SACRAMENTO — Gov. Gavin Newsom’s effort to lower prescription drug prices for Californians got its first outside participant Wednesday, as Los Angeles County said it would join the collective drug-purchasing system that Newsom is developing for state agencies.

Newsom traveled to a county-run rehabilitation center in Downey to make the announcement in recognition of his first 100 days in office, a milestone he hit this week.

Newsom signed an executive order the day he took office to establish a single purchaser of prescription drugs by 2021 for the 13 million recipients of Medi-Cal, the state’s health care program for the poor, as well as other agencies that buy drugs for state employees, retirees, hospitals and inmates.

He encouraged local governments and private companies to participate in the system, which he argued would give them all greater leverage to negotiate lower prices for pharmaceuticals. California spends more than $8 billion annually on prescription drugs for Medi-Cal alone, according to the Legislative Analyst’s Office.

“Honestly, I thought it would take years to convince other folks to join our effort,” Newsom said Wednesday. “And here we are in L.A. It took 100 days.”

Los Angeles County spends nearly $250 million a year on prescription drugs just at its public hospitals and clinics. Janice Hahn, chair of the county Board of Supervisors, said Los Angeles and the state would be “more effective when we work together.”

Representatives for San Francisco Mayor London Breed, Oakland Mayor Libby Schaaf and the Alameda County Board of Supervisors did not immediately respond to questions about whether they were considering joining the drug-purchasing collective.

Newsom said Wednesday that he had spoken with other governors about his plan. According to his office, the governors of Rhode Island, Illinois and Colorado approached Newsom at a conference in Washington, D.C., in February to discuss setting up their own systems for bulk prescription drug purchases and potentially combining forces across state lines.

“This was the number one topic of conversation when he went to the National Governors Association meeting,” spokesman Nathan Click said.

Alexei Koseff is a San Francisco Chronicle staff writer. Email: alexei.koseff@sfchronicle.com Twitter: @akoseff