, the person either gained weight or failed to lose the amount of weight expected from prolonged physical activity. This points to fluid overload rather than salt depletion as the core problem. The body’s ADH levels also tend to stay elevated during extended exercise, which compounds the issue by preventing the kidneys from clearing excess water. The practical takeaway: drinking to thirst rather than forcing fluids on a set schedule substantially reduces the risk.
Vomiting, Diarrhea, and Heavy Sweating
Any condition that causes significant fluid loss from the body can pull sodium levels down, especially when those losses are replaced with plain water or other low-sodium fluids. Prolonged vomiting, severe diarrhea, and excessive sweating all flush sodium out. The kidneys try to compensate, but if the losses are large enough or last long enough, they can’t keep up. This type of low sodium is particularly common in people with stomach viruses, food poisoning, or heat-related illness who drink large amounts of water without replacing electrolytes.
Low-Protein Diets and Heavy Beer Drinking
One of the more unusual causes is a severely restricted diet. When someone consumes very little protein and very little salt, such as a “tea and toast” diet sometimes seen in elderly individuals living alone, the kidneys lose their ability to excrete excess water efficiently. That’s because the kidneys need a minimum amount of dissolved particles (mostly from protein breakdown) to carry water out in the urine. Without enough of these particles, even a modest amount of extra fluid can overwhelm the system.
Heavy beer drinking creates the same problem through a slightly different path. Beer contains almost no sodium and negligible protein. The calories in beer also suppress muscle breakdown, which further reduces the particles available for the kidneys to work with. This condition, sometimes called beer potomania, can cause severe hyponatremia. The kidney’s capacity to clear excess water drops dramatically, so even normal fluid intake on top of heavy beer consumption can tip the balance.
Symptoms That Signal a Problem
Mild hyponatremia often flies under the radar. You might feel slightly off, a bit foggy, or unusually tired, but nothing alarming. As sodium drops further into the moderate range, symptoms become harder to ignore: persistent nausea, headache, muscle weakness, and confusion. Severe hyponatremia below 120 mEq/L is a neurological emergency. The brain swells because water shifts into brain cells, and this can produce seizures, loss of consciousness, and, in extreme cases, brain herniation. Raising sodium by just 4 to 6 mEq/L in an acute crisis can reverse impending brain swelling and stop seizure activity, which is why rapid identification matters.
The speed at which sodium drops is just as important as the number itself. A gradual decline over days or weeks gives the brain time to adapt, so someone with a sodium of 118 who got there slowly may feel relatively okay. The same level reached in under 48 hours can be life-threatening. This is why new-onset confusion, severe headache, or a seizure in someone at risk for low sodium always warrants urgent evaluation.