Drug Interactions between HalfLytely and Bisacodyl and trimethoprim

MONITOR CLOSELY: The use of trimethoprim in combination with other potassium-sparing drugs or potassium salts may increase the risk of hyperkalemia. Trimethoprim inhibits sodium reabsorption and potassium excretion by blocking sodium channels in the renal distal tubules. Studies of patients treated with standard and high dosages of trimethoprim-sulfamethoxazole compared to similar controls treated with other antibiotics indicate that reversible increases in serum potassium are fairly common with trimethoprim use. Although generally asymptomatic, severe hyperkalemia including metabolic acidosis, paralysis, nonoliguric renal failure, and ventricular arrhythmia have been reported. Risk factors for developing hyperkalemia include use of high dosages of trimethoprim (e.g., for the treatment of MRSA skin infections or Pneumocystis jiroveci pneumonia (PCP) in AIDS patients); renal impairment or age-related decline in renal function; aldosterone or adrenal insufficiency; concomitant use of drugs that increase the risk of hyperkalemia (e.g., ACE inhibitors, angiotensin II receptor blockers, aldosterone antagonists; potassium-sparing diuretics); diets with potassium-rich foods (e.g., tomatoes, raisins, figs, baked potatoes, bananas, papayas, pears, cantaloupe, mangoes); and use of potassium salt substitutes.

MANAGEMENT: Serum potassium and sodium levels as well as renal function should be closely monitored during coadministration of trimethoprim with other potassium-sparing drugs or potassium salts, particularly in patients receiving high-dose or long-term trimethoprim treatment and in patients with renal impairment, diabetes, old age, severe or worsening heart failure, or dehydration. A dosage reduction of trimethoprim is recommended in renal dysfunction (50% reduction for CrCl between 15 and 30 mL/min). Patients should be given dietary counseling to avoid excessive intake of potassium-rich foods and salt substitutes, and advised to seek medical attention if they experience signs and symptoms of hyperkalemia such as nausea, vomiting, weakness, listlessness, tingling of the extremities, paralysis, confusion, weak pulse, and a slow or irregular heartbeat. Trimethoprim should be discontinued if hyperkalemia occurs.

ADJUST DOSING INTERVAL: By increasing gastric pH, antacids may reduce the resistance of the enteric coating of bisacodyl tablets, resulting in earlier release of bisacodyl and gastric irritation and dyspepsia.

MANAGEMENT: The administration of antacids and bisacodyl should be separated by at least one hour.

GENERALLY AVOID: Concomitant use of stimulant laxatives (e.g., bisacodyl, sodium picosulfate) may increase the risk of serious gastrointestinal adverse effects associated with certain osmotic laxatives (e.g., polyethylene glycol (PEG), oral sulfate solution), such as colonic mucosal ulcerations or ischemic colitis. There have been isolated case reports of ischemic colitis occurring with the use of PEG-based bowel cleansing products in combination with higher dosages of bisacodyl (usually greater than 10 mg). Bisacodyl can cause colonic ischemia due to transient reduction in splanchnic blood flow. When administered in conjunction with an osmotic laxative such as PEG, increased intramural pressure secondary to increased peristalsis may lead to ischemic colitis and perforation.

MANAGEMENT: The manufacturers for some osmotic bowel cleansing products recommend avoiding the concurrent use of stimulant laxatives. However, stimulant laxatives, in particular bisacodyl and sodium picosulfate, are sometimes used with PEG in certain bowel cleansing regimens to help reduce dose volume and improve patient tolerability and acceptance. Please consult individual product labeling for specific recommendations and guidance. Patients using osmotic bowel cleansing products and stimulant laxatives who present with sudden abdominal pain, rectal bleeding, or other symptoms of ischemic colitis should be evaluated promptly.