What do i do if i have vitamin b12 overdose?

i accidentally overdosed on b12 ( apparently) i take a daily vitamin that has 18 mcg of b12 in it already but also take the b12 vitamin alone which has 500 mcg in it. apparently im allergic or something because i have little bumps all over my back and arms and chest. so im not sure if i should just take benadryl which already did... or go to the hospital.