How long will it take for cortisone to help my posion ivy?

As early as 6 hours: In severe cases of poison ivy/oak/summac, topical steroids may not work too well because of the blistering/fluid which reduces the absorption, so you may be given oral form. Oral Prednisone is very well absorb and can start taking effect as early as 6 hours. Calamide may help dry it out a bit quicker. The best treatment is prevention--learn your poisons and avoid them. Consult doc. Good luck.