14 C.F.R. § 135.41
Carriage of narcotic drugs, marihuana, and depressant or stimulant drugs or substances
If the holder of a certificate operating under this part allows any aircraft owned or leased by that holder to be engaged in any operation that the certificate holder knows to be in violation of § 91.19(a) of this chapter, that operation is a basis for suspending or revoking the certificate.