If you are an independent contractor, then the relationship between you and the company or person hiring you is governed by contract: if the contract or service agreement, etc. you agreed to allows for a drug test without cause, they can do this; if it does not, then they can't. (Note: contracts can be oral or verbal, or can be found from emails and correspondence describing the working relationship; it does not have to be a single "obvious" contract.) Of course, when a given project or working assignment, etc. is up, the client hiring you is free to make it a term of any new contract that you submit to drug tests; and if you do not want to, your recourse (assuming you can't get them to reconsider) is to not take the contract.
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