According to a New York Times report Wednesday, Melgen is viewed as a generous campaign contributor whose close relationship with Menendez has been scrutinized by federal prosecutors. A spokesman for Menendez declined to comment on the federal data about Melgen's Medicare billings, the Times report said.
In a statement to CNBC, Melgen's attorney, Kirk Ogrosky, denied claims that Melgen defrauded Medicare, adding that Melgen provided a substantial amount of free services and drugs to patients who couldn't pay.
"At all times, Dr. Melgen billed in conformity with Medicare rules,'' Orgosky said. "While the amounts in the CMS data release appear large, the vast majority reflects the cost of drugs. The facts are that doctors receive 6 percent above what they pay for drugs, the amount billed by physicians is set by law, and drug companies set the price of drugs, not doctors.''
Melgen's office did not immediately return CNBC's request for comment.
—By CNBC's Katie Little