Something I have been wondering for a while. Say that you inject some phsyco that give you a bonus restance of +25% and then on top off that also some med-X that also gives +25% in damage resistance. Will that result in a total of +50% damage resistance or will it still be 25% thus wasting the med-X?

They stack with each other, tho its +25 defense not +25%. See: Psycho gives +25 defense and +25% damage bonus (yea, damage bonus is in percent). Most drugs stack with each other. See: psychobuff and bufftats together give you a whooping +6 bonus to STR.