For most people, castor oil can be taken internally or used externally without bothersome side effects. However, overdoing internal use of the oil can lead to diarrhea, nausea, and stomach pain. There is one strong exception, though: if you are pregnant, you should not use castor oil without the approval of your doctor because the oil can stimulate labor. In fact, this was one of the primary uses of castor oil in centuries past.