The Collaborative International Dictionary of English v.0.48:

Precautionary \Pre*cau"tion*a*ry\, a.
   Of or pertaining to precaution, or precautions; as,
   precautionary signals.
   [1913 Webster]

WordNet (r) 3.0 (2006):

precautionary
    adj 1: taken in advance to protect against possible danger or
           failure; "gave precautionary advice"; "I would take
           precautionary steps to keep him away" [syn:
           precautionary, precautional]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "precautionary":
   foreboding, forehanded, forerunning, foreseeing, foresighted,
   forethoughted, forethoughtful, forewarning, portentous,
   precautional, precautious, precursive, precursory, premonitory,
   prodromal, prodromic, provident, provisional