You have a right to privacy. The drug testing co. cannot disclose to the employer what prescribed meds you are period. When the lab calls you then disclose to the doctor you're legal Rx and the phone # of you doctor who prescribes it. If it checks out the drug test is sent back to the employer as a negative.
Some labs will have forms that ask what meds you're on before you take the test.I write on it a not to have the lab doctor call me for that information when the test is done and I will give them the info. I live in a very small place where everyone knows everyone so that why I do it that way.
Many people are misinformed or don't understand that you do not have to disclose your private life to your employer. The drug labs have faced many law suits over disclosing legal med info to employers. It's either a negative or a positive which in that case it depends on which state you reside if the negative drug info can be shared with the employer.
It's a real crap shoot when you decide to disclose meds to your employer. Some are understanding and don't have a concern with it and others will regard you as damaged goods and a danger to the patients (which of course you're not).
Let your doctor know you are taking a drug screen test so he will know that they will be calling. The only responsibility the drug company has to the employer is to let them know the results of the screening. If you don't want the employer to know you don't have to tell them because the drug testing co certainly can't (not legally anyway).
I don't know what state you live in but here the hospitals have to send the employees out to a drug testing facility not affiliated with them for employee testing. Also, if you're so worried and you say you don't take the med on a regular basis why not just stop taking a week prior to testing? I believe that half life is 12 hours and so it would take a longer time then 3 or 4 days to get out of the system, of course depending on the dose as well. Anyway...