The Collaborative International Dictionary of English v.0.48:

Soporific \Sop`o*rif"ic\ (?; 277), a. [L. sopor a heavy sleep
   (akin to somnus sleep) + facere to make. See Somnolent,
   Fact.]
   Causing sleep; tending to cause sleep; soporiferous; as, the
   soporific virtues of opium.
   [1913 Webster]

   Syn: Somniferous; narcotic; opiate; anodyne.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Soporific \Sop`o*rif"ic\, n.
   A medicine, drug, plant, or other agent that has the quality
   of inducing sleep; a narcotic.
   [1913 Webster] Soporose

WordNet (r) 3.0 (2006):

soporific
    adj 1: sleep inducing [syn: soporific, soporiferous,
           somniferous, somnific, hypnogogic, hypnagogic]
    2: inducing mental lethargy; "a narcotic speech" [syn:
       narcotic, soporiferous, soporific]
    n 1: a drug that induces sleep [syn: soporific, hypnotic]