Low sodium, medically called hyponatremia, causes symptoms that range from mild nausea and fatigue to seizures and coma, depending on how far levels drop and how quickly. Normal blood sodium falls between 135 and 145 mmol/L, and symptoms typically begin once levels dip below 135. The tricky part is that early symptoms are vague enough to be mistaken for dozens of other problems.
Early Symptoms That Are Easy to Miss
When sodium drops only slightly, into the 130 to 134 mmol/L range, symptoms tend to be nonspecific. You might feel nauseous without an obvious reason, generally unwell, or more fatigued than usual. Headaches, muscle cramps, and a sense of weakness are also common at this stage. Many people chalk these feelings up to stress, poor sleep, or a mild bug and never think to check their sodium.
These early signs matter because they’re your window to catch the problem before it escalates. If you’re taking a medication known to lower sodium (more on that below) or you’ve been drinking large volumes of water, persistent nausea, headaches, or cramping should prompt a conversation with your doctor rather than a shrug.
Moderate Symptoms: When the Brain Gets Involved
At sodium levels between 125 and 129 mmol/L, symptoms shift from vague discomfort to noticeable neurological changes. Confusion, difficulty concentrating, and a sluggish or “foggy” feeling become more prominent. You may feel unusually drowsy, unsteady on your feet, or have trouble finding the right words in conversation.
The reason the brain is so sensitive to low sodium comes down to water balance. Your brain sits inside a rigid skull with very little room to spare. When sodium in the blood drops, water flows into brain cells by osmosis, causing them to swell. The brain tries to compensate by pushing fluid out and shedding internal salts within a few hours, but there’s a limit to how much swelling it can absorb. When sodium falls faster than the brain can adapt, symptoms worsen quickly.
Severe Symptoms That Need Emergency Care
Below 125 mmol/L, and especially below 120, the situation becomes dangerous. Severe symptoms include:
- Seizures, which can occur even in people with no history of epilepsy
- Severe confusion or delirium
- Loss of consciousness or coma
- Vomiting
- Difficulty breathing
At very low levels, around 115 mmol/L, the brain swelling can become severe enough to cause herniation, where brain tissue is forced downward through the base of the skull. This is life-threatening. In one review of adults hospitalized with prolonged seizures, 10% had a metabolic cause like low sodium as the primary trigger, and the mortality rate in that group reached 40%.
How Quickly Sodium Drops Matters as Much as How Far
A person whose sodium drifts down to 125 over the course of a week may feel relatively mild symptoms, while someone whose sodium crashes to 125 over a few hours can develop seizures. The speed of onset is critical. When sodium falls gradually (over 48 hours or more), the brain has time to shed water and adapt. When it drops rapidly, the brain swells before those protective mechanisms kick in.
This is why chronic low sodium, which is far more common, often produces subtler symptoms like persistent fatigue, unsteadiness, and difficulty concentrating. These symptoms are easy to dismiss, but even mild chronic hyponatremia is linked to higher fall risk in older adults and measurably worse outcomes during hospitalization. A large study of nearly 284,000 hospitalized patients found that 30-day mortality was 35% higher in those with even mildly low sodium compared to matched patients with normal levels. For the most severely low readings, mortality was more than three times higher.
Common Causes and Who’s Most at Risk
Several medications are well-established triggers. About 3 in 10 people taking thiazide diuretics (a common blood pressure medication) develop low sodium at some point during treatment, sometimes with serious consequences. Certain antidepressants, particularly SSRIs and related drugs, carry a marked risk, especially in the first few weeks after starting. Seizure medications like carbamazepine and oxcarbazepine cause low sodium in anywhere from 5% to over 30% of users, depending on the study.
Beyond medications, other common causes include heart failure, liver disease, kidney problems, severe vomiting or diarrhea, and a condition called SIADH, where the body produces too much of a hormone that causes water retention. SIADH can be triggered by lung infections, brain injuries, certain cancers, and some of the same medications listed above.
Older adults are especially vulnerable because aging kidneys are less efficient at regulating sodium, and older adults are more likely to be on multiple medications that affect sodium balance.
Low Sodium in Athletes and Heavy Exercisers
Endurance athletes face a specific form of low sodium caused by drinking far more fluid than the body loses through sweat. This is called exercise-associated hyponatremia, and it’s triggered by overconsumption of water or other low-sodium fluids during prolonged activity. The primary cause is simply drinking beyond thirst.
Sodium-containing sports drinks don’t prevent the problem if you’re overdrinking, because they’re still low enough in sodium to dilute your blood when consumed in excess. Weight gain during an event is a reliable signal that fluid intake has exceeded losses. Contrary to popular belief, drinking more than your thirst dictates has not been shown to reduce fatigue, muscle cramps, or heat illness risk. Women who are menstruating may face slightly higher risk due to hormonal effects on how the brain handles swelling.
Symptoms in athletes follow the same progression as other forms: early nausea and headache, then confusion and disorientation, then seizures or loss of consciousness. The key difference is context. If you’ve just finished a marathon and feel confused, nauseated, and bloated, low sodium should be considered alongside heat exhaustion. Treatment in the field for mild cases is simply stopping fluid intake. Severe cases with altered mental status require emergency medical attention.
How Low Sodium Is Treated
Treatment depends entirely on severity and the underlying cause. For mild cases, the approach is often straightforward: restricting fluid intake so the body can rebalance on its own, or addressing the medication or condition responsible. If a diuretic is the culprit, switching or stopping the drug may be enough.
For moderate cases, salt tablets or other methods to gently raise sodium may be used alongside fluid restriction. In severe or rapidly developing cases with neurological symptoms, concentrated saline solution is given intravenously to raise sodium levels quickly enough to reduce brain swelling.
There’s an important catch with treatment: correcting sodium too fast carries its own serious risk. When the brain has adapted to low sodium over days by shedding internal salts, a rapid rise in blood sodium can pull water out of brain cells too aggressively, damaging the protective coating around nerve fibers. This is why doctors monitor correction rates carefully, typically aiming for a slow, controlled increase rather than a rapid fix.