People who used a shampoo with 5% tea tree oil reduced their dandruff 41% and significantly improved the itchiness and greasiness of their scalp, according to a study from the Journal of the American Academy of Dermatology. "You can safely add a small amount of tea tree oil to your own shampoo to get a similar effect," says Sobel. "Just use sparingly, no more than a few teaspoons per bottle, to prevent overdrying of your scalp.