Each Medicare plan has a list of preferred drugs, known as a formulary. Under federal law, patients can request coverage of other drugs that may be medically necessary. But many insurers do not have procedures to handle such requests, auditors said.

John H. Wells, the compliance officer at Bravo Health, defended the company’s record, but he said: “The appeals and grievance process is very complex. It is very difficult for any plan to be fully compliant. In many cases, the government’s guidance is unclear, so it’s impossible for a business to know what to do.”

These findings were typical of the deficiencies described in Medicare audit reports:

¶UnitedHealth, which serves more than six million Medicare beneficiaries, did not have an “effective program” to supervise its marketing representatives, agents and brokers. In some cases, United improperly denied claims without giving any explanation to beneficiaries. Peter L. Ashkenaz, a company spokesman, said, “We terminated a few agents and brokers for misrepresenting our products.”

¶WellPoint, one of the nation’s largest insurers, had “a backlog of approximately 354,000 claims” at certain Medicare plans offered through its UniCare subsidiary. The company’s call center took an average of 27 minutes to answer phone calls from its members and 16 minutes to answer calls from health care providers. More than half the callers hung up before speaking to a company representative. Karen Brown, a spokeswoman for WellPoint, had no immediate comment.

¶In March, Sierra Health Services ended drug coverage for more than 2,300 Medicare beneficiaries with H.I.V./AIDS, saying they had not paid their premiums. In many cases, the premiums had been paid, and beneficiaries had canceled checks to prove it. Sierra initially refused to reinstate them, but eventually agreed to do so after repeated requests from federal officials. Peter O’Neill, a vice president of Sierra, said this particular drug plan, which attracted people with very high drug costs, would not be offered in 2008.

¶Humana, which covers more than 4.5 million people on Medicare, promised to investigate every complaint about its marketing practices, but it received so many complaints that it could not keep up. Many beneficiaries said they had received incorrect information from Humana agents. Medicare officials said some agents had not been adequately trained or supervised. Thomas T. Noland Jr., a senior vice president of Humana, said the company had taken “corrective action to improve the situation.”

¶Humana did not always tell beneficiaries about changes in its list of covered drugs. In some cases, Humana did not explain its reasons for denying claims and did not inform beneficiaries of their appeal rights.