What to do if an employer that I took a pre-employment drug screen for is sharing the results with their customers?

It really depends on the nature of the drug screen and how they are disclosing the information.  Generic, pre-employment drug screens are not automatically covered by HIPPA (which is basically a federal privacy act for medical information), however, some drugs screens or discussions regarding drugs screens can be considered medical information subject to HIPPA or the ADA.  So if they are telling customers details (like the condition you had the required medication), then you could potentially sue the employer for HIPPA or ADA violations.  This can be a very fact specific ruling-- which is why most smart employers will maintain the confidentiality of drug test results-- and just as a matter of policy-- will not share results.  If your screen was more than a routine screen or if the employer is giving out a variety of details as to when and why you tested positive, then you may have a privacy or discrimination suit under one of the laws mentioned above.  You would need to consult with an employment law attorney to see how the facts of your case fit in with the requiremtns of HIPPA.