The area I grew up in had lots of poison oak...in the late 1960’s my Mom gave us some stuff that came in a small one-serving “ampoule” that you opened and mixed with water and then drank. I cannot find any reference to any such product on the WWW...it looked like poison oak “juice”/oil.
Being kids and running around playing on the whole property we got poison oak a lot and she was looking for a “preventative”...Calamine lotion got expensive after awhile.
Fels Naptha Soap ain't the same stuff today. They removed the Napthalene from the formula to comply with California loons. So it looks and smells about the same, but it ain't worth squat.