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Adjective: soporiferous  ,só-pu'ri-fu-rus
  1. Sleep inducing
    "The soporiferous effects of the medication made driving dangerous";
    - soporific, somniferous, somnific
     
  2. Inducing mental lethargy
    "a soporiferous speech";
    - narcotic, soporific

See also: depressant, lame, lameo [informal], meh [informal], uninspiring, uninteresting