Sodium acid phosphate with sodium phosphate

How it works

The osmotically active phosphate salts draw water into the bowel lumen, increasing intraluminal volume to stimulate peristalsis and produce evacuation.

Prescribing in practice

• It can cause clinically significant fluid and electrolyte disturbances including hyperphosphataemia, hypocalcaemia and dehydration, with a risk of acute phosphate nephropathy, so it is contraindicated or used with great caution in renal impairment, heart failure and the elderly.
• Adequate oral hydration must be maintained throughout, and concomitant drugs affecting fluid or electrolyte balance such as diuretics, ACE inhibitors and lithium increase the risk.
• It is contraindicated in suspected bowel obstruction, ileus or active inflammatory bowel disease.

Monitoring

Assess renal function, hydration and electrolytes before use in at-risk patients and ensure generous fluid intake during bowel preparation.

Counselling the patient

• Drink the recommended clear fluids generously to avoid dehydration.
• Report dizziness, palpitations, severe vomiting or markedly reduced urine output.
• Tell the team about kidney or heart problems and any water tablets you take.

Evidence & guidelines

MHRA advice highlights the risk of severe electrolyte disturbance and renal injury with oral phosphate bowel-cleansing preparations, supporting careful patient selection.