tachyphylaxis /tak′ēfəlak″sis/ [Gk, tachys + phylax, guard] , 1. (in pharmacology) a phenomenon in which the repeated administration of some drugs results in a rapidly appearing and marked decrease in effectiveness. 2. (in immunology) a rapidly developing immunity to a toxin because of previous exposure, such as from previous injection of small amounts of the toxin. Also called mithridatism.
