GlaxoSmithKline is to pay $3bn in fines, in what is the largest healthcare fraud settlement in US history.
It follows charges that it marketed drugs for unauthorised uses, held back safety data, and cheated the government's Medicaid program.
GSK was also accused of paying kickbacks to doctors, paying for expensive trips and other benefits, in order to gain their support for the drugs the company was pushing.
The pharmaceuticals giant admitted to charges that it had promoted anti-depressants Paxil and Wellbutrin for uses not approved for by US regulators, including treatment of children and adolescents.
The company also conceded charges that it held back data and made unsupported safety claims over its diabetes drug Avandia.
Altogether it will pay $1bn in criminal fines and forfeitures over charges relating to the three drugs.
In addition, the company will pay $1.7bn in civil fines for illegal promotion of those drugs as well as others; paying kickbacks in their marketing; and making unsubstantiated claims about Avandia's safety and efficacy.
Separately, GSK is being fined $300 million to settle charges it underpaid rebates it owed to the Medicaid program.
The charges said that from 1998 to 2003 GSK promoted Paxil for use in treating depression in patients under 18 without approval from the US Food and Drug Administration.
Likewise, in 1999-2003 the company illegally promoted Wellbutrin for things like weight loss, sexual dysfunction treatment, and other uses.
The company also "spent millions of dollars" to encourage doctors to promote off-label uses of the drug.
Carmen Ortiz, the US Attorney for Massachusetts, which took part in the case, said physicians were given vacations in Hawaii, pheasant hunts in Europe and tickets to concerts as inducements to support GSK drugs.
The alleged kickbacks also involved the drugs Zofran, Imitrex, Lotronex, Flovent, and Valtrex, the Justice Department said.
The case involved several industry insider informants, who first reported the wrongdoings 10 years ago and who are able to share in the financial settlement in the case, according to lawyers.
As part of the settlement, GSK agreed to be monitored by government officials for five years.
GSK said in a statement it would pay the fines through existing cash resources.