Hyponatremia from SSRIs: Low Sodium and Confusion Risk in Older Adults

When you start an SSRI for depression, you expect to feel better. But for some people-especially older adults-the medication can quietly lower sodium levels in the blood, leading to confusion, dizziness, and even hospitalization. This isn’t rare. It’s one of the most common and dangerous side effects of antidepressants, yet most patients and even some doctors don’t know about it until it’s too late.

What Is Hyponatremia, and Why Does It Happen with SSRIs?

Hyponatremia means your blood sodium is too low-below 135 mmol/L. Sodium helps control fluid balance, nerve function, and muscle contraction. When it drops, water floods into cells, including brain cells. That’s when confusion, headaches, nausea, and seizures start.

SSRIs like citalopram, sertraline, and fluoxetine trigger this by overstimulating serotonin receptors in the brain. This tricks the body into releasing too much antidiuretic hormone (ADH). Normally, ADH tells your kidneys to hold onto water. Too much ADH means you keep drinking water but don’t pee it out. Your blood gets diluted. Sodium levels drop. It’s not about dehydration-it’s about water overload.

This usually happens within two to four weeks of starting the SSRI or increasing the dose. That’s why symptoms often get missed. People think they’re just adjusting to the medication, or worse, they blame aging or dementia.

Who’s Most at Risk?

If you’re over 65, you’re at dramatically higher risk. Studies show 13.9% to 18.6% of older adults on SSRIs develop hyponatremia. That’s nearly 1 in 5. Women are affected more often-65% of cases are female. People with low body weight (under 60 kg), kidney problems, or who take diuretics like hydrochlorothiazide are also at high risk.

Diuretics make things worse. They cause you to lose sodium through urine, so your body tries to compensate by holding onto water. Add an SSRI on top, and your body gets stuck in a dangerous loop: less sodium, more water, diluted blood. One study found the risk jumps 4.2 times when SSRIs and diuretics are combined.

Even if you’re healthy, age alone changes how your kidneys handle fluids. By 70, your kidney function naturally slows. Your body doesn’t flush out extra water as easily. That’s why SSRIs are now listed as potentially inappropriate for older adults in the American Geriatrics Society’s 2023 Beers Criteria.

Which SSRIs Carry the Highest Risk?

Not all SSRIs are the same. Citalopram has the highest risk-2.37 times more likely to cause hyponatremia than other antidepressants. Sertraline and fluoxetine are close behind. Paroxetine is a bit lower, but still risky.

Here’s the breakdown based on 2024 data:

• Citalopram: Odds ratio 2.37
• Sertraline: Odds ratio 2.15
• Fluoxetine: Odds ratio 1.98
• Paroxetine: Odds ratio 1.82

Compare that to mirtazapine, an antidepressant that doesn’t work like an SSRI. It has a risk level of just 0.47-less than half the risk of SSRIs. Bupropion (Wellbutrin) is also safer, with almost no link to low sodium.

This isn’t just theory. In one study, for every 1,000 people treated with SSRIs, about 18.6 developed hyponatremia. With mirtazapine, it was only 6.5. That’s a number needed to harm (NNH) of 82. In plain terms: if you switch 82 older patients from an SSRI to mirtazapine, you prevent one case of dangerous low sodium.

Real Stories: When Symptoms Are Mistaken for Something Else

A 78-year-old woman in Melbourne started sertraline for depression. Ten days later, she became confused, slurred her speech, and fell. Her family thought it was early dementia. Blood tests showed sodium at 118 mmol/L-severely low. She spent five days in intensive care.

Another case: an 82-year-old woman on citalopram became disoriented, stopped eating, and didn’t recognize her own children. Her son, who posted on Reddit, said the doctor didn’t mention hyponatremia. Sodium was 122 mmol/L. She was hospitalized. The SSRI was stopped. It took four days for her sodium to return to normal.

These aren’t outliers. A 2023 survey found only 28.7% of patients were warned about this risk before starting SSRIs. And doctors? A study showed 63.4% didn’t know the typical onset window-two to four weeks. So symptoms get ignored. Or worse, treated as dementia.

What Should You Do? Testing, Monitoring, and Safer Alternatives

Here’s what actually works:

1. Test sodium before starting. Get a blood test within 7 days before beginning any SSRI.
2. Test again at 2 weeks. That’s when levels usually drop. Don’t wait until you feel bad.
3. High-risk patients? Test monthly for the first 3 months. Especially if you’re over 65, on diuretics, or have kidney issues.
4. Watch for early signs. Headache, nausea, feeling off, fatigue. These come before confusion or seizures.
5. Ask about alternatives. If you’re over 65, mirtazapine is safer. Bupropion is another option. Both work for depression without the sodium risk.

Dr. Elena Martinez, lead author of the 2024 meta-analysis, says: “Mirtazapine should be first-line for elderly patients.” That’s not a suggestion-it’s evidence-based care.

And if hyponatremia happens? Mild cases (sodium 125-134) often fix themselves with fluid restriction (800-1000 mL/day) and stopping the SSRI. Severe cases (below 125) need hospital care-IV saline, slow correction, and monitoring to avoid brain damage from too-fast correction.

Why This Is Still Underdiagnosed

Most people don’t know hyponatremia exists. Primary care doctors don’t test for it. Families assume confusion is normal aging. Elderly patients don’t report mild nausea or fatigue-they think it’s just getting older.

Studies estimate 37.8% of mild cases go undetected in primary care. That means thousands of older adults are walking around with low sodium, at risk of falls, cognitive decline, or sudden collapse. And because the symptoms mimic dementia, many are misdiagnosed, put on more meds, and spiral further.

The FDA now requires SSRI labels to warn about hyponatremia. But warnings on a pill bottle don’t help if no one reads them-or if the doctor doesn’t mention it.

What’s Changing in 2026?

Prescribing patterns are shifting. Between 2018 and 2023, SSRI use in people over 65 dropped 22.3%. Mirtazapine prescriptions for that group rose 34.7%. That’s because doctors are finally listening to the data.

The European Medicines Agency is reviewing SSRI safety, with results due by late 2025. The first clinical algorithm for managing SSRI-induced hyponatremia was published in March 2024-giving doctors clear steps to follow.

By 2027, mirtazapine is expected to be used in over 42% of antidepressant prescriptions for older adults. That’s up from less than 30% in 2023. The trend is clear: safer options are winning.

Bottom Line: Don’t Assume It’s Just Depression

SSRIs save lives. But they’re not risk-free. For older adults, the risk of low sodium is real, avoidable, and often ignored. If you or a loved one is on an SSRI and starts feeling confused, sluggish, or nauseous-get a blood test. Don’t wait. Sodium levels can drop fast.

Ask your doctor: “Has my sodium been checked since I started this medication?” If they say no, ask for a test. If they say it’s not necessary, ask about mirtazapine or bupropion. There are safer choices.

Depression is serious. But so is brain swelling from low sodium. You don’t have to choose between treating one and risking the other.