Before the development of medical expense insurance, patients were expected to pay health care costs out of their own pockets, under what is known as the fee-for-service business model. During the middle-to-late 20th century, traditional disability insurance evolved into modern health insurance programs. One major obstacle to this development was that early forms of comprehensive health insurance were enjoined by courts for violating the traditional ban on corporate practice of the professions by for-profit corporations. State legislatures had to intervene and expressly legalize health insurance as an exception to that traditional rule. Today, most comprehensive private health insurance programs cover the cost of routine, preventive, and emergency health care procedures. They also cover or partially cover the cost of certain prescription and over-the-counter drugs. Insurance companies determine what drugs are covered based on price, availability, and therapeutic equivalents. The list of drugs that an insurance program agrees to cover is called a formulary. Additionally, some prescriptions drugs may require a prior authorization before an insurance program agrees to cover its cost.