The Stamford, Connecticut-based company and three executives admitted that they falsely claimed OxyContin was less addictive, less subject to abuse, and less likely to cause withdrawal symptoms than rival pain medications.

The U.S. Food and Drug Administration had not approved those claims.

"Purdue (Frederick) put its desire to sell OxyContin above the interests of the public," Assistant Attorney General Peter Keisler said in a statement. "Purdue abused the drug approval process which relies on drug manufacturers to be forthright in reporting clinical data and, instead, misled physicians about the addiction and withdrawal issues involved with OxyContin."

OxyContin, prescribed for patients with moderate to severe pain, is now regulated as a controlled substance with the same addictive potential as morphine.

Of the $634.5 million settlement, $276 million will be forfeited to the United States, $160 million allocated to federal and state government agencies to resolve false claims for government healthcare programs and $130 million will go to resolving private civil claims.

Additional amounts will be paid to the Virginia Attorney General's Medicaid Fraud Control Unit and the Virginia Prescription Monitoring program.

The guilty pleas follow a $19.5 million settlement the related manufacturer of OxyContin, Purdue Pharma LP, made with 26 states and the District of Columbia this week over allegations it failed to adequately disclose abuse risks posed by the powerful narcotic.

Purdue Pharma had also settled a civil case brought by its insurer in June for $200 million.