Drug Interactions between divalproex sodium and HalfLytely and Bisacodyl

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.

Limited data suggest that concurrent administration of antacids may increase the bioavailability of valproic acid. The mechanism of interaction is unknown. In seven healthy volunteers, coadministration of a single 500 mg dose of valproic acid one hour after breakfast and an antacid containing aluminum-magnesium hydroxide (dose equal to 160 mEq of neutralizing capacity) one and three hours after meals and at bedtime on the same day resulted in a mean 12% increase (range 3% to 28%) in the total area under the concentration-time curve (AUC) of valproic acid compared to administration alone. These changes are unlikely to be of clinical importance, and no special precautions appear to be necessary. Equivalent doses of antacids containing either aluminum hydroxide-magnesium trisilicate or calcium carbonate also increased the AUC of valproic acid, but the differences were not statistically significant.