Stimulant laxatives, including medications such as Dulcolax and Ex-Lax, provide nearly instant relief from constipation, according to Medical News Today. Other categories of fast, effective constipation medicines include hyperosmotic laxatives, which typically work in 30 minutes to three hours, and saline laxatives, which may take 30 minutes to six hours.
Colace is a common brand name for a type of laxative called a stool softener, which typically works within 12 to 72 hours, according to Medical News Today. Fiber supplements and fiber consumed in food, rather than medicine or supplements, may also help relieve constipation in the same time frame. Ingestion of mineral oil may produce effective constipation relief within six to eight hours by lubricating stools for easier passage through the intestine.
One hyperosmotic laxative, also called an osmotic laxative, is polyethylene glycol, available under the brand name MiraLAX, according to WebMD. Saline laxatives include milk of magnesia and Epsom salt, while hypersmotic laxatives include glycerin suppositories, according to Medical News Today.
Stimulant laxatives act on the lining of the intestine to help it pass the stool along more quickly, and they may also increase the stool's water content, making it easier to pass, says WebMD. The body can become dependent on stimulant laxatives to maintain regular defecation, so WebMD does not recommend frequent use of this type of medicine, which it also warns may cause diarrhea or cramping.