WASHINGTON — A St. Louis pharmaceutical company has agreed to pay $313 million in civil and criminal penalties and plead guilty to felony charges of obstructing a Food and Drug Administration investigation and other misconduct.

The company, Forest Pharmaceuticals Inc., also agreed to plead guilty to distributing a misbranded drug in connection with its off-label promotion of the antidepressant Celaxa. The drug was touted for use by children and adolescents when it had been approved for use only by adults.

In addition, the company will plead guilty to distributing an unapproved new drug, a count stemming from its promotion of a medication called Levothroid for hypothyroidism, according to the settlement announced Wednesday by the FDA and the Justice Department.

Levothyroid has now been approved by the FDA and currently is sold by Forest Pharmaceuticals, but the criminal charge stems from the company’s marketing of the drug between 2001 and 2003, before it was approved by the agency. The marketing counts are misdemeanors.

The civil penalty also involved off-label promotion of the antidepressant Lexapro.

The obstruction charge resulted from false statements made by Forest Pharmaceuticals employees during a 2003 FDA inspection.

In a statement, Forest said it had set aside money in anticipation of the settlement and anticipated no additional charges to company earnings.

“”We are pleased to bring closure to this long-running investigation,”” said Howard Solomon, chairman and CEO of Forest Laboratories, Forest Pharmaceutical’s New York-based parent.

The agreement is the latest in a series of hefty settlements by pharmaceutical companies involving off-label marketing of drugs growing out of an FDA crackdown..

In the last two years, Eli Lilly, Pfizer and Astra-Zeneca have settled cases for more than $500 million each.

Earlier this month, Irvine, Calif.-based Allergan Inc. agreed to pay $600 million to settle civil and criminal charges involving off-label promotion of the wrinkle treatment Botox.

Once a drug has been approved for a specific use, doctors are free to prescribe it for other purposes. But drug manufacturers are not permitted to promote it for unapproved uses.

Like several earlier settlements, the Forest Pharmaceuticals case involved whistleblowers. Three whistleblowers will share $14.6 million from a civil settlement totaling $149 million. The remainder of the financial penalty applies to the criminal case.

Forest Pharmaceuticals also agreed to a five-year corporate integrity agreement in which its marketing practices will be monitored by federal authorities.

The guilty pleas and settlement are subject to approval by the U.S. District Court in Massachusetts.