According to Mayo Clinic, rectal stimulants like Dulcolax and Pedia-Lax are some of the fastest-acting laxatives. These types of laxatives work by triggering rhythmic contractions in the intestinal muscles. As these contractions start to happen, the muscles eliminate stool from the body.
Mayo Clinic warns, however, that prolonged use of any kind of laxative is not recommended. Rectal stimulants are known to cause cramping, stomach discomfort and rectal irritation. WebMD suggests that fiber is the best and most natural laxative. Although it does not act quickly, adding fiber to a diet helps avoid constipation in the future. Fiber occurs naturally in fruits and vegetables, and it is available in supplements like Fiber-Lax and Benefiber.