Sodium acid phosphate (Sodium dihydrogen phosphate dihydrate) is a phosphate salt used primarily as a urinary acidifier and a source of phosphate ions. In the Indian context, it is commonly prescribed for the management and prevention of calcium-containing renal stones, particularly in patients with persistently alkaline urine. It works by lowering urinary pH, increasing the solubility of calcium phosphate and calcium oxalate, and inhibiting further stone formation. It also serves as a phosphate supplement in conditions of hypophosphatemia.
Adult: For urinary acidification/stone prophylaxis: 1.936 gm (one tablet/capsule/sachet equivalent) orally, 3 to 4 times daily, diluted in a full glass of water. Dose must be individualized based on urinary pH monitoring. For hypophosphatemia: As directed by physician, based on serum phosphate levels.
Note: Must be taken with a full glass (240 mL) of water. Can be taken with or after food to minimize GI upset. The tablet/capsule/granules must be completely dissolved or dispersed in water before ingestion. Do not swallow dry. Maintain high fluid intake (at least 2.5-3 liters per day) unless contraindicated.
Sodium acid phosphate provides an acid load (H+ ions) to the body. When ingested, it dissociates into sodium and dihydrogen phosphate ions. The dihydrogen phosphate ion (H2PO4-) can act as a weak acid, donating a hydrogen ion. This increases the hydrogen ion concentration in the extracellular fluid, which is then excreted by the kidneys, primarily via the sodium-hydrogen exchanger (NHE3) in the proximal tubule and through titratable acid excretion. This process lowers urinary pH.
Pregnancy: Category C: Animal reproduction studies have not been conducted. Use only if clearly needed and potential benefit justifies potential risk to the fetus. Electrolyte imbalances can affect both mother and fetus. Monitor electrolytes.
Driving: Unlikely to affect ability. However, dizziness or electrolyte imbalance-induced weakness reported rarely; patients should be cautioned.
| Potassium-Sparing Diuretics (Spironolactone, Amiloride) | Increased risk of hyperkalemia, especially if the phosphate preparation contains potassium. | Major |
| ACE Inhibitors (Ramipril, Enalapril) / ARBs (Losartan) | Increased risk of hyperkalemia. | Moderate |
| Antacids containing Aluminum, Calcium, or Magnesium | Form insoluble complexes, reducing phosphate absorption. Separate administration by at least 2 hours. | Moderate |
| Corticosteroids (Prednisolone) | May promote sodium and fluid retention, exacerbating the sodium load from the drug. | Moderate |
| Digoxin | Hypokalemia or hypercalcemia (from hypophosphatemia correction) can potentiate digoxin toxicity. Monitor levels. | Moderate |
| Thyroid Hormones (Levothyroxine) | Phosphate salts may impair absorption of levothyroxine. Administer at least 4 hours apart. | Moderate |
| Bisphosphonates (Alendronate) | Divalent cations can interfere with absorption. Administer at least 2 hours apart. | Moderate |
| Tetracycline & Fluoroquinolone Antibiotics | Decreased antibiotic absorption due to chelation. Administer at least 2-3 hours apart. | Moderate |