“We accept responsibility for those past misstatements and regret that they were made,” the statement said.

The time period covered by the guilty pleas runs from late 1995, when the Food and Drug Administration approved OxyContin for sale, to mid-2001, when Purdue Pharma, faced with both public criticism and regulatory scrutiny, dropped its initial marketing claims for the drug.

Federal officials said that internal Purdue Pharma documents show that company officials recognized even before the drug was marketed that they would face stiff resistance from doctors who were concerned about the potential of a high-powered narcotic like OxyContin to be abused by patients or cause addiction.

As a result, company officials developed a fraudulent marketing campaign designed to promote OxyContin as a time-released drug that was less prone to such problems. The crucial ingredient in OxyContin is oxycodone, a narcotic that has been used for many years. But unlike other medications like Percocet that contain oxycodone along with other ingredients, OxyContin is pure oxycodone, with a large amount in each tablet because of the time-release design.

The drug has proven to be valuable in treating serious, long-lasting pain.

Purdue Pharma acknowledged in the court proceeding today that “with the intent to defraud or mislead,” it marketed and promoted OxyContin as a drug that was less addictive, less subject to abuse and less likely to cause other narcotic side effects than other pain medications.

For instance, when the painkiller was first approved, F.D.A. officials allowed Purdue Pharma to state that the time-release of a narcotic like OxyContin “is believed to reduce” its potential to be abused.

But according to federal officials, Purdue sales representatives falsely told doctors that the statement, rather than simply being a theory, meant that OxyContin had a lower potential for addiction or abuse than drugs like Percocet. Among other things, company sales officials were allowed to draw their own fake scientific charts, which they then distributed to doctors, to support that misleading abuse-related claim, federal officials said.