I took 3 20mg tablets of prednisone ended up in the er about 2 hrs after(im allergic) gave 2 shots benadryl (diphenhydramine) & ativan how long til pred.out of system?

Unlikely allergic: I presume your reactions were from the prednisone which may cause hyperactivity, depression, anxiety, insomnia, palpitation, flushing occasionally in a few people when given at this dose. I would expect these side effects to decline after about a day and subside altogether in 2-3 days. The problem may also be from the disease for which the prednisone was prescribed. Not allergic