How do I write a prescription for chlorthalidone 12.5 mg once daily in the morning?

How to Write a Prescription for Chlorthalidone 12.5 mg Once Daily

Write the prescription as: Chlorthalidone 12.5 mg tablet, take one tablet by mouth once daily in the morning with food, dispense #30 tablets, with appropriate refills based on your follow-up schedule.

Essential Prescription Components

• Drug name and strength: Chlorthalidone 12.5 mg tablet 1
• Directions (Sig): Take one tablet by mouth once daily in the morning with food 1
• Quantity: Dispense #30 tablets (for 30-day supply) 1
• Refills: Typically 3-5 refills depending on your monitoring schedule 1

Critical Pre-Prescribing Requirements

Before writing this prescription, you must verify baseline laboratory values:

• Check serum sodium, potassium, and creatinine to establish baseline renal and electrolyte status 1
• Verify eGFR - chlorthalidone can be used even in advanced CKD (eGFR <30 mL/min/1.73 m²) but requires closer monitoring 2, 3
• Screen for gout history - use caution unless patient is on uric acid-lowering therapy 1

Mandatory Patient Counseling Points

When prescribing chlorthalidone 12.5 mg, counsel patients on:

• Take in the morning with food to minimize gastrointestinal upset and avoid nighttime urination 1
• Report muscle weakness, cramps, or dizziness immediately as these may indicate hypokalemia or hyponatremia 1
• Hold medication during acute illness with vomiting, diarrhea, or poor oral intake to prevent volume depletion and acute kidney injury 2
• Avoid NSAIDs as they can precipitate acute renal failure and worsen hypertension control 4

Required Follow-Up Monitoring Schedule

The most critical aspect of chlorthalidone prescribing is establishing a monitoring schedule:

• Recheck electrolytes (sodium, potassium) and creatinine within 2-4 weeks of initiation 1
• Hold chlorthalidone if sodium drops below 130 mEq/L 1
• Hold chlorthalidone if potassium drops below 3.5 mEq/L 1, 5
• Hold if acute kidney injury develops or azotemia worsens 1
• For stage 1 hypertension with 10-year ASCVD risk ≥10%: repeat BP evaluation in 1 month 1
• For stage 2 hypertension: evaluate within 1 month, often requiring combination therapy 1
• Once stable: monitor every 3-6 months 2

Dosing Rationale and Evidence

The 12.5 mg starting dose is evidence-based and optimal for most patients:

• Clinical trials demonstrate 12.5 mg is as effective as higher doses with significantly fewer adverse effects 6, 7
• The dose-response curve is flat above 25 mg - doses above 50 mg provide minimal additional benefit but substantially increase hyponatremia risk 1, 6
• Chlorthalidone carries 3-fold higher hyponatremia risk than hydrochlorothiazide due to its prolonged 24-72 hour duration of action 1
• In elderly patients with isolated systolic hypertension, 12.5 mg achieved therapeutic success with no clinically significant biochemical changes 6

Special Population Considerations

Patients with Advanced CKD (eGFR <30 mL/min/1.73 m²)

• Do NOT automatically discontinue chlorthalidone when eGFR decreases to <30 mL/min/1.73 m² 2
• Chlorthalidone remains effective for BP management even in stage 4 CKD 2, 3
• Monitor electrolytes more frequently (every 2-4 weeks initially, then every 3-6 months) 2
• Expect reversible increases in serum creatinine - this does not mandate discontinuation unless acute kidney injury develops 2

Pediatric Patients

• Initial dose: 0.3 mg/kg/day once daily 1
• Maximum dose: 2 mg/kg/day up to 50 mg/day 1
• Requires particularly vigilant electrolyte monitoring 1

Patients on Loop Diuretics

• Chlorthalidone can be safely added to loop diuretics for synergistic effect in resistant hypertension or volume overload 2
• Exercise particular caution due to increased risk of electrolyte abnormalities and volume depletion 2
• Monitor electrolytes within 1 week when combining diuretics 2

Common Prescribing Pitfalls to Avoid

• Never start at 25 mg or 50 mg - these doses increase adverse effects without proportional benefit 1, 6, 7
• Never prescribe without establishing a monitoring schedule - hyponatremia and hypokalemia can develop insidiously 1
• Never combine with potassium-sparing diuretics in significant CKD (eGFR <45 mL/min) due to hyperkalemia risk 2
• Never fail to counsel on holding during acute illness - this is a leading cause of preventable acute kidney injury 2
• Never prescribe to patients with active gout unless they are on uric acid-lowering therapy 1

Sample Prescription Format

Chlorthalidone 12.5 mg tablet
Sig: Take one tablet by mouth once daily in the morning with food
Disp: #30 tablets
Refills: 3
Follow-up: Recheck electrolytes and creatinine in 2-4 weeks