I believe they have the right to say that a person cannot work in certain positions when they are taking certain kinds of medications.
I wouldn't want to be an OR or ER nurse who's having trouble with side effects that cause a loss of concentration, for example.
The idea is to avoid potential problems r/t those unwanted side effects, not wait until something happens and then react. That's why pilots aren't allowed to fly after they've taken certain types of drugs......you should see the regulations they've got (
http://www.leftseat.com/medcat1.htm )
Would you care be a bit more specific?