What Is Hyponatremia? The Hidden Danger of Drinking Too Much Water

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What is hyponatremia, and why does it matter to someone who exercises? You’ve been told to stay hydrated and drink plenty of water your whole life, and that’s sound advice, mostly. But there’s a lesser-known danger at the opposite extreme, one that has harmed and even killed endurance athletes: drinking too much. Hyponatremia is what happens when that goes wrong.

In plain terms, hyponatremia is low blood sodium. And the part that surprises everyone is the mechanism: for athletes, it’s usually caused by taking in more water than your body can handle, not by sweating out too much salt. Which means the intuitive response to feeling unwell, drink more water, can be exactly the wrong move.

I’m Inna, a NASM-certified personal trainer, and I want to explain this clearly, because it’s one of the few hydration topics where getting it wrong is dangerous. This is educational, not medical advice. I’m a trainer, not a doctor. For the broader hydration picture, our guide to the best electrolytes for women is the anchor. This one is about the risk at the far end of “drink more.”

What Is Hyponatremia, Exactly?

Hyponatremia is defined as a blood sodium concentration below 135 mmol/L. When it drops considerably lower, below around 120, it’s considered severe and dangerous.

To understand why low sodium is a problem, you need to know what sodium does. It’s the main regulator of the water balance between your blood and your cells. When blood sodium falls too low, the balance tips, and water shifts out of the blood and into your cells, causing them to swell. In most tissues that’s bad enough. The real danger is your brain. Brain cells swelling inside the fixed space of your skull raises pressure in the head, which is what makes severe hyponatremia life-threatening.

So this isn’t a minor “your levels are a little off” situation at its serious end. Mild hyponatremia can be subtle and manageable, but severe hyponatremia is a medical emergency precisely because of what happens to the brain. That distinction between mild and severe runs through everything that follows.

What Causes Low Blood Sodium

Hyponatremia has several possible causes, and it helps to see the whole map before zooming in on the one that matters most for athletes.

• Overdrinking (dilutional): taking in more fluid than your kidneys can excrete, diluting your blood sodium. This is the athlete’s version.
• Medical conditions: kidney, heart, or liver problems, and hormonal conditions like SIADH, can all disturb sodium balance.
• Medications: certain diuretics, some antidepressants, and other drugs can lower sodium.
• Prolonged heavy sweating replaced with water only: losing sodium in sweat and replacing just the fluid, not the sodium.
• Illness: significant vomiting or diarrhea can contribute.

The medical causes are firmly doctor territory, and if you have a condition or take medications that affect sodium, that’s a conversation with your physician, not something to manage yourself. For this article, we’re focused on the cause most relevant to active women. If you’re trying to make sense of vague symptoms more broadly, our guide to signs of electrolyte imbalance in women covers the wider picture.

Exercise-Associated Hyponatremia: the One Athletes Need to Know

For active women, the version that matters is exercise-associated hyponatremia, or EAH, and its cause is the counterintuitive heart of this whole topic.

EAH is driven primarily by overdrinking, meaning too much water or low-sodium fluid during prolonged exercise, more than your kidneys can excrete. As you keep drinking beyond what your body can process, you progressively dilute your blood sodium. Note what this is not: it’s not primarily caused by sweating out sodium. The dominant driver is fluid in, not salt out.

This matters because the standard hydration message, “drink as much as you can, stay ahead of your thirst,” is exactly the behavior that causes EAH. Well-meaning athletes who force fluids on a rigid schedule during a marathon, thinking they’re preventing dehydration, can drink themselves into dangerously low sodium instead. The longer the event, the greater the opportunity to overdrink. Our guide to the best electrolytes for runners women covers how to hydrate for long efforts without tipping into this.

Why Women Are Flagged as Higher Risk (Honestly)

Women runners are frequently named as a higher-relative-risk group for EAH, and I want to give you the honest version rather than a scary headline.

The higher-risk factors include female sex, lower body weight, slower pace, events lasting over about four hours, easy access to fluid along the course, and NSAID use. A lot of newer or recreational marathoners check several of those boxes.

But here’s the nuance the headlines skip: when researchers adjust for body size and finishing time, the pure sex difference often shrinks or loses statistical significance. In other words, it’s not that being a woman is inherently dangerous. It’s that the risk factors, smaller body size and slower finishing times giving more hours to overdrink, cluster in ways that catch women more often. That’s a meaningful distinction. The takeaway isn’t fear. It’s awareness that if you’re smaller and slower and have water available at every mile, you have more opportunity to overdrink, so drink-to-thirst discipline matters more for you.

