More than 500 volunteers were patch tested with dilutions of purified poison oak urushiol in acetone. Of this group, 69 subjects participated in three experiments that conclusively showed that large amounts of urushiol taken over a period of three or more months produced measurable hyposensitization, as detected by repeated patch testing. No systemic toxicity was observed, but pruritus ani was common, and occasional skin eruptions were seen when too much drug was given. These cutaneous symptoms and signs disappeared once the subject had become hyposensitized.
Epstein WL, Baer H, Dawson CR, Khurana RG. Poison Oak HyposensitizationEvaluation of Purified Urushiol. Arch Dermatol. 1974;109(3):356-360. doi:10.1001/archderm.1974.01630030016002