The Collaborative International Dictionary of English v.0.48:

Dormitive \Dor"mi*tive\, a. [Cf. F. dormitif, fr. dormire to
   sleep.]
   Causing sleep; as, the dormitive properties of opium.
   --Clarke. -- n. (Med.) A medicine to promote sleep; a
   soporific; an opiate.
   [1913 Webster]