Dr. Daryl Westerback, 55, was arrested on June 5 for driving under the influence of prescription drugs after he was spotted on the road by a narcotics detective.
Westerback is now being investigated for overprescribing drugs to his patients. Authorities are looking into at least one death related to his practice.
"We began looking into him and found that one of his patients last year died of a fatal overdose, so we're now opening the case and investigating that death," said Capt. Don Aguilar with the Ventura County Sheriff's Department.
Westerback, a psychiatrist, is also accused of treating patients while he was impaired by prescription drugs.
"We started watching him and found that he was also under the influence of opiates while seeing patients. We found that he was operating a psychiatry office and was treating pain-management patients without an exam room, without all the typical methods of good medicine," said Aguilar.
Officials say they are investigating at least 10 doctors for operating criminally in Ventura County. They say these doctors appear to be nothing more than drug dealers peddling prescription medications for profit.
Ventura County sheriff officials say federal law enforcement is helping with a special task force to crack down on the growing illegal use of prescription drugs like Oxycontin, Vicodin and more.
"This has been created out of a prescription drug problem we've been seeing in the county, specifically in East County," said Aguilar.
Detectives said Westerback's license to prescribe medication has been suspended and he remains free on bail.
Westerback was also arrested for driving under the influence of drugs and felony child endangerment after being involved in a car crash in Thousand Oaks on March 8, according to Ventura County Assistant Sheriff Gary Pentis.