I read an article in one of the outdoor sport magazines about poison oak/poison ivy and it had a very interesting remark. Once you've gotten it bad, you can break out without actually coming in contact with any of the stuff.....This struck a note with me because I get poison oak bad and would swear up and down I had not come into contact with any but here's a rash!
Can't help with what medication to use since the good stuff doesn't work for you but here's what I do.
-Hyper alert for the plant so I can avoid touching it.
-It's tough but if you pet your dog or cat, don't touch any exposed areas prior to washing your hands with soap. It's an oil so they can easily being carrying it around on their coat. FYI, I even wash my hands after cleaning my horses feet just on the off chance his legs brushed up against a plant.
-When you come inside from working, trail riding, whatever you did that might bring you in contact with the nasty stuff. Strip, toss your clothes in the washing machine (so you don't have to touch them again) and immediately jump in the shower. This right here as seriously cut down on my getting poison oak.
I can't say this enough, what the plants exude is an oil and doesn't dissipate without using soap. That means you can put on a pair of gloves you hadn't used in a year or pick up a tool out of the shed and end up getting some on you. Heck, I read where a botanist pulled a branch out of storage where it had sat for ~100 years and ended up getting a rash.