It can be a lucrative business. Getting the doctor to write you a prescription for pain killers doesn't usually cost much, but a diagnosis of 'work-related stress' would go for more since, by German law, your employer is then obligated to send you to a resort and pay for daily massages for up to six weeks to help you 'recover'. Black markets crop up in ALL socialist systems.
I can't imagine how Germany exists with foolishness such as this. How absolutely unproductive. Let's face it people have a tendency to be slackers when there is no incentive to be productive. This type of systems begs to be abused. No wonder Germany has a sluggish economy. I would hate living in a prison mediocrity such as the one the Germans have created for themselves.