There are no studies to support the use of castor oil for preventing or reversing gray hair. Indeed, there is no way to reverse gray hair: “Once the melanocyte stem cells in the bulge region of the follicle are depleted it is irreversible,” Zenovia Gabriel, MD, told us. Howard Sobel, MD, says that while there is no way to prevent going gray, managing your stress, avoiding smoking, and making sure you're getting vitamins D and B12 can help keep hair from graying prematurely.