N.Y. Comp. Codes R. & Regs. tit. 9 § 4236.4
A trainer shall be responsible at all times for the condition of all horses trained by him. No trainer shall start or permit a horse in his custody, care or control to be started if he knows, or he might have known or have cause to believe, that the horse has received any drug or other restricted substance that could result in a positive test. The trainer shall be held responsible for any positive test unless he can show by substantial evidence that neither he nor any employee nor agent was responsible for the administration of the drug or other restricted substance. Every trainer must guard each horse trained by him in such manner and for such period of time prior to racing the horse so as to prevent any person whether or not employed by or connected with the owner or trainer from administering any drug or other restricted substance to such horse contrary to this Part.
N.Y. Comp. Codes R. & Regs. Tit. 9 § 4236.4