tach·y·phy·lax·is 
(tăk′ə-fĭ-lăksĭs)
(tăk′ə-fĭ-lăksĭs)n. pl. tach·y·phy·lax·es (-fĭ-lăksēz)
1. Rapid desensitization to a toxic substance produced by inoculation with a series of small doses.
2. A rapidly decreasing response to a drug following administration of the initial doses.