CHARLESTON — An influential U.S. senator is checking up on South Carolina doctors who have billed millions of dollars in prescriptions to the financially struggling, taxpayer-funded Medicaid program.

U.S. Sen. Charles Grassley, an Iowa Republican, requested data from each state this year listing which doctors write the most prescriptions for eight common drugs covered by Medicaid, the federal health program for the poor. The reports were intended to “ensure that taxpayer dollars are appropriately spent,” Grassley wrote in a letter to state officials.

The Palmetto State’s report, released to The (Charleston) Post and Courier, identifies a handful of doctors who have written thousands of prescriptions for painkillers and anti-psychotics over the past two years. While many of the claims are legitimate, state Department of Health and Human Services officials confirmed this week that some doctors on the list are under investigation for fraud.

Kathleen Snider, the state agency’s compliance chief, declined to say which doctors are under review because their cases are open. State health departments are responsible for monitoring Medicaid prescription rates and billing irregularities.

Among the doctors getting the most reimbursements were a Columbia psychiatrist who wrote about 3,900 prescriptions for the drugs in question in 2008 and 2009. The doctor billed about $1.3 million to Medicaid, according to a Post and Courier review of the data.

A family doctor in Summerville billed about $635,000 for writing nearly 2,400 prescriptions for antipsychotics and painkillers during that time.

A psychiatrist with an Augusta address wrote more than 1,300 prescriptions, billing nearly $720,000 over the two years.

A Sumter family doctor billed more than $500,000 for writing about 860 prescriptions.

Grassley, a member of the Senate Finance Committee, which oversees Medicare and Medicaid, requested the state reports after discovering a Florida provider wrote 96,685 prescriptions for mental health drugs in a 21-month period.

Although the report shows no Palmetto State doctors approached that figure, Grassley took South Carolina’s data into consideration when he wrote to U.S. Secretary of Health and Human Services Kathleen Sebelius last month. His letter detailed states’ findings and encouraged the federal department to “step up efforts to monitor providers that are outliers” in both the Medicaid and Medicare systems.

A spokeswoman for Grassley said Friday Sebelius has not yet responded.

The states’ data does not indicate illegal activity, but shows that “there are very often providers that prescribe certain drugs at significantly higher rates than their peers,” Grassley wrote in his letter.

He continued, “This may be because a particular physician has a specific expertise or patient population, but it might also suggest overutilization or even health care fraud.”

Grassley also noted that the top prescriber for a particular drug often writes several times more prescriptions than the 10th highest prescriber. This was the case for several of South Carolina’s lists.

For example, a Greenville area neurologist wrote 100 prescriptions for Oxycontin in 2009 — 10 times more prescriptions that the No. 10 prescriber on the list.

The No. 1 prescriber of Xanax, a Greenville psychiatrist, wrote 1,073 prescriptions in 2009, while the number 10 prescriber wrote 63, according to the data.

Snider, of the S.C. Department of Health and Human Services, said over the past several years, the state has enacted data-mining surveillance systems to target Medicaid doctors who over-prescribe drugs.

While prescription drug abuse strains the system, Snider said other examples of fraud — billing for duplicate tests, extra hours or phantom patients — cause even more wasteful payouts because they can be harder to detect.