Q: How are employers asking this and not violating ADA?  
Q: How can an applicant handle this in a way that won't hinder their ability to be hired by the company?

Backstory:  
This has come up 3 times for me in the last several weeks and has me questioning what I thought I knew.  Since I am no longer actively hiring folks, perhaps the laws have changed or there are loops I was unaware of.  I have had 3 people share with me that at their drug screen they were asked to disclose their prescription list to the employer or to sign off on something releasing the list to their employers.  One was concerned because she is prescribed ADHD medication, one depression/anxiety medication, and the third isn't on anything but felt the question violated ADA (She is a therapist).  

My understanding has always been that if someone tests positive for a controlled substance, they are simply required to give the collection facility a valid prescription and that screen then becomes a negative test.  I understand that some positions (ie: operating heavy equipment) might have different requirements, but I have never worked with those types of positions nor were the 3 in question those types of positions.  

I found this and it is helpful but I would love some real world experiences of others to shed light on this matter for my clients and colleagues. 
http://www.workforce.com/2016/09/06/ada-prescription-meds-need-know/