Castor oil increases the luster of hair, according to a research study published in the Journal of Cosmetic Science in 2003. As of 2014, it has not been shown to stimulate hair growth.
Castor oil should only be used externally by applying it to hair as a hot oil treatment. The treatment is made by heating the oil in a double boiler and adding essential oils or herbs to improve the scent. Any herb pieces are strained out, and the treatment is applied to the hair by brush starting at the bottom. The hair is wrapped in plastic and a warm towel for 30 minutes and then shampooed twice to remove the oil.