Selsun Blue is an over-the-counter dandruff shampoo containing selenium sulfide as the active ingredient. Selenium sulfide is an antifungal agent that is effective in treating dandruff, seborrheic dermatitis, tinea versicolor, and tinea capitis. Its mechanism of action involves inhibiting fungal growth and slowing the proliferation of skin cells. Selsun Blue should be applied to the scalp or affected areas of the skin and left on for several minutes before rinsing for proper treatment of various fungal infections. While generally safe as directed, in large amounts selenium sulfide can potentially cause toxicity in humans.