But the inspector general's report concluded that the $53-billion program left the door open to fraud from the beginning.
Before the new analysis, there were apparently no comprehensive data about pharmacies' typical billing patterns, or types of questionable billings.
"The program has limited safeguards in place and is vulnerable to fraud, waste and abuse," the report said.
For example, the private insurers who serve as program middlemen are encouraged to report fraud, but they are not required to do so.
"Because (insurers) are on the front lines of detecting fraud, waste and abuse ... a significant vulnerability exists when (they) are not required to report this information," the report found.
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