58 Pa. Code § 183.357
A trainer shall be responsible for the condition of the horses trained by him. No trainer shall start a horse or permit a horse in his custody to be started if he knows or if by the exercise of reasonable care he might have known or have cause to believe, that the horse has received a drug, stimulant, sedative, depressant, medicine or other substance that could result in a positive test. Every trainer shall guard or cause to be guarded each horse trained by him in a manner and for a period of time prior to racing the horse so as to prevent a person from administering a drug, stimulant, sedative, depressant or other substance resulting in a positive test.
58 Pa. Code § 183.357
This section cited in 58 Pa. Code § 183.361 (relating to winnings when rule is violated).