A Southern California doctor has pleaded guilty to writing prescriptions for more than 120,000 opioid pills over a six-year span, including to an impaired driver who struck and killed a bicyclist, federal prosecutors said.

In his plea agreement, Dr. Dzung Ahn Pham of Tustin admitted distributing the pills without a legitimate medical purpose in exchange for cash and insurance payments. He pleaded guilty Friday to conspiracy to distribute controlled substances, the Orange County Register reported.

Pham faces up to 20 years in federal prison when he is sentenced on Jan. 6, 2023, the newspaper said.

From Jan. 1, 2013 and Dec. 17, 2018, Pham wrote prescriptions for around 53,000 Oxycodone pills, 68,000 hydrocodone pills and 29,000 pills of amphetamine salts using 18 different patient names, according to his plea agreement.

Pham’s record of prescribing large amounts of pills led a CVS pharmacy to stop accepting prescriptions from him when he couldn’t justify the number of pills patients were picking up, prosecutors said when charges were filed in 2018.

Pham conspired with Jennifer Thaoyen Nguyen, 51, a licensed pharmacist who also has agreed to enter a guilty plea for the same felony charge later this month, court records show.

Pham directed his patients to Nguyen’s pharmacy to fill his prescriptions because he knew other pharmacies would not, according to court documents.

A man who fatally struck an off-duty firefighter training on his bike for a triathlon told investigators he was on drugs prescribed by Pham, prosecutors said. Several prescription bottles with Pham’s name were found in the driver’s car.

Stephen Scarpa was convicted of second-degree murder in the death of Costa Mesa fire Capt. Mike Kreza. Pham was not charged in the death.