PUTREFACTIVE is a 12 letter word that starts with P.
\Pu`tre*fac"tive\, a. [Cf. putr['e]factif. See {Putrefy}.] 1. Of or pertaining to putrefaction; as, the putrefactive smell or process. --Wiseman. 2. Causing, or tending to promote, putrefaction. -- {Pu``tre*fac"tive*ness}, n.
WordNet Dictionary