Laxatives are primarily used to alleviate constipation, a common gastrointestinal disorder characterized by infrequent bowel movements and difficulty passing stools. They work by various mechanisms to increase the volume or frequency of bowel movements. The primary modes of action of laxatives include increasing stool bulk, softening the stool, stimulating intestinal motility, and osmotically drawing water into the intestines.

Osmotic or saline laxatives, like magnesium hydroxide or milk of magnesia and polyethylene glycol (PEG), draw water into the intestines from surrounding tissues. This increased water content softens the stool, expands the bowel, and promotes bowel movement. They are particularly effective for short-term relief of acute constipation and bowel preparation for medical procedures like gastric surgeries or endoscopies. Lubricant laxatives coat the stool, making it easier to pass. Mineral oil is an example of this type. Stool softeners, also known as emollient laxatives, include agents like docusate sodium (Colace) and glycerine suppositories. They are administered orally or through the rectum. They may take a few days to show their effectiveness. They allow water and lipids to penetrate the stool, making it softer and easier to pass.