Well, I have had to deal with an allegation of that here in Canada, where I worked for some years as an OH&S Compliance officer. We have specific legislation here to address impairment, and although it took a certain approach to effectively deal with the individial involved, the operator found themselves off the machine and eventually out the door due to thier unwillingness to address their substance abuse problem.

As Norfolk boy says Have you a drug and alcohol policy . The easiest solution is to send him home as unfit to work then let HR deal with it. Like in many cases if you are involved with HR a verbal warning and move on.