The Dangerous Overlap With Dehydration

This is the single most important practical point in the article, so I’m giving it its own section.

The early symptoms of hyponatremia, meaning nausea, headache, dizziness, and feeling generally unwell, overlap almost perfectly with the symptoms of dehydration. And that overlap sets a trap. Picture a woman two hours into a hot race, feeling nauseated and headachy. She assumes she’s dehydrated, because that’s what everyone worries about, so she does the “responsible” thing and drinks more water. If her problem was actually hyponatremia, she just made it worse.

So burn this into memory: feeling unwell during prolonged exercise does not automatically mean “drink more water.” If anything, reflexively chugging water when you feel off on a long effort can be dangerous. Sodium is the missing piece as often as fluid is, and overdrinking is a real possibility. Our guide to sodium for women athletes covers how to think about sodium during training rather than defaulting to more water.

Symptoms: Mild vs Severe (and When It’s an Emergency)

Symptoms fall into two categories, and knowing the difference tells you whether to adjust or to get help immediately.

Milder, earlier symptoms can include:

• Nausea
• Headache
• Dizziness or feeling off-balance
• Bloating or puffiness
• Fatigue and general malaise

Severe symptoms are a medical emergency and need immediate care:

• Repeated vomiting
• A severe or worsening headache
• Marked confusion or altered mental state
• Seizures
• Loss of consciousness

If you or someone with you shows the severe signs, especially after a long endurance event, that’s an emergency. Call for help and get medical care immediately. Do not wait it out, and do not try to fix it with more water. I’m keeping this calm on purpose, but I won’t soften the line: severe hyponatremia can be fatal, and it’s treated in a hospital, not on the sidelines.

How to Prevent Hyponatremia

The good news is that prevention is simple, and it’s the same advice that steers you away from the overdrinking trap.

Drink to thirst. This is the consensus guidance for endurance athletes, and it protects you from both dehydration and overdrinking at once. Your thirst is a remarkably good regulator, better than a fixed schedule or a giant marked bottle telling you to finish it by a certain mile. Drinking “ahead of thirst,” the old advice, is exactly what contributed to EAH cases.

Include sodium on long or hot efforts. For sustained exercise, replacing some sodium helps, though remember that sodium attenuates the risk, it doesn’t guarantee prevention. No salt tablet makes it safe to overdrink. Sodium and sensible fluid intake work together.

Don’t swing to dehydration either. The answer to overdrinking isn’t to stop drinking. Real dehydration impairs performance and carries its own risks. The target is the sensible middle, reached by drinking to thirst rather than by either forcing or restricting fluids. Our guides to whether women need electrolytes every day and the best electrolytes for hot yoga both cover hydrating sensibly in demanding conditions.

What to Do If You Suspect It

If you suspect hyponatremia, particularly if you feel unwell after prolonged exercise and have been drinking heavily, the first thing to do is stop drinking plain water. Adding more fluid to already-diluted blood makes it worse.

Then get help. For mild symptoms, that means not pushing on blindly and seeking medical advice. For severe symptoms such as confusion, severe headache, vomiting, or seizures, it means emergency care, immediately. I’m deliberately not giving you home treatment steps beyond this, because hyponatremia is treated by medical professionals who can measure your sodium and correct it safely. Correcting low sodium too fast can itself be harmful, which is exactly why it’s a job for a hospital, not a water bottle and good intentions. Your role is to recognize it, stop overdrinking, and get proper help.

It’s Not Just Athletes

While EAH is the version most relevant to this audience, hyponatremia isn’t only an exercise problem. It also arises from medical conditions, from medications, and it’s more common in older adults, whose bodies regulate sodium and fluid less robustly.

These causes are all medical matters. If you have a chronic condition, take medications that can affect sodium, or care for an older adult who seems confused or unwell, low sodium is one of many possibilities a doctor can check. I mention it only so you know the condition has a broader life than the marathon course, not so you diagnose it yourself. Bloodwork and a physician are how it’s actually identified.

Do You Need to Worry in Everyday Life?

Let me put the everyday risk in proportion, because I don’t want you eyeing your morning glass of water with suspicion.

For normal daily life, meaning drinking when you’re thirsty, having water with meals, and staying hydrated in ordinary ways, hyponatremia is rare. Your kidneys are perfectly capable of handling a normal, thirst-guided intake of fluid. The risk concentrates in specific situations: prolonged endurance exercise where people deliberately overdrink, and particular medical or medication contexts. It is not a reason to fear ordinary hydration.

