If they ask you to do a drug test, they will supply you with the info. to get it done. If they do not feel a test is needed (likely if they found all allegations untrue), then you are clear. Follow up with the department to find out who your caseworker is, and then follow up with him/her to make sure there is nothing further they need from you and the "complaint" has been deemed unsubstantiated/closed.
Responses are for general information purposes only, and are based on the extremely limited facts given. A consultation with an attorney experienced in the area of law(s) indicated in the question is highly recommended. Information and advice given here should not be relied upon for any final action or decision, as the information is limited by its nature to the question asked and the fact(s) presented in that question. THIS RESPONSE DOES NOT CREATE AN ATTORNEY/CLIENT RELATIONSHIP, particularly considering that the names of the parties are unknown.