The Collaborative International Dictionary of English v.0.48:

Posology \Po*sol"o*gy\, n. [Gr. ? how much + -logy: cf. F.
   posologie.] (Med.)
   The science or doctrine of doses; dosology.
   [1913 Webster]

WordNet (r) 3.0 (2006):

posology
    n 1: the pharmacological determination of appropriate doses of
         drugs and medicines