Poison Ivy, Oak, and Sumac Dermatitis: What Is Known and What Is New? Poison ivy, poison oak, and poison sumac are the most common causes of clinically diagnosed allergic contact dermatitis in North America. Approximately 50% to 75% of the US adult population is clinically sensitive to poison ivy, oak, and sumac. We reviewed the botany and history of these plants; urushiol chemistry and pathophysiology, clinical features, and the prevalence of allergic contact dermatitis caused by these plants; and current postexposure treatment and preventive methods, including ongoing investigations in the development of a vaccine (immunotherapy). Although extensive efforts have been made to develop therapies that prevent and treat contact dermatitis to these plants, there lacks an entirely effective method, besides complete avoidance. There is a need for a better therapy to definitively prevent allergic contact dermatitis to these plants.