hosehead,

If you fail your polygraph interrogation with the agency that has offered you employment, that information may very well be shared with your current employer and have adverse consequences for you.

If you lied about your drug use in a previous background investigation and were to admit it now, it is conceivable that this information could be used against you. Everything you say to your polygrapher becomes, in essence, a matter of public record. Even if agencies are more tolerant of past drug use nowadays, a de facto confession to having lied in the course of a prior background investigation may not go over well.

Of course, the ethical choice here is yours. But before you make it, you should be fully aware that your polygrapher will be lying to and attempting to deceive you during the course of your pre-employment polygraph interrogation, as you'll discover in Chapter 3 of The Lie Behind the Lie Detector.