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

Why Your Antidepressant Might Be Lowering Your Sodium Levels

It’s not just about feeling better emotionally. When you start an SSRI like sertraline or citalopram, your body is doing something unexpected-holding onto too much water. And that’s not harmless. This can drop your blood sodium levels dangerously low, leading to confusion, dizziness, even seizures. It’s not rare. In people over 65, up to 1 in 6 may develop this problem within weeks of starting the medication. Most don’t know it’s even possible.

Hyponatremia means your blood sodium is below 135 mmol/L. Normal is 135-145. When it falls below 125, you’re in danger. At 118, as one 78-year-old woman experienced after starting sertraline, you end up in the ICU. Her symptoms? Confusion, nausea, stumbling. Doctors thought it was dementia. It wasn’t. It was the drug.

How SSRIs Cause Low Sodium

SSRIs work by boosting serotonin in the brain. That’s good for mood. But serotonin also acts on the hypothalamus-the part of your brain that controls how much water your kidneys hold onto. Too much serotonin = too much antidiuretic hormone (ADH) = your body keeps water instead of flushing it out.

This isn’t dehydration. It’s water intoxication. Your blood gets diluted. Sodium drops. Your brain swells slightly. That’s why you get headaches, confusion, or trouble walking. It’s not mental decline. It’s chemical.

Not all SSRIs are equal. Citalopram has the highest risk-over twice as likely as some others. Sertraline and fluoxetine follow close behind. Paroxetine is lower, but still risky. Why? It’s about how tightly they bind to the serotonin transporter. The tighter the grip, the more ADH gets released.

Who’s Most at Risk

This isn’t a problem for everyone. But for some, the risk is sky-high:

• Age 65+: Your kidneys don’t handle water the same way. Risk jumps to 14-19%.
• Women: Over 65% of cases occur in women, likely due to body size and hormonal differences.
• Low body weight: Under 60 kg? Your blood volume is smaller. Less water to dilute.
• On diuretics: Thiazide diuretics (like hydrochlorothiazide) make this risk 4 times worse. Many older adults take them for high blood pressure.
• Renal impairment: If your eGFR is under 60, your kidneys can’t clear water efficiently.
• Starting or increasing dose: The biggest spike in risk happens in the first 2-4 weeks.

One man in his 80s took citalopram for depression. Two weeks later, he stopped recognizing his grandchildren. His sodium? 122 mmol/L. He was hospitalized. The doctor didn’t connect the dots until the family mentioned the new pill.

What Symptoms to Watch For

Early signs are easy to miss. They look like aging, stress, or just "getting older." But they’re not.

• Headaches that won’t go away
• Nausea or loss of appetite
• Feeling unusually tired or weak
• Mild confusion or trouble remembering things
• Walking unsteadily or feeling off-balance
• Muscle cramps or twitching

By the time someone is having seizures or slipping into a coma, it’s too late. The warning signs are subtle. If you or a loved one starts an SSRI and begins acting "not themselves," check sodium levels. Don’t wait.

How Doctors Should Be Testing

It’s not complicated. But too many doctors skip it.

The American Psychiatric Association says: Test sodium before starting an SSRI, then again at 2 weeks. For high-risk patients, test monthly for the first 3 months.

Here’s what they’re looking for:

• Serum sodium < 135 mmol/L
• Urine sodium > 30 mmol/L (your kidneys are holding onto salt)
• Urine osmolality > 100 mOsm/kg (your body is concentrating urine too much)
• No signs of dehydration or heart failure (it’s euvolemic SIADH)

A 2023 survey found that 63% of primary care doctors didn’t know the typical onset window was 2-4 weeks. That’s why cases get missed. It’s not the patient’s fault. It’s a system gap.

What to Do If Sodium Drops

What happens next depends on how low it goes.

Mild hyponatremia (125-134 mmol/L):

• Stop the SSRI immediately
• Limit fluids to 800-1000 mL per day
• Recheck sodium in 48 hours
• Recovery usually happens in 3-4 days

Severe hyponatremia (<125 mmol/L):

• Hospitalization required
• IV hypertonic saline (3%) given slowly-no more than 6-8 mmol/L increase in 24 hours
• Too fast correction can cause brain damage (osmotic demyelination)
• Recovery takes days to weeks

One woman in Oxford had her sodium drop to 118 after starting sertraline. She was confused, couldn’t stand without help. They stopped the drug and gave her saline. It took 5 days for her to return to normal. She didn’t go back on any SSRI.

Better Alternatives for Older Adults

There are antidepressants that don’t carry this risk.

Mirtazapine is the clear winner. Studies show it has less than half the risk of SSRIs. In fact, for every 1,000 older adults treated with mirtazapine, only 6-7 develop hyponatremia. With SSRIs? 18-20. That’s a number needed to harm (NNH) of 82. Meaning, if you switch 82 elderly patients from an SSRI to mirtazapine, you prevent one case of dangerous low sodium.

Bupropion is another good option. It doesn’t affect serotonin the same way. Risk is low-about the same as placebo. It’s not ideal for everyone (can cause insomnia or anxiety), but it’s safe for sodium.

SNRIs like venlafaxine and duloxetine? Still riskier than mirtazapine. Tricyclics like amitriptyline? Also risky. Stick to mirtazapine or bupropion for older patients.

The American Geriatrics Society’s 2023 Beers Criteria lists SSRIs as potentially inappropriate for seniors because of hyponatremia. Mirtazapine is recommended instead.

Why This Is Still Underdiagnosed

Most cases go unnoticed.

A 2023 survey found only 29% of patients were told about this risk before starting SSRIs. Doctors assume confusion means dementia. Families think it’s just aging. Emergency rooms see "unexplained falls" and don’t test sodium.

One study found 38% of mild cases in primary care are never caught. That means hundreds of thousands of older people are walking around with low sodium-increasing their risk of falls, fractures, and long-term cognitive decline.

It’s not just a side effect. It’s a silent crisis.

What You Can Do

If you or someone you care for is on an SSRI and over 65:

1. Ask for a baseline sodium test before starting the drug.
2. Ask for a repeat test at 2 weeks.
3. Watch for any change in thinking, balance, or appetite.
4. If symptoms appear, get sodium checked immediately-don’t wait.
5. Ask if mirtazapine or bupropion could be a safer alternative.

Don’t assume the doctor knows. Ask. Push. Keep a symptom diary. This isn’t paranoia. It’s prevention.

Antidepressants save lives. But they shouldn’t trade one risk for another. With the right monitoring and alternatives, you can treat depression without putting someone’s brain at risk.

What’s Changing in 2025

The FDA now requires SSRI labels to warn about hyponatremia. The European Medicines Agency is reviewing safety data-results expected by fall 2025. In the U.S., prescriptions for SSRIs in patients over 65 dropped 22% between 2018 and 2023. Mirtazapine prescriptions for that group rose 35%.

By 2027, mirtazapine could be the top choice for older adults needing antidepressants. That’s progress. But it’s not fast enough.

Every year, $1.27 billion is spent in the U.S. treating SSRI-induced hyponatremia-mostly in hospitals and ERs. Most of it could have been avoided with a simple blood test.

Final Thought

Depression in older adults is real. So is the risk of these drugs. You don’t have to choose between feeling better and staying safe. With the right test, the right drug, and the right questions, you can avoid the silent danger of low sodium. Ask for the blood test. Ask about alternatives. Your brain is worth it.