LAXATIVE

any drug that stimulates the bowel's smooth muscles or affects the stool's consistency to promote the evacuation of feces. Laxative, cathartic, purgative, and drastic denote increasing intensity of effect in promoting defecation, a gradation sometimes achieved simply by increasing the dosage of a single drug. Laxatives produce their effect by several mechanisms. Irritant, or stimulant, laxatives act directly on the muscles of the intestine, stimulating the wavelike muscular contractions (peristalsis) that result in defecation. The stimulants include cascara sagrada, senna, castor oil, bisacodyl, and phenolphthalein. Bulk-forming laxatives promote evacuation by increasing the stool's water content and bulk volume. The bulk laxatives include the hydrophilic (water-attracting) colloids, which consist of such indigestible fibrous matter as dietary bran, agar, psyllium seeds, methylcellulose, and polycarbophil. The action of saline laxatives is not completely understood, but it is certain that they draw water into the intestine from adjacent tissues. The increased water content and bulk of the stool are believed to stimulate evacuation. Saline laxatives include magnesium sulfate (Epsom salts), magnesium hydroxide (milk of magnesia), magnesium citrate, and such phosphate salts as disodium phosphate. Glycerin is another substance that attracts water into the stool. Lubricant laxatives make the bowel walls and stool moist and slippery, thus facilitating defecation. The principal lubricant laxatives are mineral oil and certain vegetable oils. Stool-softening laxatives such as dioctyl sulfosuccinate help liquids mix into the stool and thus prevent dry, hard stools from forming. Laxatives are secondary in importance to a high fibre diet in correcting simple intestinal constipation. Other valid uses include prevention of strain when anorectal disorders or diseases of arterial vessels exist. They also alleviate constipation caused by various illnesses or drug treatment. A secondary use is the elimination of bowel contents prior to surgery.