Employee’s performance is slipping because of medication–what are our options?

Q: A long-term employee has been working part time for a year now due to double knee replacement surgery. The employee must use painkillers, but does not take them when at work. Our concern is that in recent months, she has not always displayed good judgment, like she did before. In addition, she sometimes shakes all over and is not able to concentrate well during a conversation. Due to the fact that we are a mental health agency, we are concerned that she may be addicted to the painkillers. We are curious as to what our options are other than the steps we have already taken.  – Lisa, Alaska