cathartic cathartic in medicine, a cathartic is a substance that accelerates defecation. this is similar to a laxative, which is a substance that eases defecation, usually by softening feces. it is possible for a substance to be both a laxative and a cathartic. however, agents such as psyllium seed husks increase the bulk of the feces. cathartics such as sorbitol, magnesium citrate, magnesium sulfate, or sodium sulfate were previously used as a form of gastrointestinal decontamination following poisoning via ingestion. they are no longer routinely recommended for poisonings. high-dose cathartics may be an effective means of ridding the lower gastrointestinal tract of toxins; however, they carry a risk of electrolyte imbalances and dehydration. catharsis can be an effect of pesticide poisonings, such as with elemental sulfur.