You cannot ask any medically-related questions on a job application or during a pre-job offer interview. You can only inquire about medical, worker’s comp, and drug testing issues AFTER a job offer is made; this is normally a conditional job offer.
Once the employee is on board, medical and drug questions and tests can be addressed. Be sure to keep that information in a separate file – in a separate location – apart from the general personnel file. Check with your legal counsel to ensure compliance on this and other issues.