Contact with poison ivy (Toxicodendron radicans) is one of the most widely reported ailments at poison centers in the United States and this plant has been introduced throughout the world where it occurs with other allergenic members of the cashew family (Anacardiaceae). Approximately 80% of humans develop dermatitis upon exposure to the carbon-based active compound urushiol. It is not known how poison ivy might respond to increasing concentrations of atmospheric carbon dioxide (CO2) but previous work done in controlled growth chambers shows that other vines exhibit large growth enhancement from elevated CO2. Rising CO2 is potentially responsible for the increased vine abundance that is inhibiting forest regeneration and increasing tree mortality around the world. In this 6-year study at the Duke University Free-Air CO2 Enrichment experiment we show that elevated atmospheric CO2 in an intact forest ecosystem increases photosynthesis water use efficiency growth and population biomass of poison ivy. The CO2 growth stimulation exceeds that of most other woody species. Furthermore high-CO2 plants produce a more allergenic form of urushiol. Our results indicate that Toxicodendron taxa will become more abundant and more \"toxic\" in the future potentially affecting global forest dynamics and human health.