Most departments require polygraph tests & ask about drug use. If PHs are schedule III, thats a drug. You will be in possesion, regardless of when/how you got it. They also do thorough background checks (depending on department). Not sure if they would check your credit card history & tie that to PH or ancillaries purchases.
I have a few cop friends who had to take the polygraph at the academy. They had used illegal drugs in the past, but denied it in the test. They never saw the results, but go into the PD without a problem. Go figure...
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I have a Top Secret/SCI security clearance, and so does everyone I've worked with over the past four and a half years. There isn't going to be anyone checking credit card history. That's ridiculous. If you're asked if you've done illegal drugs in the past, and you have done some PHs that you purchased at GNC or over the internet, then you answer no. Case closed.
And there certainly isn't going to be anyone over searching your apartment/house/car for bottles of supplements. That will never happen. In fact drug tests you take for security clearances here (and in the military) don't even register steroids, much less pro-hormones/supplements.