The fastest-acting laxative varies depending on the individual, but one fast-acting laxative is the drug bisacodyl when administered as a rectal suppository, according to MedHelp. Dulcolax is an over-the-counter, brand-name example of the drug. The drug is prescribed for constipation.
Bisacodyl, a stimulant laxative, works by increasing intestinal fluids and salts, reports WebMD. The suppository form brings on a bowel movement within 15 to 60 minutes. Although the rectal suppository is the fastest method of bringing relief, some sufferers may find that an oral form of the medication is a more pleasant, if slower, form of taking the medicine. Laxatives are used to address symptoms such as constipation and hard, infrequent or incomplete evacuation of stools.