It says on their web site that it removes the oil that causes the rash allowing the skin to heal. So, the way I understand it is that all along you were still harboring the oils somewhere on your person and the ivy off removed it. It also says not to use it 3 days after using hydrocortosne preparations...(shot and prescription that you mentioned?)
Ivy off is recommended to use just after exposure or use it later to unlock the oils.
Day 19? To tell you the truth, I don't think the ivy off helped you.
By the way, ivy off contains mineral spirits, an item that has been a topic on this site in the past.The ivy off pre- contact towelettes contain alcohol or witch hazel, another item discussed in treating the oils- not the rash.
You had to either be re-exposing yourself to the oils for the ivy off to work or your rash finally subsided.
I'm just saying...
