Xenobiotics that promote intestinal evacuation are referred to as laxatives, cathartics, purgatives, promotility agents, and evacuants. Using different doses, the same xenobiotic may often accomplish any or all of these tasks, but with different side effect profiles. Laxatives promote a soft-formed or semifluid stool within 6 hours to 3 days, depending on the xenobiotic and the dose used. Cathartics promote a rapid, watery evacuation within 1 to 3 hours.37 The term purgatives is used to relate the force associated with bowel evacuation. Promotility agents stimulate gastrointestinal (GI) motor function via the enteric nervous system by affecting acetylcholine, serotonin, motilin, or intestinal chloride channels. Evacuants are commonly used to cleanse the bowel before a procedure, with an onset of action of as little as 30 to 60 minutes, but typically requiring 4 hours for a more complete effect. Although it is far from perfect, the most effective process of evacuating the intestinal tract in poisoned patients is referred to as whole-bowel irrigation (WBI). WBI is typically accomplished using polyethylene glycol 3350 (PEG), which is added to a balanced electrolyte lavage solution (PEG-ELS).
The traditional classification of laxatives into the categories of bulk-forming, softener or emollient, lubricant, stimulant or irritant, saline, hyperosmotic, and evacuant is largely empirical. Bulk-forming agents include high-fiber products such as methylcellulose, polycarbophil, and psyllium; softeners or emollients include docusatecalcium. Mineral oil is the sole lubricant. None of these three classes of cathartics is used therapeutically in medical toxicology because their onsets of action are often delayed for several days. In addition, softeners cause an increase in intestinal permeability for a few hours and may therefore increase the absorption of some xenobiotics.78 Mineral oil may enhance the absorption of lipid-soluble xenobiotics and aspiration could result in a lipoid pneumonia.102
Stimulant or irritant laxatives include anthraquinones (sennosides, aloe, and casanthranol), diphenylmethane (bisacodyl), and castor oil. Abdominal discomfort, cramping, and tenesmus are common early manifestations. Long-term use produces bowel habituation and damage to intestinal tissue. Thus, stimulant and irritant laxatives are rarely used today in medical toxicology because of their significant GI side effects.
Saline (meaning salt) cathartics, which include magnesium citrate, magnesium hydroxide, magnesium sulfate, sodium phosphate, and sodium sulfate, are used infrequently and cautiously in medical toxicology. Hyperosmotic agents, including sorbitol and lactulose, are also occasionally considered in poisoned patients.
When different cathartics were compared with respect to time to first stool and number of stools,38,46,68,69,90 sorbitol produced 10 to 15 watery stools and the most abdominal cramping before catharsis. Sorbitol produced stools in the shortest amount of time but with the highest incidence of nausea, vomiting. generated gas, abdominal cramping, and increased flatus.42,43,71 The nauseating sweetness of sorbitol resulted in patient preference of magnesium citrate. In comparison, the first bowel movement typically occurs about 1 hour after the start of WBI with PEG-ELS.