While castor oil works as a laxative when taken internally, when used externally, the benefits are completely different. Castor oil promotes lymph flow, boosts white blood cell production and soothes away aches and pains. As an added bonus, castor oil also helps to prevent infection. To use castor oil on a bruise, apply castor oil to a piece of cotton, place the cotton onto the bruise and wrap it with gauze. Allow it to sit until the pain subsides and you will find there is very little discoloration. Castor oil will also prevent the skin from peeling as the bruise heals.