A 60 milliliter dose of castor oil appears to induce labor within 24 hours of ingestion in more than half the women who use it when their pregnancy is at full term, WebMD reports. Using castor oil to induce labor works in some cases but not in others, states About.com.
JustMommies notes that castor oil is a laxative, so most women who ingest castor oil to induce labor experience diarrhea. Doctors believe that as the castor oil stimulates the bowels, it leads to cramping and tightening of the intestinal muscles that spreads to the uterus.
Castor oil has an unpleasant taste and often causes nausea, vomiting and abdominal cramping, warns JustMommies. The safety and efficacy of using castor oil to induce labor is a matter of debate, and the site advises pregnant women to consult with their health care provider before using castor oil.