Drug Interactions between HalfLytely and Bisacodyl and Quin-Release

MONITOR: Coadministration with drugs that can increase urinary pH such as antacids, carbonic anhydrase inhibitors, or urinary alkalinizers may decrease the urinary excretion of quinidine. The proposed mechanism is increased renal tubular reabsorption due to reduced ionization of quinidine in alkaline urine. In four healthy subjects given quinidine 200 mg every 6 hours, renal clearance of quinidine was reduced by an average of 50% during coadministration with sodium bicarbonate and acetazolamide 500 mg twice a day. Average urinary quinidine level was 115 mg/L at urinary pH below 6, but fell to 13 mg/L at urinary pH above 7.5. Likewise, average quinidine urinary excretion rate fell from 103 to 31 mcg/minute with increasing pH. In another six healthy subjects studied under the same conditions, increased serum quinidine levels were observed in five subjects during urine alkalinization, and prolongations of the QT interval were reported in three of the five. Since only about 20% of a quinidine dose is typically eliminated unchanged by the kidney, the clinical significance of this interaction is unknown. A case report describes a woman who developed toxicity in association with a threefold increase in serum quinidine levels after taking eight Mylanta antacid tablets (each containing 200 mg aluminum hydroxide gel, 200 mg magnesium hydroxide, and 20 mg simethicone) everyday for a week. However, the patient also drank over a liter of orange and grapefruit juice daily, the latter of which can inhibit quinidine metabolism and may have contributed to the toxic drug levels. In pharmacokinetic studies, single doses of aluminum hydroxide alone had no significant effect on quinidine pharmacokinetics or urinary pH, although the possibility of a significant interaction in occasional patients cannot be ruled out.

MANAGEMENT: Caution is advised if quinidine is used with agents that can increase urinary pH. Quinidine levels may need to be monitored more closely following addition or discontinuation of these agents, and the quinidine dosage adjusted as necessary. Patients should be advised to seek medical attention if they experience symptoms that could indicate quinidine toxicity such as tinnitus, hearing loss, visual disturbances, diarrhea, headache, dizziness, palpitations, syncope, or irregular heartbeats.

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MONITOR: Bowel cleansing as well as overuse of certain laxatives may cause electrolyte loss and increase the risk of torsade de pointes ventricular arrhythmia in patients treated with drugs that prolong the QT interval. Electrolyte disturbances including hypokalemia and hypomagnesemia have been reported with laxative abuse and are known risk factors for torsade de pointes associated with QT interval prolongation.

MANAGEMENT: Patients treated with drugs that prolong the QT interval should exercise caution when self-medicating with laxatives. The recommended dosage and duration of use should not be exceeded. Patients treated with lactulose for more than six months should be monitored periodically for electrolyte imbalance. Patients should be advised to seek prompt medical attention if they experience symptoms that could indicate the occurrence of torsade de pointes such as dizziness, lightheadedness, fainting, palpitation, irregular heart rhythm, shortness of breath, or syncope.

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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.

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MONITOR: Bowel cleansing as well as overuse of certain laxatives may cause electrolyte loss and increase the risk of torsade de pointes ventricular arrhythmia in patients treated with drugs that prolong the QT interval. Electrolyte disturbances including hypokalemia and hypomagnesemia have been reported with laxative abuse and are known risk factors for torsade de pointes associated with QT interval prolongation.

MANAGEMENT: Patients treated with drugs that prolong the QT interval should exercise caution when self-medicating with laxatives. The recommended dosage and duration of use should not be exceeded. Patients treated with lactulose for more than six months should be monitored periodically for electrolyte imbalance. Patients should be advised to seek prompt medical attention if they experience symptoms that could indicate the occurrence of torsade de pointes such as dizziness, lightheadedness, fainting, palpitation, irregular heart rhythm, shortness of breath, or syncope.

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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.

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