Workman comp aproved my surgery can they denie me after finding drugs in my blood stream after doing my pre su?

i got hurt in oct 11 2010 its now april 14 2011. my doctor says i have to get surgery on my shoulder. workmans comp has approved my surgery and everything is up to date. i have to now get a pre-surgical physical. can workmans comp denie me if i have drugs in my blood system even though the injury happen 7months ago?