Asking Employees to Disclose a Prescription

Hello, 
Can you ask an employee to disclose that they are using a prescription drug that could impair their ability to do their job or their safety? I know that you cannot ask for specifics as to the diagnosis or details, but with legalization of marijuana on the horizon, if an employee has a prescription for medical marijuana, asking employees to disclose that they have this prescription gives us the ability to accommodate them, if necessary.
Thanks