Ah, castor oil. Some women do take castor oil to help get begin labor, but if it was that easy to induce labor every woman would be doing it, wouldn't she? Castor oil acts as a laxative. If your body is ready to go into labor, or is already in early labor, some people believe that castor oil can tip you over the edge and get the labor going stronger, although none of this is proved by science. This is what it comes down to: Some people say Mexican food will put you into labor, some people say castor oil, and some people say going for a car ride on a bumpy road (among a million other myths). Although we know a lot about the science of childbirth, no one knows why labor begins. And no one knows how to start labor in a woman whose body is not ready to begin labor.
You asked about harmful effects of castor oil. Because castor oil induces intestinal cramps, you could be uncomfortable and could have diarrhea, but in healthy women there should be no real harmful effects. So my advice is this: If you want to take castor oil to induce labor, ask your doctor or midwife about this. If you have the okay from your health care provider, then decide whether to try it. Or, you could wait until your body is ready to go into labor on its own timetable. That would be my choice.