Can my employer ask me to validate my prescription of Adderal if I made a med error regarding oxycodone, but only tested positive for my adderall?

I errantly took a pill of oxycodone home with me, but brought it back the same evening to return it. I was drug tested as a result of this error, but only tested positive for my prescription of Adderall. However, my employer had already received a validation of this medication a year and a half prior to this incident. They have informed me that due to the lack of testing available in house, they have to place me on administrative leave until #1: I can validate an active prescription and #2: An independent lab can confirm that my serum levels match my prescription. Can they do this every time I make an error that's unrelated to my prescription. I have been there 3-1/2 years and only made a mistake 2 times.