You don't have to tolerate cranky employees

Q If an employee is on some kind of medicine for his back and we notice he has mood swings and can be hostile toward his manager, what rights do we have as the employer? May we require him to bring in a doctor's note? This could be a long-term issue with him.

A Obviously, hostility toward a manager is a disciplinary issue, so begin there. From your question, I can't tell whether the employee has a disability. If he raises the medication issue, advise him that continued hostility toward his manager won't be tolerated. Tell him that if his doctor can suggest an accommodation for his condition, you will consider it. Of course, the Americans with Disabilities Act (ADA) may come into play, so proceed carefully.