14 CFR §120.33

1. (a)This section applies to individuals who perform a function listed in subpart E of this part for a certificate holder or operator. For the purpose of this section, an individual who performs such a function pursuant to a contract with the certificate holder or the operator is considered to be performing that function for the certificate holder or the operator.
2. (b)No certificate holder or operator may knowingly use any individual to perform, nor may any individual perform for a certificate holder or an operator, either directly or by contract, any function listed in subpart E of this part while that individual has a prohibited drug, as defined in this part, in his or her system.
3. (c)No certificate holder or operator shall knowingly use any individual to perform, nor shall any individual perform for a certificate holder or operator, either directly or by contract, any safety-sensitive function if that individual has a verified positive drug test result on, or has refused to submit to, a drug test required by subpart E of this part and the individual has not met the requirements of that subpart for returning to the performance of safety-sensitive duties.