By Charles Ornstein and Tracy Weber, ProPublicaThis piece originally appeared at ProPublica.

When the agency that runs Medicare announced last week that it would take action against doctors who prescribe abusively in its massive drug program 2013 perhaps banning them 2013 it raised an interesting question.

What exactly constitutes “abusive” prescribing?

On this point, the Centers for Medicare and Medicare and Medicaid Services (CMS) is treading carefully, refusing to get pinned down by numerical thresholds for specific drugs. Instead Medicare will consider a variety of factors in deciding whether a physician’s drug choices pose a “threat to the health or safety” of seniors and the disabled.

“In our view, if a physician or eligible professional repeatedly and consistently fails to exercise reasonable judgment in his or her prescribing practices, we should have the ability to remove such individuals from the Medicare program,” officials wrote in the 678-page document proposing changes.

“Honest physicians and eligible professionals who engage in reasonable prescribing activities would not be impacted by our proposal,” they wrote.

In stories last year, ProPublica detailed lax oversight of the Medicare drug program, known as Part D. The series showed that federal regulators’ failure to keep watch over the program has enabled doctors to prescribe massive quantities of inappropriate medications, has wasted billions on needlessly expensive drugs and has exposed the program to rampant fraud. Part D cost taxpayers $62 billion in 2012.

In the “proposed rule,” CMS said it would weigh eight factors in determining whether a health professional poses a threat. These include:

Whether patients’ diagnoses support using the medications prescribed.

Whether providers wrote prescriptions to patients they could not have seen 2013 such as those who are dead or were out of state at the time of billed office visits.

Whether providers prescribed excessive volumes of painkillers and other controlled substances linked to overdoses.

Whether disciplinary actions have been taken against providers by state regulators or Medicaid programs for the poor.

Whether providers have been sued for malpractice, including the number and type of such lawsuits, and whether those suits resulted in judgments or settlements.

Medicare said it would not base its decision on any single factor. “Nonetheless, there are certain criteria that, if met, would weigh heavily and perhaps decisively towards a finding that a revocation is justified,” the agency said.

Medicare’s case-by-case strategy is one ProPublica also determined was the best method when assessing Part D prescribing data it obtained from the program. The prescribing practices of doctors, reporters found, could not be judged by numbers alone 2013 whether overall or concentrated on specific medications. Often what looked troubling in the data had a real-life explanation. For example, in some cases, when physicians’ annual tally of prescriptions topped 150,000 2013 an inconceivable amount 2013 it turned out they specialized in nursing home care and their totals included prescriptions by others in their practices.

In order to identify physicians prescribing in unusual ways, ProPublica compared them to others in their specialty and state, looked at changes in their prescribing patterns from one year to the next, researched their backgrounds and disciplinary histories, and examined their preference for drugs with a high risk of abuse or misuse.

Reporters were able to spot the outliers. One Miami psychiatrist gave hundreds of elderly dementia patients antipsychotic drugs, despite a black-box warning against doing so. And an Oklahoma doctor gave an Alzheimer’s medication to scores of autistic and developmentally disabled young adults, despite a lack of evidence that it would help them.

Neither doctor had ever been questioned by Medicare.

In submitting the proposed new rules, CMS said it lacked the legal authority to take action against physicians who prescribed improperly, unless they had been formally excluded from Medicare, a step typically taken only after a criminal conviction.

As a result, the document said, “This means, in many cases, that the prescriber can continue prescribing drugs that will be covered under Part D.”

Two senators pushing CMS to do more about abusive prescribing released statements praising the agency’s proposed new rules. Sen. Thomas Carper, D-Del., chairman of the Senate Homeland Security and Governmental Affairs Committee, called them “common sense reforms.”

Sen. Tom Coburn, R-Okla., the top Republican on the panel, said “the vast majority of physicians” want to help patients, but “where there is proof of abuse or fraud, CMS should take necessary actions to protect patients and taxpayers.”

Others greeted the proposals with more tempered reactions.

Dr. Ardis Dee Hoven, president of the American Medical Association, said the group is reviewing the proposal to ensure CMS “does not compromise appropriate prescribing or exceed its statutory authority.”

“Responsible prescribing of pharmaceutical drugs is a fundamental aspect of medical practice and the American Medical Association has zero tolerance for harming the health and safety of patients,” Hoven said in a statement.

Physicians who appropriately and safely prescribe medications “should not be targets of misguided government enforcement and driven out of practice,” she added

The agency will take comments on the proposed rules until March 7. They are slated to take effect on Jan. 1, 2015.