Section II. CATHARTIC AGENTS
Not all cathartics have the same mechanism of action. In fact, there are several
categories of cathartics, each has a particular mechanism of action. These categories
are bulk-forming cathartics, lubricant cathartics, stimulant cathartics, emollient cathartics
(also known as stool softeners), and hyperosmotic cathartics.
IMPORTANT INFORMATION FOR PERSONS TAKING CATHARTICS
Persons who take cathartics should be told of the importance of drinking extra
fluids. In fact, a person who is taking a laxative should drink at least six to eight full
glasses of fluid (each glass should be equal to 8 fluid ounces, 240 milliliters). This extra
fluid helps the cathartic to produce its effects faster. Certain cathartics (e.g., those in
the bulk-forming category) require fluid in addition to the six to eight glasses of fluid they
should be drinking. This additional fluid should be taken when ingesting the cathartic.
MECHANISMS OF ACTION OF CATHARTICS
Each category of cathartics has its own particular mechanism of action. The
mechanisms of action are important because the physician may select a particular
agent because of the specific favorable results obtained as a direct effect of a
mechanism of action.
a. Bulk-Forming Cathartics. These cathartics absorb water and provide bulk
for the gastrointestinal tract. The increased bulk provides stimulation to the bowels
b. Lubricant Cathartics. Lubricant cathartics increase the fluid level in the
small intestines. They do this by coating the surfaces of the stool and the intestines.
This coating results in decreased absorption of water and increase in the volume of
water in the intestines. This effect also eases the flow of stool through the intestines by
c. Stimulant Cathartics. Stimulant cathartics increase the rate of peristalsis in
the intestine by directly acting on the smooth muscle of the intestine.
d. Emollient Cathartics. Emollient cathartics reduce the surface film tension of
the stool. This allows for fluids to penetrate the stool and thus to make the stool softer.
e. Hyperosmotic Cathartics. Hyperosmotic cathartics are concentrated
solutions of substances which draw water into the intestine. Increased water content of
the stool further stimulates peristalsis.