So the practical message isn’t “drink less” in daily life. It’s “drink to thirst,” which happens to protect you in both directions. If you’re using electrolyte products as part of that, our guide to the best electrolyte powder for women covers choosing sensibly.

What Is Hyponatremia Telling Us About Hydration?

Step back and what is hyponatremia really teaching us? That hydration is a balance, not a contest to maximize water intake.

The whole condition exists because “more water is always better” is a myth. Beyond a point, more water isn’t more hydrated. It’s diluted. Your body doesn’t reward you for flooding it. It rewards you for meeting its needs and no more. That’s why the modern guidance moved away from forcing fluids and toward drinking to thirst, and why sodium earns a place alongside water on long efforts rather than water alone.

If you take one lesson from all of this, let it be that hydration has two ditches, not one. Everyone warns you about dehydration. Far fewer warn you about the other side. Drink to thirst, include sodium when it matters, and stop treating water as something you can’t overdo. Our comparison of electrolytes vs sports drinks covers when plain water needs company.

Common Myths About Hyponatremia

Myth: “You can’t drink too much water”

You absolutely can. Overdrinking is the main cause of exercise-associated hyponatremia. Water is essential, but beyond what your kidneys can handle, more of it dilutes your blood sodium dangerously.

Myth: “Hyponatremia just means you need more salt”

Not quite. For athletes it’s usually about too much fluid rather than too little salt. Sodium helps, but the core problem is often overdrinking. More salt on top of continued overdrinking isn’t the fix.

Myth: “Salt tablets prevent it”

Sodium attenuates the risk but doesn’t guarantee prevention. No salt tablet makes it safe to overdrink. Drinking to thirst does more to protect you than any pill.

Myth: “It only happens to elite athletes”

Recreational, slower athletes are actually at higher relative risk, because slower finishing times mean more hours to overdrink. It’s not an elite-only problem. In some ways it’s the opposite.

Myth: “If you feel bad on a long run, drink more water”

This is the dangerous one. Mild hyponatremia mimics dehydration, so reflexively drinking more water when you feel off during long exercise can worsen it. Don’t assume more water is the answer.

Myth: “Staying maximally hydrated is always safest”

Hydration is a balance. Maximal water intake isn’t the safest state. It’s how overdrinking happens. Drinking to thirst, not to a maximum, is the safe approach.

FAQ

What is hyponatremia in simple terms?

It’s low blood sodium, below 135 mmol/L. Sodium controls the water balance between your blood and cells, so when it drops too low, water moves into cells and they swell, which is dangerous when it affects the brain.

Can you really drink too much water?

Yes. Drinking more fluid than your kidneys can excrete, especially during long exercise, dilutes your blood sodium and can cause hyponatremia. Water is essential, but it’s possible to overdo it.

What are the first signs of hyponatremia?

Early signs are often nausea, headache, dizziness, bloating, and general malaise, which overlap with dehydration and are easy to misread. Severe signs like confusion, vomiting, or seizures are a medical emergency.

Does taking salt prevent hyponatremia?

Sodium reduces the risk but doesn’t guarantee prevention if you overdrink. It works alongside sensible fluid intake, not as a license to drink unlimited water. Drinking to thirst matters more.

Is hyponatremia an emergency?

Severe hyponatremia is. Confusion, severe headache, repeated vomiting, seizures, or loss of consciousness require immediate medical care. Milder cases still need medical advice and stopping any overdrinking. It’s diagnosed and treated by professionals.

How do runners avoid hyponatremia?

Drink to thirst rather than forcing fluids on a schedule, include sodium on long or hot efforts, and don’t reflexively drink more water when you feel unwell mid-run. Balance beats maximizing water intake.

The Bottom Line

Hyponatremia is low blood sodium, and for active women it’s most relevant through overdrinking during long exercise, with the counterintuitive truth that more water causes it rather than cures it. Its mild symptoms mimic dehydration, which is the dangerous trap that leads people to drink more and worsen it. Prevent it by drinking to thirst rather than forcing fluids, including sodium on long or hot efforts, and never assuming more water is always the answer. Severe symptoms such as confusion, severe headache, vomiting, and seizures are a medical emergency that needs immediate care. In everyday life, thirst-guided drinking keeps you safe. The real risk lives in endurance events and specific medical situations, not your daily glass of water.

This article is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare professional before starting any new supplement regimen.