Can any of the listed medications—including divalproex sodium, sertraline, atorvastatin, milk of magnesia, fleet enema, bisacodyl, oxybutynin, cetirizine, mirtazapine, B‑complex/B12, and clonazepam—cause an increase in serum sodium?

Fleet Enema (Sodium Phosphates) is the Primary Culprit for Elevated Sodium

Among the medications listed, Fleet Enema (sodium phosphates) is the only agent that can directly cause hypernatremia (elevated serum sodium), while several others—particularly sertraline, mirtazapine, and divalproex sodium—are well-documented causes of hyponatremia (low sodium), not elevated sodium.

Medications That Can ELEVATE Sodium

Fleet Enema (Sodium Phosphates) - HIGH RISK

• Mechanism: Direct sodium load from absorption of sodium phosphate salts across the rectal mucosa
• Clinical significance: Can cause acute hypernatremia, particularly with:
  • Repeated dosing
  • Prolonged retention
  • Underlying renal impairment
  • Dehydration
• Monitoring: Check serum sodium if multiple enemas used or if patient develops altered mental status, seizures, or signs of volume overload

Medications That LOWER Sodium (Opposite Effect)

Sertraline - MODERATE TO HIGH RISK

• Mechanism: Syndrome of inappropriate antidiuretic hormone secretion (SIADH) 12
• Incidence: OR = 4.15 for hyponatremia 2
• Onset: Typically within first few weeks of therapy
• Risk factors: Elderly patients, female sex, concurrent diuretics

Mirtazapine - MODERATE RISK

• Mechanism: SIADH 345
• Incidence: 3.26% (95% CI 3.06-3.45%) 4
• Onset: Mean 6.5 days, can occur with doses as low as 7.5 mg daily 5
• Clinical presentation: Confusion (57%), somnolence (42%), altered speech (28%) 4
• Mean sodium nadir: 117 mEq/L 45

Divalproex Sodium - LOW TO MODERATE RISK

• Mechanism: SIADH (less commonly reported than SSRIs) 61
• Clinical note: FDA label recommends monitoring sodium when combined with sertraline 1

Clonazepam - MINIMAL DIRECT RISK

• No direct sodium-altering effects documented 7
• May contribute indirectly through drug interactions or altered mental status affecting fluid intake

Medications With NO Significant Sodium Effects

• Atorvastatin: No sodium alterations
• Oxybutynin: No direct sodium effects (anticholinergic effects may affect fluid balance indirectly)
• Cetirizine: No sodium alterations
• B-Complex/B-12: No sodium effects
• Milk of Magnesia: Magnesium-based; no sodium load
• Bisacodyl: Stimulant laxative; no sodium effects

Clinical Management Algorithm

For ELEVATED sodium (hypernatremia):

1. Discontinue Fleet Enema if multiple doses given
2. Check serum sodium, BUN/creatinine
3. Assess volume status and renal function
4. Provide free water replacement if hypernatremic
5. Switch to alternative bowel regimen (bisacodyl, milk of magnesia already on board)

For DECREASED sodium (hyponatremia) - More Likely Scenario:

1. Check baseline and serial sodium levels given multiple SIADH-risk medications (sertraline 150 mg total daily, mirtazapine 7.5 mg)
2. If sodium <135 mEq/L: assess volume status, urine osmolality, urine sodium
3. If sodium 120-125 mEq/L: fluid restriction to 1,000 mL/day, hold diuretics if applicable 89
4. If sodium <120 mEq/L: more severe fluid restriction, consider albumin infusion, discontinue offending antidepressants 89
5. Correction rate: Target 4-6 mEq/L per 24 hours, not exceeding 8 mEq/L per 24 hours to prevent osmotic demyelination syndrome 810

Critical Pitfalls

• Do not confuse hyponatremia with hypernatremia: The antidepressants (sertraline, mirtazapine) cause LOW sodium, not high
• Polypharmacy risk: This patient has TWO hyponatremia-inducing antidepressants (sertraline 150 mg total, mirtazapine 7.5 mg), increasing cumulative risk 24
• Elderly/female patients: Highest risk for antidepressant-induced hyponatremia 235
• Rapid correction danger: Overcorrection of chronic hyponatremia (especially with sodium phosphate enemas in a hyponatremic patient) risks osmotic demyelination syndrome 810

Bottom line: Fleet Enema is the only medication that elevates sodium. The antidepressants do the opposite—they lower sodium through SIADH. Monitor sodium levels closely given the combination of sertraline and mirtazapine, both established hyponatremia risks.